Interface circuit of a mems gas sensor and method of controlling the same

Abstract

The application discloses an interface circuit of a MEMS gas sensor and a control method thereof, and belongs to the technical field of sensing circuit design. The interface circuit comprises a reference voltage current generating module, a gear switching module, a resistance conversion voltage module, a bias current generating module and a post-processing module. The resistance conversion voltage module is used to convert the equivalent resistance corresponding to the MEMS gas sensor into a voltage. The gear switching module is used to change the voltage amplification coefficient and the current mirror ratio by switching the state, so as to realize the voltage gear detection. Therefore, a wider resistance detection range can be realized under the same power consumption. Meanwhile, the voltage is converted into a period. In the case that the PVT is the same, the error term can be eliminated by subtracting the periods of T1 and T2. Finally, the period T corresponding to the equivalent resistance of the MEMS gas sensor is obtained, so that the detection precision of the equivalent resistance corresponding to the MEMS gas sensor is improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of sensing circuit design, and more particularly, to an interface circuit of a MEMS gas sensor and a control method thereof. BACKGROUND

[0002] With the continuous maturity of MEMS gas sensor technology, the research of gas identification technology has become a trend in the field of artificial intelligence. In a gas identification system, in order to accurately identify the gas, the system not only needs a gas sensor, but also needs a detection circuit to provide support. With the continuous improvement of the performance of the MEMS gas sensor, its sensitivity is continuously increased, and the resistance value change range also becomes wider and wider, and accordingly the detection circuit also needs to have a wide range, higher resolution and linearity. Moreover, portable and low-cost gas identification systems are the main concern nowadays, so the common resistance voltage division method and Wheatstone bridge method detection circuit cannot meet the requirements of the existing MEMS gas sensor.

[0003] The RTF structure is to convert the equivalent resistance value corresponding to the MEMS gas sensor into the frequency of the oscillating square wave. Compared with the traditional detection method, it does not need ADC, has fewer components and low cost. The output is directly a digital signal, which is convenient for direct measurement. The common RTF structure fixes the voltage across the equivalent resistance corresponding to the MEMS gas sensor as the reference voltage. According to Ohm's law, the charging current changes with the equivalent resistance value corresponding to the MEMS gas sensor. When the sensor resistance is large, the charging current is small, and the delay of the comparator in the circuit is large; when the sensor resistance is small, the charging current is large, and the power consumption is also large; ultimately leading to limited measurement range and poor measurement accuracy. SUMMARY

[0004] In view of the above defects or improvement needs of the prior art, the present application provides an interface circuit of a MEMS gas sensor and a control method thereof, which aims to solve the technical problems of limited measurement range and poor measurement accuracy of the interface circuit of the MEMS gas sensor.

[0005] To achieve the above-mentioned purpose, according to one aspect of the present application, an interface circuit of a MEMS gas sensor is provided, comprising: a reference voltage current generation module, a gear switching module, a resistance conversion voltage module, a bias current generation module and a post-processing module.

[0006] The reference voltage current generation module is used to convert the input voltage into a first reference voltage, a second reference voltage, a reference current, a common-mode reference voltage, a charging current and a third reference voltage.

[0007] The gear switching module comprises a first comparator, a second comparator, a state machine and a lookup table; a high-level input end of the first comparator is connected to a first reference voltage, a low-level input end of the second comparator is connected to a second reference voltage, and a low-level input end of the first comparator and a high-level input end of the second comparator are connected to an output end of the voltage conversion frequency module; the first comparator and the second comparator are used for outputting 1 when the voltage of the high-level input end is greater than the input end of the low-level input end, and outputting 0 in the opposite case; the state machine is connected to the output end of the first comparator and the output end of the second comparator, and is used for outputting a jump to the next state when the input end signal is 00; the output state remains unchanged when the input end signal is 01 or 11; the input end of the lookup table is connected to the output end of the state machine, and is used for generating a current mirror enable signal, a capacitor switching array enable signal and an enable signal Buffer-EN under different gears according to the output state of the state machine;

[0008] The bias current generation module is used for generating a bias current I REF under different gears according to the reference current I bais output by the reference voltage current generation module;

[0009] The resistance conversion voltage module is used for generating a voltage amplification ratio β under different gears by using the capacitor switching array enable signal and the enable signal Buffer-EN, and converting the equivalent resistance R S of the MEMS gas sensor into a stable level signal V H under the action of the third reference voltage and the voltage amplification ratio β;

[0010] The post-processing module is connected to the reference voltage current generation module and the resistance conversion voltage module, and is used for converting the stable level signal V H and a common-mode reference voltage into oscillation waveforms Q1 and Q2; and performing period subtraction on the two groups of oscillation waveforms Q1 and Q2 to obtain the equivalent resistance R S of the MEMS gas sensor corresponding to a period T.

