Sensor interface circuit and sensor module

Abstract

This disclosure relates to sensor interface circuits and sensor modules. One aspect of the sensor interface circuit includes a frequency synchronization circuit connectable to a sensor. The frequency synchronization circuit comprises: a reference voltage source generating a reference voltage; a current source connected to the reference voltage source and generating a current using the reference voltage; a voltage difference detection circuit having a first input node connected to the reference voltage source, a second input node connected to the current source, and an output node, wherein one of the voltages received at the first input node and the second input node corresponds to the detection value of the sensor, and a control voltage is generated based on the difference between the voltages received at the first input node and the second input node; a voltage-controlled oscillation circuit connected to the output node of the voltage difference detection circuit and generating an oscillation signal based on the control voltage; and a frequency impedance conversion circuit connected between the voltage-controlled oscillation circuit and the second input node of the voltage difference detection circuit, converting the frequency of the signal corresponding to the oscillation signal into impedance.

Description

[0001] This application is a patent application filed in China with international application PCT / JP2021 / 020031 (international application date: May 26, 2021), and the aforementioned international application is incorporated herein by reference. Technical Field

[0002] This invention relates to sensor interface circuits and sensor modules. Background Technology

[0003] The sensor interface circuit with an oscillation circuit oscillates according to the signal level of the sensor, generating and outputting an oscillation signal with a frequency corresponding to the sensor signal. At this time, the sensor interface circuit needs to convert the sensor signal into a frequency with high precision (see, for example, Kaede Miyauchi, Taichi Taguchi, Yosuke Ishikawa, Hiroyuki Ito, Masashiro Michi, Kazuya Masu, and Noboru Ishihara, “Prototype Evaluation Results of Low-Power Wireless Sensor Terminal Module Using Radio Frequency Backscattering”, 2018 General Conference of the Society for Electronics, Information and Communications Technology, Japan, March 20-23, 2018, B-18-17, p. 361). Summary of the Invention

[0004] For example, in an interface circuit, in order to convert the sensor signal into a frequency with high accuracy, if a crystal or other original oscillator is used, a current corresponding to the oscillation signal of the original oscillator will continuously flow, and the current consumption may increase. Therefore, it is desirable to reduce the current consumption.

[0005] The present invention is proposed in view of the above circumstances, and its purpose is to provide a sensor interface circuit and sensor module that can reduce current consumption.

[0006] To address the aforementioned issues and achieve the objective, a side-related sensor interface circuit of the present invention includes a frequency synchronization circuit that can be connected to a sensor. The frequency synchronization circuit comprises: a reference voltage source that generates a reference voltage; a current source connected to the reference voltage source that generates a current using the reference voltage; a voltage difference detection circuit having a first input node connected to the reference voltage source, a second input node connected to the current source, and an output node, wherein one of the voltages received at the first input node and the second input node corresponds to the detection value of the sensor, and a control voltage is generated based on the difference between the voltages received at the first input node and the second input node; a voltage-controlled oscillation circuit connected to the output node of the voltage difference detection circuit that generates an oscillation signal based on the control voltage; and a frequency impedance conversion circuit connected between the voltage-controlled oscillation circuit and the second input node of the voltage difference detection circuit that converts the frequency of the signal corresponding to the oscillation signal into impedance.

[0007] According to the present invention, the effect of reducing current consumption is achieved. Attached Figure Description

[0008] Figure 1 This is a diagram illustrating the communication system structure that incorporates a sensor module with a sensor interface circuit related to the first embodiment.

[0009] Figure 2 This is a diagram showing the structure of a sensor module including the sensor interface circuit related to the first embodiment.

[0010] Figure 3 This is a diagram showing the structure of the frequency synchronization circuit in the first embodiment.

[0011] Figure 4 This is a diagram illustrating the operation of the frequency impedance transformation circuit in the first embodiment.

[0012] Figure 5 This is a waveform diagram showing the operation of the frequency synchronization circuit in the first embodiment.

[0013] Figure 6 This is a diagram showing the structure of a sensor module including the sensor interface circuit related to the second embodiment.

[0014] Figure 7 This is a diagram showing the structure of the frequency synchronization circuit in the second embodiment.

[0015] Figure 8 This is a waveform diagram showing the operation of the frequency synchronization circuit in the second embodiment. Detailed Implementation

[0016] Hereinafter, embodiments of the sensor interface circuit will be described in detail with reference to the accompanying drawings. In the following embodiments, parts with the same reference numerals will be assumed to perform the same operations, and repeated descriptions will be omitted as appropriate.

[0017] (First Implementation)

[0018] The sensor interface circuit in the first embodiment is a circuit that converts the sensor signal into a frequency. It is considered that when this circuit is applied to IoT (Internet of Things) technology, it is configured to enable communication using radio frequency backscatter communication, where the signal corresponding to the frequency converted by the sensor interface circuit is used.

[0019] For example, a sensor module 100 including sensor interface circuit 1, such as Figure 1 It is constructed as shown. Figure 1This is a diagram illustrating the structure of a communication system 300 that includes a sensor module 100 with sensor interface circuit 1.

[0020] In the communication system 300, multiple sensor modules 100-1 to 100-n and an information collection terminal 200 are configured to perform radio frequency backscatter communication. n is an integer greater than or equal to 1. The information collection terminal 200 can transmit RF signals to each sensor module 100. Each sensor module 100 can use the RF signal to transmit a signal corresponding to the sensor's detection value to the information collection terminal 200.

[0021] Each sensor module 100 includes a sensor 2, a sensor interface circuit 1, an impedance transformation circuit 4, and an antenna 5. The sensor interface circuit 1 is electrically connected between the sensor 2 and the antenna 5. The sensor interface circuit 1 has an oscillation circuit and an RF switch. The frequency of the oscillation circuit changes due to the signal from the sensor 2, and the oscillation signal controls the on / off state of the RF switch, thereby enabling radio frequency backscatter communication. That is, the sensor interface circuit 1 causes a change in the impedance of the RF switch side observed from the antenna 5, and based on the sensor information, it reflects and absorbs the RF signal from the information collection terminal 200 and transmits it to the information collection terminal 200. Thus, the sensor interface circuit 1 can reduce current consumption while simultaneously transmitting sensor information signals to the information collection terminal 200.

[0022] The oscillation frequency of an oscillating circuit is formed, for example, by any one or a combination of an amplifier, resistor, capacitor, inductor, and delay element. In the case of an oscillating circuit that is a relaxation oscillating circuit consisting of an inverter, a resistor R, and a capacitor C, the oscillation period T is theoretically expressed by the following formula 1.

[0023] T=2·ln3·(R·C)+Td...Equation 1

[0024] In Equation 1, Td represents the inverter's delay time. To achieve high stability with respect to temperature using this relaxation oscillation circuit, it is desirable that the temperature coefficients of the resistor R, capacitor C, and delay time Td be close to zero. Since the delay time Td is the inverter's delay time, it varies significantly with changes in power supply voltage and temperature, making it difficult to achieve high stability with respect to temperature.

[0025] As a specific example where temperature stability becomes an issue, consider the application of detecting changes in frequency. By connecting a sensor to a relaxation oscillation circuit, the change in oscillation frequency can be obtained, for example, based on the change in the sensor's resistance. The change in sensor resistance and the change in oscillation frequency are correlated. For example, when the sensor signal changes by 1%, the frequency changes by 1%. If the relaxation oscillation circuit oscillates at 100 kHz, the frequency change becomes 1 kHz. To detect this 1 kHz change with, for example, 64 grayscale values, it is desirable to generate an oscillation frequency with an accuracy corresponding to a change of 1 kHz / 64 = 15.625 Hz.

[0026] To achieve high precision in the oscillation frequency of the oscillator circuit, a frequency synchronization circuit is considered. For example, the frequency synchronization circuit is a frequency negative feedback circuit consisting of a primary oscillator, a frequency comparator circuit, a frequency divider circuit, and a voltage-controlled oscillator (VCO). The frequency difference between the oscillation frequency of the VCO and the reference oscillation frequency of the primary oscillator is detected, and the VCO is voltage-controlled to bring the frequency difference close to zero. As a result, the VCO oscillates with frequency accuracy at the reference oscillation frequency, thus mitigating the characteristic requirements of a voltage-controlled oscillator circuit.

[0027] In this frequency synchronization circuit, a crystal oscillator or resonator can be used as the original oscillator to generate the reference frequency. However, it is difficult to use a crystal oscillator or resonator as the original oscillator in the sensor interface circuit 1 for the following reasons.

[0028] For example, in IoT applications, the sensor interface circuit 1 requires low power consumption. However, if a primitive oscillator such as a crystal oscillator or resonator is used, a current corresponding to the oscillation signal of the primitive oscillator will continuously flow. As a result, the current consumption of the sensor interface circuit 1 increases, and there is a possibility that it will be difficult to meet the requirement of low power consumption.

[0029] Each sensor module 100 sometimes uses RF signals received from the information collection terminal 200 to generate electricity, but the time from power generation to operation is limited due to the limitations of the radio wave law. If a primitive oscillator such as a crystal oscillator or resonator is used in the frequency synchronization circuit, the lock-in time until the frequency of the reference frequency signal stabilizes is long, and therefore there is a possibility that it is difficult to meet the limitations of the radio wave law.

[0030] Compared to circuit elements such as transistors, primitive oscillators, such as crystal oscillators or resonators, are often physically larger. Therefore, if primitive oscillators such as crystals are used in frequency synchronization circuits, the sensor interface circuit 1 is prone to large-scale operation, and there is a possibility that it will be difficult to meet the actual installation size requirements required in technologies such as IoT.

[0031] Compared to circuit components such as transistors, primitive oscillators such as crystal oscillators or resonators are more expensive. Therefore, if primitive oscillators such as crystal oscillators or resonators are used in the frequency synchronization circuit, the cost of sensor interface circuit 1 is likely to increase.

[0032] Therefore, in the first embodiment, the frequency synchronization circuit is configured to perform frequency locking loop feedback operation in the voltage domain instead of the frequency domain in the sensor interface circuit 1, thereby achieving high precision of the frequency synchronization circuit without using the original oscillator.

[0033] Specifically, in sensor interface circuit 1, a frequency synchronization circuit generates a reference voltage instead of a reference frequency signal. The reference voltage can be generated using circuit elements such as resistors. The frequency synchronization circuit converts the oscillation frequency of the oscillation signal generated by the oscillation operation into an impedance. For example, by using a switched capacitor circuit, the oscillation frequency of the oscillation signal can be converted into an impedance. The converted impedance is further converted into a voltage using a current corresponding to the sensor's detected value. The converted voltage is the voltage corresponding to the sensor's detected value. Feedback operation of the frequency-locking loop is performed so that the difference between the voltage corresponding to the sensor's detected value and the reference voltage is close to zero. Thus, frequency synchronization can be achieved in the voltage domain, minimizing impact on the accuracy of the reference voltage and obtaining a high-precision and robust oscillation frequency. As a result, the frequency synchronization circuit can be made highly accurate without using the original oscillator. Therefore, for example, the current consumption of sensor interface circuit 1 can be reduced, meeting the requirements for low power consumption. The locking time of the frequency-locking loop can be shortened, satisfying the limitations of the radio wave method. It can suppress the enlargement of sensor interface circuit 1, meet the actual installation size requirements of IoT technology, and therefore does not degrade the accuracy of oscillation frequency, allowing for integration into integrated circuits. Furthermore, compared to using a raw oscillator, it can reduce the cost of sensor interface circuit 1.

[0034] More specifically, the sensor interface circuit 1, including the frequency synchronization circuit 10, can... Figure 2 It is constructed as shown. Figure 2 This is a diagram illustrating the structure of a sensor module 100, including a sensor interface circuit 1.

[0035] In sensor module 100, sensor interface circuit 1 is connected between sensor 2, impedance transformation circuit 4, and antenna 5. Impedance transformation circuit 4 is electrically connected between sensor interface circuit 1 and antenna 5.

[0036] Sensor 2 is a resistive sensor, including a variable resistive element R whose resistance changes equivalently according to its detected value. SENS Variable resistor element R SENSOne end is connected to terminal 1a of sensor interface circuit 1, and the other end is connected to ground potential.

[0037] Impedance transformation circuit 4 includes impedance transformer 4a and impedance transformer 4b. Impedance transformer 4a and impedance transformer 4b are connected in parallel between sensor interface circuit 1 and antenna 5. One end of impedance transformer 4a is connected to antenna 5, and the other end is connected to terminal 1b of sensor interface circuit 1. One end of impedance transformer 4b is connected to antenna 5, and the other end is connected to terminal 1c of sensor interface circuit 1.

[0038] The sensor interface circuit 1 includes a frequency synchronization circuit 10, an LPF (Low Pass Filter) 7, an RF switch 6, a power generation circuit 8, and a voltage control circuit 9.

[0039] The power generation circuit 8 is electrically connected between terminal 1c and voltage control circuit 9. The power generation circuit 8 has storage elements such as capacitors, and transmits RF signals from antenna 5 through impedance transformer 4b and terminal 1c.

[0040] A voltage control circuit 9 is electrically connected between the power generation circuit 8 and the frequency synchronization circuit 10. During charging, the voltage control circuit 9 cuts off the power supply to the frequency synchronization circuit 10, causing charge to accumulate in the storage element. During discharging, the charge accumulated in the storage element is used to supply power to the frequency synchronization circuit 10. Thus, a power supply potential V can be supplied to the input node 10a of the frequency synchronization circuit 10. DD .

[0041] The input node 10a of the frequency synchronization circuit 10 is electrically connected to the voltage control circuit 9, the input node 10b is electrically connected to the sensor 2 via terminal 1a, and the output node 10c is electrically connected to the LPF7. The frequency synchronization circuit 10 operates based on the detected value of the sensor 2 (e.g., the variable resistor R). SENS The frequency synchronization circuit 10 performs oscillation based on the resistance value of the resistor. At this time, the frequency synchronization circuit 10 performs feedback operation of the frequency locking loop in the voltage domain.

[0042] Frequency synchronization circuit 10 generates reference voltage V REF At the same time, the frequency synchronization circuit 10 sets the oscillation frequency F of the oscillation signal generated by the oscillation operation. OUT Transformed into impedance. Frequency synchronization circuit 10 uses a variable resistor element R. SENS The resistance value and the corresponding current I SENS The transformed impedance is further transformed into a voltage V. SENS The transformed voltage V SENS The voltage corresponding to the sensor's detected value is determined by the frequency synchronization circuit 10, which performs a frequency-locking loop feedback operation to ensure that the voltage V corresponding to the sensor's detected value is... SENSand reference voltage V REF The difference is close to zero. Therefore, frequency synchronization can be achieved with high precision in the voltage domain.

[0043] For example, such as Figure 2 As shown, the frequency synchronization circuit 10 includes a voltage-controlled oscillation circuit 11, a frequency divider circuit 12, a frequency impedance transformation circuit 13, a reference voltage source 14, a current source 15, a voltage difference detection circuit 16, and a filter 17. The voltage difference detection circuit 16, filter 17, voltage-controlled oscillation circuit 11, frequency divider circuit 12, and frequency impedance transformation circuit 13 are connected in a loop. This loop connection constitutes a frequency-locked loop. Furthermore, in the frequency synchronization circuit 10, the sensor 2 is connected to the current source 15, and the current I flowing through the current source 15... SENS The frequency synchronization circuit 10 changes based on the detected value of sensor 2. It can be called a current-varying frequency synchronization circuit.