[0011] Further, the resistance conversion voltage module comprises:

[0012] a mirror current source, a first transmission gate, a voltage buffer, a switched capacitor amplifier C1, C2, C3, a switched capacitor unit gain buffer, a second transmission gate, a third transmission gate, a capacitor switching array and switches Sw1, Sw2, Sw3, Sw4, Sw5 and Sw6;

[0013] The output end of the mirror current source is connected with one end of the equivalent resistance, the other end of the equivalent resistance is grounded, and the resistance value of the equivalent resistance is converted into a voltage value; the output end of the equivalent resistance is connected with the input end of the first transmission gate and the voltage buffer; under the small resistance gear, the first transmission gate enables the output end, and the input end of the switched capacitor amplifier C1 is connected with the output end of the first transmission gate; under the large resistance gear, the voltage buffer enables the output end, and the voltage buffering is realized; the output end of the voltage buffer is connected with the input end of the switched capacitor amplifier C1; the output end of the switched capacitor amplifier C1 is connected with the input end of the second transmission gate, and the output end of the switched capacitor unit gain buffer is connected with the input end of the third transmission gate; the capacitor switching array is connected in parallel between the two ends of the switched capacitor amplifier C1; one end of the capacitor C2 is connected with the capacitor C1, and the other end is connected with the output end of the switched capacitor amplifier A1; one end of the capacitor C3 is connected with the negative input end of the switched capacitor unit gain buffer, and the other end is connected with the switch Sw4; one end of the switch Sw1 is connected with the output end of the voltage buffer, and the other end is connected with the switch Sw3; one end of the switch Sw3 is connected with the switch Sw1, and the other end is grounded; the switch Sw2 is connected in parallel on the capacitor C2; one end of the switch Sw4 is connected with the output end of the switched capacitor amplifier A1, and the other end is connected with the capacitor C3; one end of the switch Sw5 is connected with the capacitor C3, and the other end is connected with the output end of the switched capacitor unit gain buffer A2; one end of the switch Sw6 is connected with the capacitor C3, and the other end is connected with the output end of the switched capacitor unit gain buffer A2.

[0014] The enable end of the capacitor switching array inputs the capacitor switching array enable signal α1…αj, only one enable end is effective at any time to select a group of capacitor arrays, determine the capacitor C1 under the current gear, and determine the output end of the voltage amplification mirror current source under the current gear and the output end of the equivalent resistance, and the resistance value of the equivalent resistance is converted into a voltage value coefficient C1 / C2.

[0015] Further, when clock clk+ is high, switches Sw1, Sw2 are closed, Sw3 is open, C1 is pre-charged to Vout1-Vcm, when A1 open loop gain reaches the first threshold, due to virtual short, the output of the switched capacitor amplifier C1 is Vcm, when clock clk+ is low, switches Sw3 is closed, Sw1, Sw2 are open, the charge is redistributed between C1, C2, the output of the switched capacitor amplifier C1 is Vout2=(C1 / C2)*Vout1; when clock clk- is high, switches Sw4, Sw6 are closed, Sw5 is open, C3 is pre-charged to Vout2-Vcm, when A2 open loop gain reaches the second threshold, due to virtual short, the output of the switched capacitor unit gain buffer is Vcm; when clk- is low, switch Sw5 is closed, Sw4, Sw6 are open, due to the conservation of the total amount of charge on the capacitor C3, the output of the switched capacitor unit gain buffer is Vout2; the second transmission gate transmits Vout2 during clk+ is low, and the third transmission gate transmits Vout2 during clk- is low; the stable voltage level V H , V H = V CM + βI REF · R S , where R S is the equivalent resistance of the sensor, β=k*C1 / C2 is the product of the current amplification coefficient k and the voltage amplification coefficient C1 / C2, k is the width-length ratio of the current mirror tube of the resistance conversion voltage module and the current source tube of the bias current module, V CM is the third reference voltage.

[0016] Further, the post-processing module comprises, in sequence: a voltage conversion frequency module, a counter module, and a hardware delay elimination module.

[0017] The voltage conversion frequency module is configured to convert the stable voltage level signal V H into an oscillating waveform Q1, and convert the third reference voltage into an oscillating waveform Q2.

[0018] The counter module is configured to record a period T1 of the oscillating waveform Q1 and a period T2 of the oscillating waveform Q2, respectively.

[0019] The hardware delay elimination module is configured to subtract the period T1 of the oscillating waveform Q1 and the period T2 of the oscillating waveform Q2 to obtain the equivalent resistance R S of the MEMS gas sensor corresponding to the period T.