[0044] A reference voltage source 14 is connected in parallel with a current source 15 and a voltage difference detection circuit 16. The output node 14a of the reference voltage source 14 is connected to the control node 15c of the current source 15 and the input node 16a of the voltage difference detection circuit 16. The reference voltage source 14 generates a reference voltage. For example, the reference voltage source 14 is as follows: Figure 3 As shown, a reference voltage V is generated by resistor voltage division. REF . Figure 3 This diagram illustrates the structure of the frequency synchronization circuit 10. The reference voltage source 14 has multiple resistive elements 141 and 142. One end of resistive element 141 is connected to ground potential, and the other end is connected to resistive element 142. One end of resistive element 142 is connected to resistive element 141, and the other end is connected to the power supply potential V. DD Connection. If the resistance value of resistor 141 is set to R, and the resistance value of resistor 142 is set to R, then the reference voltage source 14 can generate the reference voltage V shown in Equation 2. REF .

[0045] V REF ={R / (R+R)}×V DD =V DD / 2...Equation 2

[0046] exist Figure 3 The example shown illustrates a structure with a resistor voltage division ratio of 1 / 2, but the reference voltage source 14 can also be configured according to the required reference voltage V. REF The value is formed by the values ​​of other resistor voltage division ratios that are greater than 0 and less than 1.

[0047] Figure 2 The reference voltage source 14 shown will transmit the reference voltage V REF The current is supplied to the current source 15 and the voltage difference detection circuit 16 respectively.

[0048] Current source 15 is electrically connected to reference voltage source 14, voltage difference detection circuit 16, and frequency impedance transformation circuit 13, and can be electrically connected to sensor 2. Input node 15a of current source 15 is connected to voltage control circuit 9, input node 15c is connected to reference voltage source 14, input node 15d is connected to sensor 2, and output node 15b is connected to input node 16b of voltage difference detection circuit 16. Current source 15 generates a current I corresponding to the detected value of sensor 2. SENS It generates and flows to the input node 16b of the voltage difference detection circuit 16.

[0049] For example, current source 15 can be like Figure 3 It is configured as shown. The current source 15 includes transistor 151, transistor 152, and differential amplifier circuit 153. Transistor 151 is electrically connected to the power supply potential V. DD Between the input node 16b of the voltage difference detection circuit 16 and the transistor 151, which is, for example, a PMOS transistor, with its source at the power supply potential V. DD The drain of transistor 152 is connected to the input node 16b of the voltage difference detection circuit 16, and the gate is connected to the output node 153c of the differential amplifier circuit 153. Transistor 152 can be electrically connected to the power supply potential V. DD Between sensor 2 and transistor 152. Transistor 152 is, for example, a PMOS transistor, with its source at the power supply potential V. DD Connection, drain and variable resistor R in sensor 2 SENS One end of the gate is connected to the output node 153c of the differential amplifier circuit 153 and the gate of the transistor 151. The differential amplifier circuit 153 has an input node 153a, an input node 153b, and an output node 153c. Input node 153a is electrically connected to the reference voltage source 14 and receives the reference voltage V from the reference voltage source 14. REF Input node 153b is electrically connected to node 154 between transistor 152 and sensor 2. Output node 153c shares a common connection with the gates of transistors 151 and 152.

[0050] That is, transistors 151 and 152 form a current mirror circuit via differential amplifier circuit 153. Using the feedback loop of differential amplifier circuit 153 → transistor 152 → node 154 → differential amplifier circuit 153, differential amplifier circuit 153 controls the gate voltages of transistors 151 and 152 so that the potential of node 154 is close to the reference voltage V. REF They are equal. Therefore, in the variable resistor element R... SENS Current I flowing in SENS 'It becomes the same as the following formula 3.'

[0051] I SENS '=VREF / R SENS ...Equation 3

[0052] If the reflection coefficient of the current reflector circuit is assumed to be 1, then the current I flowing from the current source 15 to the input node 16b of the voltage difference detection circuit 16 is... SENS It becomes the following expression, 4.

[0053] I SENS =I SENS '=V REF / R SENS ...Equation 4

[0054] As shown in Equation 4, the current I of current source 15 SENS According to the resistance value R of sensor 2 SENS Changes occur due to variations in current I. SENS The resistance value R of sensor 2 is indicated. SENS The changes.

[0055] Figure 2 The voltage-controlled oscillation circuit 11 shown is electrically connected between the voltage difference detection circuit 16 and the output node 10c, and is also electrically connected between the filter 17 and the frequency divider circuit 12. The input node 11a of the voltage-controlled oscillation circuit 11 is electrically connected to the output node 16c of the voltage difference detection circuit 16 via the filter 17, and the output node 11b is electrically connected to the output node 10c via the frequency divider circuit 12. The voltage-controlled oscillation circuit 11 operates based on the control voltage V received from the voltage difference detection circuit 16 via the filter 17. CTRL It performs an oscillation action to generate a voltage V that is similar to the control voltage. CTRL The corresponding frequency F SENS The oscillation signal.

[0056] For example, voltage-controlled oscillation circuit 11, such as Figure 3 As shown, it can be constructed using a relaxation-type oscillation circuit. The voltage-controlled oscillation circuit 11 has an inverter chain 111, a variable resistor element 112, and a capacitor element 113. The inverter chain 111 includes multiple stages of inverters Inv1 to Inv3 connected in a ring. Each inverter Inv is constructed, for example, by inverter-connecting NMOS transistors and PMOS transistors. The number of stages of the inverters Inv is an odd number, for example, 3 stages. The output node of the first-stage inverter Inv1 is electrically connected to the input node of the next-stage inverter Inv2. The output node of the final-stage inverter Inv3 is electrically connected to the output node 11b of the voltage-controlled oscillation circuit 11 and the input node of the first-stage inverter Inv1, respectively. The variable resistor element 112 is electrically connected in series with the multiple stages of inverters Inv1 to Inv3 in the inverter chain 111. Figure 3The diagram illustrates a structure in which a variable resistor element 112 is electrically connected between the output node of inverter Inv2 and the input node of inverter Inv3. A capacitor element 113 is connected in parallel with inverter Inv in inverter chain 111 and with the variable resistor element 112. Figure 3 The diagram illustrates a structure in which capacitor element 113 is connected in parallel with respect to the series connection of inverter Inv2 and variable resistor element 112 in the second stage.

[0057] In the voltage-controlled oscillation circuit 11, the variable resistor element 112 receives the control voltage V at the control node. CTRL The variable resistor element 112 adjusts according to the control voltage V. CTRL And let its resistance value R VCO The value changes. If we assume the capacitance of capacitor element 113 is C... VCO The resistance value R through the variable resistor element 112 is... VCO The time constant R of the variable resistor element 112 and the capacitor element 113 changes, thus affecting their relationship. VCO ×C VCO The oscillation frequency F of the voltage-controlled oscillation circuit 11 changes. SENS Based on the time constant R VCO ×C VCO This is determined by the time constant R. VCO ×C VCO According to the control voltage V CTRL As the time constant changes, the voltage-controlled oscillator circuit 11 generates a time constant R that is the same as the changed time constant. VCO ×C VCO The corresponding frequency F SENS The oscillation signal. It should be noted that the variable resistor element 112 can, for example, have its drain connected to the output node of inverter Inv2, its source connected to the input node of inverter Inv3, and its gate subject to a control voltage V. CTRL It is composed of NMOS transistors.

[0058] Figure 2 The voltage-controlled oscillator circuit 11 shown will have a frequency F SENS The oscillation signal is supplied to the frequency divider circuit 12.

[0059] Frequency divider circuit 12 is electrically connected between voltage-controlled oscillator circuit 11 and output node 10c. Input node 12a of frequency divider circuit 12 is electrically connected to output node 11b of voltage-controlled oscillator circuit 11, and output node 12b is electrically connected to output node 10c. Frequency divider circuit 12 divides the oscillation signal received at input node 12a to generate a frequency F. OUT The oscillation signal is supplied to LPF7 and frequency impedance transformation circuit 13 respectively. Frequency F OUT It can also be frequency FSENS The frequency is half of the frequency. At this time, the frequency divider circuit 12 can adjust the duty cycle of the oscillation signal to, for example, around 50%.

[0060] For example, frequency divider circuit 12, such as Figure 3 The circuit is configured as shown to perform a 2-fold frequency division. The frequency divider circuit 12 includes a flip-flop 121 and an inverter 122. The data input node D of the flip-flop 121 is connected to the output node of the inverter 122, the clock node CK is connected to the output node 11b of the voltage-controlled oscillation circuit 11, and the data output node Q is connected to the input node of the inverter 122, LPF7, and the frequency impedance transformation circuit 13, respectively. The flip-flop 121 maintains its output signal inverted synchronously with the rising edge of the oscillation signal waveform and switches the output signal. Thus, the frequency divider circuit 12 divides the oscillation signal received at the input node 12a by 2, generating a frequency F. OUT (=F SENS The frequency divider circuit 12 can adjust the duty cycle of the oscillation signal to around 50% by switching the output oscillation signal according to the period of the received oscillation signal.

[0061] Figure 2 The frequency divider circuit 12 shown will have a frequency F OUT The oscillation signal is output to LPF7 and fed back to the frequency impedance conversion circuit 13.

[0062] The frequency impedance transformation circuit 13 is electrically connected between the input node 16b of the voltage-controlled oscillation circuit 11 and the voltage difference detection circuit 16, and is also electrically connected between the input node 16b of the frequency divider circuit 12 and the voltage difference detection circuit 16. The frequency impedance transformation circuit 13 is routed to the feedback line from the output node 12b of the frequency divider circuit 12 to the input node 16b of the voltage difference detection circuit 16. The input node 13a of the frequency impedance transformation circuit 13 is connected to the frequency divider circuit 12, and the output node 13b is connected to the input node 16b of the voltage difference detection circuit 16. The frequency impedance transformation circuit 13 receives the oscillation signal from the frequency divider circuit 12 and converts the frequency F of the oscillation signal... OUT Transformed into impedance.

[0063] For example, frequency impedance transformation circuit 13, as shown Figure 3 As shown, it can be constructed using a switched capacitor circuit. A switched capacitor circuit is a circuit that limits current or voltage, similar to a resistor, by combining a switch and a capacitor. The frequency impedance transformation circuit 13 charges and discharges the capacitor according to the oscillation signal, thereby enabling the circuit impedance to become a value corresponding to the frequency of the oscillation signal.

[0064] The frequency impedance transformation circuit 13 includes capacitor element 131, capacitor element 132, switch 133, switch 134, and inverter 135. One end of capacitor element 131 is connected to ground potential, and the other end is connected to node 136 between switch 133 and switch 134. One end of capacitor element 132 is connected to ground potential, and the other end is connected to input node 16b of voltage difference detection circuit 16. One end of switch 133 is connected to input node 16b of voltage difference detection circuit 16, and the other end is connected to node 136. Its control terminal is connected to voltage controlled oscillation circuit 11 via frequency divider circuit 12. One end of switch 134 is connected to node 136, and the other end is connected to ground potential. Its control terminal is connected to inverter 135. The input node of inverter 135 is connected to voltage controlled oscillation circuit 11 via frequency divider circuit 12, and its output node is connected to switch 134.

[0065] In the frequency impedance transformation circuit 13, switches 133 and 134 are switched on and off complementaryly according to the level of the oscillation signal. As a result, capacitor 131 is charged and discharged. When the oscillation signal is at level H, switch 133 is kept off and switch 134 is kept on, the charge in capacitor 131 is discharged to ground potential, and capacitor 131 is discharged. When the oscillation signal is at level L, switch 133 is kept on and switch 134 is kept off, and the current I... SENS The corresponding charge is stored in capacitor element 131, and capacitor element 131 is charged. At this time, regardless of the level of the oscillation signal, capacitor element 132 maintains its connection with current I. SENS The corresponding charge has accumulated.

[0066] That is, the frequency impedance transformation circuit 13 uses a circuit with a frequency F OUT The oscillation signal periodically charges and discharges the capacitor element 131, thereby effectively generating a frequency F. OUT The corresponding impedance. The output voltage of the frequency impedance transformation circuit 13 is represented by the voltage V at the input node 16b of the voltage difference detection circuit 16. SENS Voltage V SENS The charging of capacitor element 131 varies in a time constant manner, but is maintained in relation to current I. SENS The corresponding charge accumulation in the capacitor element 132 is averaged and converges to a stable point.

[0067] For example, such as Figure 4 As shown, in current I SENS When I = I1, the voltage V at which it converges to the steady point SENS =V1. In current I SENS When I = I2, the voltage V at which it converges to the steady point SENS =V2. In current ISENS =I 10 When the voltage V converges to the steady point SENS =V 10 . Figure 4 The diagram illustrates the operation of the frequency impedance transformation circuit 13, with the vertical axis representing the voltage magnitude and the horizontal axis representing time. It can be seen that in the frequency impedance transformation circuit 13, if the flowing current I... SENS If the voltage V increases, it will converge to the steady point. SENS It increases almost proportionally. That is, if the capacitance of capacitor element 132 is C... AVE The capacitance value of capacitor element 131 is C. SC Then, the voltage V at input node 16b of the voltage difference detection circuit 16 when converging to the steady point is... SENS It becomes the following number, 5.

[0068] V SENS =I SENS / (F OUT ·C SC )...Equation 5

[0069] As shown in Equation 5, when converging to a stable point, the frequency F of the oscillation signal... OUT The frequency impedance transformation circuit 13 transforms the impedance to 1 / (F). OUT ·C SC Equivalently, it becomes impedance 1 / (F) OUT ·C SC One end of the circuit is connected to the input node 16b of the voltage difference detection circuit 16, and the other end is connected to the ground potential. Therefore, regarding the input node 16b of the voltage difference detection circuit 16, the current I from the current source 15... SENS Inflow equivalent impedance 1 / (F) OUT ·C SC ), thereby through the equivalent impedance 1 / (F OUT ·C SC And the current I SENS Transformed into voltage V SENS Voltage V SENS Including current I SENS This corresponds to the value detected by sensor 2. Furthermore, the voltage V... SENS Including frequency F OUT The oscillation frequency F of the voltage-controlled oscillation circuit 11 SENS correspond.

[0070] Reference voltage source 14, current source 15, frequency impedance transformation circuit 13 and Figure 2The input terminals of the voltage difference detection circuit 16 shown are connected, and the filter 17 is electrically connected to the output terminal. Input node 16a of the voltage difference detection circuit 16 is connected to the reference voltage source 14, input node 16b is connected to the current source 15 and the frequency impedance transformation circuit 13, and output node 16c is connected to the filter 17. The voltage difference detection circuit 16 receives the reference voltage V at input node 16a. REF At input node 16b, a voltage V is generated through current source 15 and frequency impedance transformation circuit 13. SENS The voltage difference detection circuit 16 detects the voltage difference based on the reference voltage V. REF and voltage V SENS The differential generation is used to control the voltage V so that the differential is reduced. CTRL '.

[0071] For example, voltage difference detection circuit 16 Figure 3 The circuit includes a differential amplifier circuit 161 as shown. The non-inverting input terminal (+) of the differential amplifier circuit 161 is connected to a reference voltage source 14, the inverting input terminal (-) is connected to a current source 15 and a frequency impedance transformation circuit 13, and the output terminal is connected to a filter 17. The differential amplifier circuit 161 operates on a reference voltage V... REF and voltage V SENS The differential voltage V is amplified to generate the control voltage. CTRL '.

[0072] Figure 2 The filter 17 shown is electrically connected between the voltage difference detection circuit 16 and the voltage-controlled oscillation circuit 11. The input node 17a of the filter 17 is connected to the voltage difference detection circuit 16, and the output node 17b is connected to the voltage-controlled oscillation circuit 11. The filter 17 receives the control voltage V from the voltage difference detection circuit 16. CTRL ', for control voltage V CTRL 'Perform filtering processing. Filter 17 will filter the control voltage V...' CTRL Supply to voltage-controlled oscillation circuit 11.

[0073] For example, such as Figure 3 As shown, filter 17 is composed of a low-pass filter. Filter 17 has a resistive element 171 and a capacitive element 172. One end of the resistive element 171 is connected to the output terminal of the differential amplifier circuit 161, and the other end is connected to one end of the capacitive element 172 and the voltage-controlled oscillation circuit 11. The other end of the capacitive element 172 is connected to ground potential. Through this structure, filter 17 controls the voltage V. CTRL Implementing low-pass filtering and smoothing can improve the smoothed control voltage V. CTRL Supply to voltage-controlled oscillation circuit 11.

[0074] In the frequency synchronization circuit 10, a frequency-locked loop is used, consisting of voltage difference detection circuit 16 → filter 17 → voltage-controlled oscillation circuit 11 → frequency divider circuit 12 → frequency impedance transformation circuit 13 → voltage difference detection circuit 16. The voltage difference detection circuit 16 controls the voltage V. CTRL Perform feedback control to make the voltage V SENS With reference voltage V REF They are equal. That is, under normal circumstances, the following equation 6 holds true.

[0075] V REF =V SENS ...Equation 6

[0076] If we substitute the number 6 into the number 5, we get the following number 7.

[0077] V REF =I SENS / (F OUT ·C SC )...Equation 7

[0078] If we are referring to frequency F OUT Solving expression 7, we obtain expression 8.

[0079] F OUT =1 / {(V REF / I SENS )·C SC}...Equation 8

[0080] If we substitute the number 8 into the number 4, we get the following number 9.

[0081] F OUT =1 / {R SENS ·C SC}...Equation 9

[0082] As shown in Equation 9, it can be seen that the frequency synchronization circuit 10 does not depend on the reference voltage V. REF The equivalent resistance value R of sensor 2 is obtained. SENS The corresponding oscillation frequency F OUT Therefore, frequency synchronization can be achieved in the voltage domain, and the reference voltage V can be obtained with minimal impact. REF The precision of the oscillation frequency F OUT That is, it is possible to obtain a high-precision and robust oscillation frequency F. OUT .

[0083] Furthermore, as shown in Equation 9, the frequency synchronization circuit 10 can obtain the power supply potential V. DD Insensitive oscillation frequency F OUT Therefore, it is possible to enhance the control over the power supply potential V. DDThis improves the tolerance to fluctuations and alleviates the characteristic requirements of the voltage control circuit 9. As a result, chip costs can be reduced when the sensor interface circuit 1 is actually mounted on a semiconductor chip.

[0084] Furthermore, as shown in Equation 9, the frequency synchronization circuit 10 can obtain an oscillation frequency F that is insensitive to transistor characteristic deviations. OUT This reduces the performance requirements on the transistor. As a result, when the sensor interface circuit 1 is actually mounted on a semiconductor chip, chip costs can be reduced.

[0085] Furthermore, as shown in Equation 9, the oscillation frequency F, which has low temperature dependence on the resistance and capacitance values, is obtained through the frequency synchronization circuit 10. OUT Thus, it is possible to obtain an oscillation frequency F that is insensitive to temperature. OUT Therefore, it is possible to improve tolerance to temperature variations using low-cost components without employing crystal oscillators or MEMS resonators. As a result, chip costs can be reduced when the sensor interface circuit 1 is actually mounted on a semiconductor chip.

[0086] Next, use Figure 5 The operation of the frequency synchronization circuit 10 is explained. Figure 5 A waveform diagram to show the operation of the frequency synchronization circuit 10.

[0087] If the frequency synchronization circuit 10 starts at time t1, then the current source 15 begins to flow in response to the resistance value R of the sensor 2. SENS = The corresponding current I of R1 SENS =I1. Correspondingly, the frequency impedance transformation circuit 13 will interact with the current I. SENS =I1 The corresponding charge is stored in capacitor element 132, and the voltage V SENS The voltage gradually increases. At this time, the oscillation signal is at level L, switch 133 is kept in the open state, switch 134 is kept in the closed state, and capacitor element 131 is in the discharging state, with its voltage V... SC It is approximately equal to the ground potential. Furthermore, the reference voltage V... REF It is kept constant, and therefore accompanied by voltage V SENS As the voltage gradually increases, the voltage difference detection circuit 16 causes the control voltage V to rise. CTRL Gradually increasing.

[0088] At time t2, the voltage-controlled oscillation circuit 11 begins oscillation, and the oscillation signal level changes to H level. Correspondingly, switch 133 is turned on, and switch 134 is turned off. One end of capacitor element 131 is connected to input node 16b, thereby increasing the voltage V. SENS It drops for a split second. Afterwards, the charge is redistributed by capacitor elements 131 and 132 and interacts with current I. SENSThe corresponding charge I1 is charged to capacitor elements 131 and 132 respectively, and the voltage V SC and voltage V SENS They gradually rose.

[0089] At time t3, the oscillation signal level changes to L. Correspondingly, switch 133 opens and switch 134 closes. Capacitor 131 is discharged, and its voltage V... SC The voltage drops to ground potential. At this time, capacitor element 132 retains its charge, thus accompanied by a current I. SENS =I1 corresponding charge is charged to capacitor element 132, voltage V SENS It continues to rise gradually.

[0090] At time t4, the oscillation signal level changes to H level. Correspondingly, switch 133 is turned on, and switch 134 is turned off. One end of capacitor element 131 is connected to input node 16b, thus increasing the voltage V. SENS It drops for a split second. Afterwards, the charge is redistributed by capacitor elements 131 and 132 and interacts with current I. SENS =I1 corresponding charge is charged to capacitor elements 131 and 132 respectively, voltage V SC and voltage V SENS They gradually rose.

[0091] At time t5, the oscillation signal level changes to L. Correspondingly, switch 133 opens and switch 134 closes. Capacitor 131 is discharged, and its voltage V... SC It is reduced to ground potential. At this time, capacitor element 132 retains its charge, and therefore is related to current I. SENS =The corresponding charge of I1 is charged to capacitor element 132, and with this, voltage V SENS It continues to rise gradually.

[0092] During timings t6 to t14, the same actions as timings t4 to t5 and timings t5 to t6 are repeated alternately, and the voltage V... SENS It varies with a sawtooth-shaped waveform while maintaining a time-averaged approximation to the reference voltage V. REF Control voltage V CTRL It gradually approaches the value V1. Accompanying this, the frequency F of the oscillation signal... OUT Approaching the resistance value R of sensor 2 SENS = R1 corresponds to the value F1.

[0093] If the voltage V is at time t14 SENS Averaged over time with reference voltage V REF If they are equal, then from time t14 to time t18, the control voltage V... CTRLThe frequency F of the oscillation signal stabilizes at a value of V1. OUT Stabilized at the resistance value R of sensor 2 SENS When the value F1 corresponding to R1 is equal to the frequency lock loop, the frequency synchronization circuit 10 enters a locked state. Therefore, the frequency synchronization circuit 10 stably outputs the resistance value R of sensor 2. SENS = Frequency F corresponding to R1 OUT =F1. Period TP1 is the period corresponding to frequency F1.

[0094] At time t18, the resistance value R of sensor 2 is determined based on changes in the state of the object being detected by sensor 2. SENS The change is R2 (>R1), and the current change is I. SENS =I2 (<I1). Correspondingly, during timings t18 to t24, the same actions as timings t4 to t5 and t5 to t6 are repeated alternately, and the voltage V SENS It varies with a sawtooth-shaped waveform and approaches the reference voltage V on an average time basis. REF Control voltage V CTRL It gradually approaches the value V2 (>V1). Accompanying this, the frequency F of the oscillation signal... OUT Approaching the resistance value R of sensor 2 SENS = R2 corresponds to the value F2 (< F1).

[0095] If the voltage V is at time t24 SENS Averaged over time with reference voltage V REF If they are approximately equal, then after timer t24, the control voltage V... CTRL The frequency F of the oscillation signal stabilizes at a value of V2. OUT Stabilized at the resistance value R of sensor 2 SENS =R2 corresponds to the value F2, and the frequency locking loop is locked again. Therefore, the frequency synchronization circuit 10 stably outputs the resistance value R of sensor 2. SENS = Frequency F corresponding to R2 OUT =F2. Period TP2 is the period corresponding to frequency F2.

[0096] As described above, in the first embodiment, in the sensor interface circuit 1, the frequency synchronization circuit 10 is configured to perform the frequency-locked loop feedback operation in the voltage domain instead of the frequency domain. Therefore, an oscillation frequency with accuracy independent of the reference voltage can be obtained, and the feedback operation of the frequency-locked loop can be easily made highly accurate in the voltage domain. Thus, the frequency synchronization circuit 10 can be made highly accurate without using a raw oscillator.

[0097] It should be noted that in the frequency synchronization circuit 10, if the duty cycle of the oscillation signal output from the voltage-controlled oscillation circuit 11 is close to 50%, the frequency divider circuit 12 can be omitted. Furthermore, if the control voltage output from the voltage difference detection circuit 16 is smooth, the filter 17 can also be omitted.

[0098] (Second Implementation)

[0099] Next, the sensor interface circuit 1j related to the second embodiment will be described. The description will focus on the parts that differ from the first embodiment.

[0100] In the first embodiment, a current-varying frequency synchronization circuit is exemplified as the feedback operation of the frequency-locking loop in the voltage domain; however, in the second embodiment, a voltage-varying frequency synchronization circuit is exemplified. In the current-varying frequency synchronization circuit, the current of the current source changes relative to the change in the sensor's detected value; conversely, in the voltage-varying frequency synchronization circuit, the voltage amplified and output by the amplifier circuit changes.

[0101] Specifically, the sensor module 100j, including the sensor interface circuit 1j, can... Figure 6 It is constructed as shown. Figure 6 This is a diagram illustrating the structure of a sensor module 100j that includes the sensor interface circuit 1j associated with the second embodiment.

[0102] Sensor interface circuit 1j replaces frequency synchronization circuit 10 (refer to) Figure 2 The frequency synchronization circuit 10j replaces the current source 15 (see reference). Figure 2 It has a current source 15j, an amplifier circuit 18j, and a resistor element 19j.

[0103] Amplifier circuit 18j is electrically connected between reference voltage source 14 and voltage difference detection circuit 16, and can be electrically connected to sensor 2. Input node 18a of amplifier circuit 18j is connected to reference voltage source 14, and input node 18b is connected to resistive element 19j and variable resistor element R. SENS The nodes are connected at node 192, and the output node 18c is connected to the voltage difference detection circuit 16.

[0104] The reference voltage source 14 is connected to the current source 15j and the amplifier circuit 18j, respectively. The output node 14a of the reference voltage source 14 is connected to the control node 15c of the current source 15j and the input node 18a of the amplifier circuit 18j. The reference voltage source 14 generates a reference voltage V. REF The amplifier circuit 18j receives the reference voltage V at input node 18a. REF '.

[0105] The variable resistance element R of sensor 2 SENS One end of the variable resistor R is connected to the voltage control circuit 9 via terminal 1d of the sensor interface circuit 1j, and the other end is connected to the amplifier circuit 18j and the resistive element 19j via terminal 1e of the sensor interface circuit 1j. SENS One end is supplied with power potential V DD One end is connected to one end of resistor element 19j. The other end of resistor element 19j is connected to ground potential. Resistor element 19j is a reference resistor element and has become a variable resistor element R. SENS The change in resistance value is based on the reference resistance value. Therefore, the power supply potential V... DD According to the variable resistor element R SENS The resistance value is the voltage V divided by the resistor. SENS The voltage V is applied to input node 18b of amplifier circuit 18j. SENS According to the resistance value R of sensor 2 SENS It changes with the change. Voltage V SENS 'Indicates the resistance value R of sensor 2 SENS The changes.

[0106] Amplifier circuit 18j amplifies the reference voltage V REF 'With voltage V SENS The differential output is the voltage V corresponding to the amplified differential. SENS Voltage V SENS The value detected by sensor 2 (e.g., the variable resistance element R) SENS The resistance value corresponds to this. That is, the voltage difference detection circuit 16 replaces the input node 16b (refer to the resistance value). Figure 3 Meanwhile, at input node 16a, the voltage V corresponding to the detection value of sensor 2 is received. SENS Replace input node 16a (refer to) Figure 3 Meanwhile, the reference voltage V is received at input node 16b. REF This differs from the first implementation method.

[0107] For example, amplifier circuit 18j Figure 7As shown, it is constructed from an instrumentation amplifier circuit. The amplifier circuit 18j has multiple drivers 181-183 and multiple resistors 184-191. The first input node of driver 181 is connected to a reference voltage source 14, the second input node is connected to one end of resistors 184 and 186 respectively, and the output node is connected to one end of resistor 188 and the other end of resistor 186 respectively. The first input node of driver 182 is connected to node 192, the second input node is connected to one end of resistors 185 and 187 respectively, and the output node is connected to one end of resistor 191 and the other end of resistor 187 respectively. The first input node of driver 183 is connected to the other end of resistor 188 and one end of resistor 189 respectively, the second input node is connected to one end of resistor 190 and the other end of resistor 191 respectively, and the output node is connected to the other end of resistor 189. The other end of resistor 190 is connected to the other ends of resistors 184 and 185 respectively. The driving forces of multiple drivers 181 to 183 can also be balanced. The resistance values ​​of multiple resistive elements 184 to 191 can also be balanced.

[0108] With this structure, the amplifier circuit 18j can amplify the voltage V. SENS 'Relative to reference voltage V REF The difference is used to generate voltage V. SENS If the resistance of resistor 19j is set to R... REF Sensor 2 (variable resistance element R) SENS The resistance value is R. SENS Then the voltage V SENS It is represented by the following numerical expression 10.

[0109] V SENS '=V DD ·{R REF / (R REF +R SENS )}...Equation 10

[0110] Furthermore, if the amplification factor of amplifier circuit 18j is assumed to be A, then the amplified voltage V SENS It is represented by the following numerical expression 11.

[0111] V SENS =(V SENS '-V REF ')·A+V REF '...Equation 11

[0112] Due to V REF '=V DD / 2, therefore, if we substitute expression 10 into expression 11, we get the following expression 12.

[0113] V SENS =(V DD ·{R REF / (R REF +R SENS )}-V DD / 2)·A+V DD / 2…Equation 12

[0114] As shown in equations 11 and 12, amplifier circuit 18j can amplify the voltage V to increase it. SENS 'Relative to reference voltage V REF The difference is used to generate voltage V. SENS .

[0115] Figure 6 The current source 15j shown is not connected to sensor 2, but will be connected to the reference voltage V. REF 'Corresponding reference current I REF The current flows to input node 16b of the voltage difference detection circuit 16.

[0116] For example, current source 15j Figure 7 As shown, it also includes a resistor element 155j. Resistor element 155j has a fixed resistance value. Using a feedback loop of differential amplifier circuit 153 → transistor 152 → node 154 → differential amplifier circuit 153, differential amplifier circuit 153 controls the gate voltage of transistor 151 and the gate voltage of transistor 152 so that the potential of node 154 is similar to the reference voltage V. REF The equality is the same as in the first embodiment. Therefore, the current source 15j is able to convert the reference voltage V... REF 'Corresponding reference current I REF The current flows to input node 16b of the voltage difference detection circuit 16. Because V REF '=V DD / 2, therefore, if the resistance value of the resistor element 155j is set to R, then the following equation 13 holds true.

[0117] I REF =(V DD / 2) / R…Equation 13

[0118] The output voltage of the frequency impedance transformation circuit 13 is represented by the voltage V at the input node 16b of the voltage difference detection circuit 16. REF Voltage V REF The charging of capacitor element 131 varies in a time constant manner, but is maintained in relation to current I. REF The corresponding charge accumulation in the capacitor element 132 is averaged and converges to a stable point. The voltage V at input node 16b of the voltage difference detection circuit 16 at the stable point is... SENSAs in the following expression 14.

[0119] V REF =I REF / (F OUT ·C SC )...Equation 14

[0120] As shown in Equation 14, when converging to a stable point, the frequency F of the oscillation signal is... OUT The frequency impedance transformation circuit 13 transforms the impedance into "1 / (F)". OUT ·C SC Equivalently, it becomes impedance "1 / (F)". OUT ·C SC One end of the current source 15j is connected to the input node 16b of the voltage difference detection circuit 16, and the other end is connected to the ground potential. Therefore, at the input node 16b of the voltage difference detection circuit 16, the current I from the current source 15j... REF Inflow equivalent impedance "1 / (F OUT ·C SC ), thus through the equivalent impedance "1 / (F OUT ·C SC "and current I" REF Transformed into voltage V REF Voltage V REF Including frequency F OUT The oscillation frequency F of the voltage-controlled oscillation circuit 11 SENS This is the same as in the first embodiment, but the difference lies in the detection value of sensor 2. Instead, the voltage V shown in Equation 12... SENS Including resistance value R SENS This corresponds to the detection value of sensor 2.

[0121] In the frequency synchronization circuit 10j, a frequency-locking loop is used, consisting of voltage difference detection circuit 16 → filter 17 → voltage-controlled oscillation circuit 11 → frequency divider circuit 12 → frequency impedance transformation circuit 13 → voltage difference detection circuit 16. The voltage difference detection circuit 16 controls the voltage V. CTRL Perform feedback control to make the voltage V SENS With reference voltage V REF The equality is the same as in the first embodiment. That is, when the feedback control functions normally, equation 6 holds true. If equation 6 is substituted into equation 14, the following equation 15 is obtained.

[0122] V SENS =I REF / (F OUT ·C SC )...Equation 15

[0123] If we substitute expressions 12 and 13 into expression 15, we get expression 16.

[0124] (V DD ·{R REF / (R REF +R SENS )}-V DD / 2)·A+V DD / 2={(V DD / 2) / R}·{1 / (F OUT ·C SC )}...Equation 16

[0125] In equation 16, let {R} REF / (R REF +R SENS If )}=RS, then equation 16 is rewritten as equation 17. RS represents the resistance value R of sensor 2. SENS The parameters that change.

[0126] (V DD RS-V DD / 2)·A+V DD / 2={(V DD / 2) / R}·{1 / (F OUT ·C SC )}...Equation 17

[0127] If we are referring to frequency F OUT Solving equation 17, we obtain equation 18.

[0128] F OUT =[{(V DD / 2) / R}·{1 / (C SC )}] / [(V DD RS-V DD / 2)·A+V DD / 2]

[0129] ={1 / (C SC ·R)}·[1 / {2·A·RS+(1-A)}]…Equation 18

[0130] As shown in Equation 18, in the sensor interface circuit 1j, the first term on the right, {1 / (C}, can be obtained from the first term on the right. SC The oscillation frequency is designed using the second term [1 / {2·A·RS+(1-A)}], which allows for the design of the sensitivity (amplification A) to the detection value of sensor 2. That is, compared to the first embodiment, a more accurate and robust oscillation frequency F can be obtained. OUT .

[0131] In addition, the operation of the frequency synchronization circuit 10j is as follows: Figure 8 As shown, it differs from the first embodiment in the following aspects. Figure 8 The waveform diagram shows the operation of the frequency synchronization circuit 10j.

[0132] exist Figure 8 In the waveform diagram, relative to Figure 5 The waveform diagram shows a sawtooth-like variation from the voltage V. SENS Replace with voltage V REF In a relatively constant case, consider the voltage V. REF Replace with voltage V SENS Voltage V SENS The resistance value R of sensor 2 SENS The resistance value R of sensor 2 remains almost constant during the constant period, but if the resistance value R of sensor 2 is... SENS The value of changes accordingly.

[0133] If the frequency synchronization circuit 10j starts at timer t31, then the current source 15j will begin to flow with the reference voltage V. REF 'Corresponding current I REF =I 11 Correspondingly, in the frequency impedance transformation circuit 13, the capacitor element 132 stores current I. REF =I 11 The corresponding charge, voltage V REF The voltage gradually increases. At this time, the oscillation signal is at level L, switch 133 remains open, switch 134 remains closed, and capacitor 131 is in a discharging state with voltage V. SC It is approximately equal to the ground potential. Furthermore, the reference voltage V... SENS The value V11 is almost maintained, therefore accompanied by voltage V REF As the voltage gradually increases, the voltage difference detection circuit 16 causes the control voltage V to rise. CTRL Gradually increase.

[0134] During timer t32, the voltage-controlled oscillation circuit 11 begins oscillation, and the oscillation signal level changes to H level. Correspondingly, switch 133 is turned on, and switch 134 is turned off. Voltage V REF The voltage drops momentarily due to the connection of one end of capacitor element 131 to input node 16b. Subsequently, the charge is redistributed by capacitor elements 131 and 132 and the current I... REF =I 11 The corresponding charges are respectively charged to capacitor elements 131 and 132, thereby increasing the voltage V. SC and voltage V REF They gradually rose.

[0135] During timer t33, the oscillation signal level becomes L level. Correspondingly, switch 133 opens and switch 134 closes. Capacitor 131 is discharged, and its voltage V... SC It is reduced to ground potential. At this time, capacitor element 132 retains its charge, and therefore is accompanied by current I. REF =I 11 The corresponding charge is charged to capacitor element 132, and the voltage V REF It continues to rise gradually.

[0136] During timer t34, the oscillation signal level becomes H level. Correspondingly, switch 133 is turned on, and switch 134 is turned off. Voltage V REF The voltage drops momentarily due to the connection of one end of capacitor element 131 to input node 16b. Subsequently, the charge is redistributed by capacitor elements 131 and 132 and the current I... REF =I 11 The corresponding charges are respectively charged to capacitor elements 131 and 132, thereby increasing the voltage V. SC and voltage V REF They gradually rose.

[0137] During timing t35, the oscillation signal level becomes L level. Correspondingly, switch 133 is opened, and switch 134 is closed. Capacitor 131 is discharged, and its voltage V... SC It is reduced to ground potential. At this time, capacitor element 132 retains its charge, and therefore is related to current I. REF =I 11 The corresponding charge is charged to capacitor element 132, accompanied by a voltage V REF It continues to rise gradually.

[0138] During timing t36 to t44, the same actions as timing t34 to t35 and the same actions as timing t35 to t36 are repeated alternately, and the voltage V REF It varies with a sawtooth-shaped waveform and approaches the voltage V on an average time basis. SENS =V 11 Control voltage V CTRL It gradually approaches the value V1. Accompanying this, the frequency F of the oscillation signal... OUT Approaching the resistance value R of sensor 2 SENS = R1 corresponds to the value F1.

[0139] If the voltage V is at time t44 REF Averaged over time with reference voltage V SENS If they are approximately equal, then from time t44 to time t48, the control voltage V... CTRL The frequency F of the oscillation signal stabilizes at a value of V1. OUT Stabilized at the resistance value R of sensor 2SENS =R1 corresponds to the value F1, and the frequency locking loop is in a locked state. Therefore, the frequency synchronization circuit 10j stably outputs the resistance value R of sensor 2. SENS = Frequency F corresponding to R1 OUT =F1. Period TP1 is the period corresponding to frequency F1.

[0140] At time t48, due to changes in the state of the object being detected by sensor 2, the resistance value R of sensor 2 will increase. SENS The change is R2 (>R1), and the voltage change is V. SENS =V 12 (<V) 11 Correspondingly, during timings t48 to t58, the same actions as timings t34 to t35 and the same actions as timings t35 to t36 are repeated alternately, with voltage V... REF It varies with a sawtooth-shaped waveform and approaches the reference voltage V on an average time basis. SENS Control voltage V CTRL It gradually approaches the value V3 (< V1). Accompanying this, the frequency F of the oscillation signal... OUT Approaching the resistance value R of sensor 2 SENS = The value F3 corresponding to R2 (> F1).

[0141] If the voltage V is at time t58 REF Average over time with voltage V SENS If they are approximately equal, then after timer t58, the control voltage V... CTRL The frequency F of the oscillation signal stabilizes at a value of V3. OUT Stabilized at the resistance value R of sensor 2 SENS =R2 corresponds to the value F3, and the frequency locking loop is in a re-locked state. Therefore, the frequency synchronization circuit 10j stably outputs the resistance value R of sensor 2. SENS = Frequency F corresponding to R2 OUT =F3. Period TP3 is the period corresponding to frequency F3.

[0142] As described above, in the second embodiment, the frequency synchronization circuit 10j in the sensor interface circuit 1j is composed of a voltage-changing type frequency synchronization circuit, so that the voltage amplified and output by the amplifier circuit 18j changes relative to the change in the detected value of the sensor 2. With the structure described above, it is also possible to obtain a voltage independent of the reference voltage V. REF The high precision of the oscillation frequency allows for easy and high-precision control of the frequency-locked loop feedback in the voltage domain.

[0143] The embodiments of the present invention have been described above, but these embodiments are provided as examples and are not intended to limit the scope of the invention. The new embodiments described above can be implemented in various other ways, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments are included within the scope and spirit of the invention, and are also included within the scope of the invention as described in the claims and its equivalents.

[0144] According to the present invention, the effect of reducing current consumption is achieved.

[0145] -Explanation of Figure Markers-

[0146] 1,1j Sensor Interface Circuit

[0147] 2 Sensors

[0148] 4 Impedance Transformation Circuit

[0149] 5 antennas

[0150] 6 RF switches

[0151] 7 LPF

[0152] 8. Power generation circuit

[0153] 9. Voltage control circuit

[0154] 10, 10J frequency synchronization circuit

[0155] 11 Voltage-controlled oscillation circuit

[0156] 12-frequency divider circuit

[0157] 13 Frequency Impedance Transformation Circuit

[0158] 14. Reference Voltage Source

[0159] 15J current source

[0160] 16 Voltage Difference Detection Circuit

[0161] 17 Filters

[0162] 18J amplifier circuit

[0163] 19J Resistor Element

[0164] 100, 100J sensor module

[0165] 200 Information Collection Terminals

[0166] 300 Communication System.

Claims

1. A sensor interface circuit, It has a frequency synchronization circuit that can be connected to the sensor. The frequency synchronization circuit includes: A reference voltage source generates a reference voltage. A current source is connected to the reference voltage source and uses the reference voltage to generate current. A voltage difference detection circuit has a first input node connected to the reference voltage source, a second input node connected to the current source, and an output node. The voltage received at the first input node and the voltage received at the second input node correspond to the detection value of the sensor. A control voltage is generated based on the difference between the voltage received at the first input node and the voltage received at the second input node. A voltage-controlled oscillation circuit is connected to the output node of the voltage difference detection circuit and generates an oscillation signal according to the control voltage. A frequency impedance transformation circuit is connected between the second input node of the voltage-controlled oscillation circuit and the voltage difference detection circuit to transform the frequency of the signal corresponding to the oscillation signal into impedance.

2. The sensor interface circuit according to claim 1, wherein, The current source can be connected to the sensor and generate current using the reference voltage and the sensor's detection value. The voltage received at the second input node by the voltage difference detection circuit corresponds to the detection value of the sensor and the frequency.

3. The sensor interface circuit according to claim 1, wherein, The sensor interface circuit also features: An amplifier circuit, connected between the reference voltage source and the voltage difference detection circuit, can be connected to the sensor to amplify the difference between the reference voltage and the sensor's detected value, and outputs a voltage corresponding to the amplified difference. The voltage received at the first input node of the voltage difference detection circuit corresponds to the detection value of the sensor, and the voltage received at the second input node corresponds to the frequency.

4. The sensor interface circuit according to claim 1, wherein, The sensor interface circuit also features: A frequency divider circuit, connected between the voltage-controlled oscillation circuit and the frequency impedance transformation circuit, divides the oscillation signal by frequency. The frequency impedance transformation circuit transforms the frequency of the frequency-divided oscillation signal into impedance.

5. The sensor interface circuit according to claim 1, wherein, The frequency impedance transformation circuit includes a switched capacitor circuit.

6. The sensor interface circuit according to claim 5, wherein, The frequency impedance transformation circuit has the following characteristics: The first capacitor element has one end connected to the first potential; The second capacitor element has one end connected to the first potential and the other end connected to the second input node of the voltage difference detection circuit; The first switch has one end connected to the second input node of the voltage difference detection circuit, the other end connected to the other end of the first capacitor element, and the control terminal connected to the voltage control oscillation circuit. The inverter is connected to the voltage-controlled oscillation circuit; and The second switch has one end connected to the other end of the first capacitor element and the other end connected to the first potential, and its control terminal connected to the inverter.

7. The sensor interface circuit according to claim 2, wherein, The current source has: The first transistor is connected between the second potential and the second input node of the voltage difference detection circuit; The second transistor can be connected between the second potential and the sensor; and The differential amplifier circuit has a first input node connected to the reference voltage source, a second input node connected to the node between the second transistor and the sensor, and an output node shared with the gates of the first transistor and the second transistor.

8. The sensor interface circuit according to claim 3, wherein, The current source has: Resistive elements; The first transistor is connected between the second potential and the second input node of the voltage difference detection circuit; A second transistor is connected between the second potential and the resistive element; and A differential amplifier circuit has a first input node connected to the amplifier circuit, a second input node connected to the node between the second transistor and the resistive element, and an output node shared with the gate of the first transistor and the gate of the second transistor.

9. A sensor module, comprising: antenna; Impedance transformation circuit; Sensors; and The sensor interface circuit according to any one of claims 1 to 8 is connected between the sensor and the antenna.

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