[0020] Further, the voltage conversion frequency module comprises: a first oscillation waveform generating group and a second oscillation waveform generating group which are identical in structure and each comprises: a PMOS current mirror, an NMOS current mirror, a first switch tube, a second switch tube, an integration capacitor, a high-level comparator, a low-level comparator and an RS flip-flop; the current mirror mirrors the charging current output by a reference current voltage module to charge and discharge the integration capacitor; the output end of the integration capacitor is connected with the negative input end V1- and the positive input end V2+ of the high-level comparator and the low-level comparator; the output voltage V H of the resistance conversion voltage module at the V1+ end and the output voltage VL of the reference voltage current generating module at the V2- end are compared to perform level inversion; the output ends of the high-level comparator and the low-level comparator are connected with the reset end R and the set end S of the RS flip-flop respectively to output two non-overlapping oscillation waveforms Q and QN; the Q end is connected with the gate of the first switch tube to control the charging of the integration capacitor; and the QN end is connected with the gate of the second switch tube to control the discharging of the integration capacitor to generate a final stable oscillation square wave.

[0021] Further, the counter module is provided with three input ends; the first input end is connected with the output end of the first waveform generating group to record the period T1 of the oscillation waveform Q1; the second input end is connected with the output end of the second waveform generating group to record the period T2 of the oscillation waveform Q2, T2 being the error term oscillation period generated by V CM ; and the third input end is connected with a reference clock; tcm is the common-mode delay error term generated by V CM ; and td is the delay error term generated by the comparator.

[0022] Further, the period T1 of the oscillation waveform Q1 is expressed as: T1 = 2C L βαR S +t cm +t d ; β = k * C1 / C2 is the product of the current amplification coefficient k and the voltage amplification coefficient C1 / C2, k being the width-length ratio of the current mirror tube of the resistance conversion voltage module and the current source tube of the bias current module, V CM is a third reference voltage, and α is a voltage amplification coefficient.

[0023] According to another aspect of the present application, a control method of an interface circuit of a MEMS gas sensor is provided, comprising:

[0024] S1: the state machine of the gear switching module outputs an initial value S0 = 0000 and enters S2;

[0025] S2: a lookup table determines corresponding current mirror enable signals β1…βi, capacitor switching array enable signals α1…αj and voltage follower enable signals Buffer-EN according to the current output of the state machine.

[0026] S4: the bias current voltage module provides corresponding bias current to the resistance conversion voltage module according to the current mirror enable signal; the capacitor switching array of the resistance conversion voltage module selects corresponding capacitor array to obtain the voltage amplification coefficient according to the corresponding enable signal;

[0027] S5: the resistance conversion voltage module outputs voltage V bias to the MEMS gas sensor by the current source I H ;

[0028] S6: V H > 1.6V and 1.3V, Shift1Shift0 = 00, the state machine of the gear switching module switches to the next state;

[0029] S7: if the state machine of the gear switching module outputs 1011, the resistance conversion voltage module outputs voltage V H under the action of the voltage amplification coefficient; otherwise, repeat S2 to S6;

[0030] S8: input V H obtained in S7 to the post-processing module to generate stable oscillation waveform Q1 and oscillation waveform Q2; under the control of the reference clock, Q1 and Q2 are counted respectively, the delay error term is eliminated by period subtraction, and the period T corresponding to the equivalent resistance of the MEMS gas sensor is generated.

[0031] Overall, compared with the prior art, the above technical solutions conceived by the present application can achieve the following beneficial effects:

[0032] (1) The present application provides an interface circuit of a MEMS gas sensor, which converts the equivalent resistance corresponding to the MEMS gas sensor into voltage V by using a resistance conversion voltage module, changes the voltage amplification coefficient and the current mirror ratio by fixing the charging current to realize the detection of different gears by the gear switching module, so as to realize a wider resistance detection range under the same power consumption. At the same time, the voltage is converted into a period to eliminate the delay, and under the same PVT, the period subtraction of T1 and T2 can eliminate the error term, and finally the period T corresponding to the equivalent resistance of the MEMS gas sensor is obtained, thereby improving the detection accuracy of the equivalent resistance corresponding to the MEMS gas sensor.

[0033] (2) The scheme takes into account that under the large resistance gear, even if the current has a slight fluctuation, the voltage will have a large deviation, so a voltage buffer is needed, which can ensure the voltage sampling accuracy under the large resistance gear. In addition, the traditional sampling circuit has charge leakage and charge injection error, so a switched capacitor amplifier is used, which can avoid charge injection effect and improve sampling accuracy by controlling the timing of the switch. Further, a simple sampling circuit introduces a large charge error and has no load capacity, so a switched capacitor unit buffer is used, which can avoid charge injection effect and improve sampling accuracy, and has a certain load capacity.

[0034] (3) The scheme takes into account that the traditional RTF is a fixed voltage, and the charging current is changed, when the sensor resistance is large, the charging current is small, and the delay of the comparator in the circuit is large. When the sensor resistance is small, the charging current is large, and the power consumption is also large. The measurement range and the measurement accuracy are limited. Therefore, the fixed charging current is used to realize the detection of different gears, which can realize a wider resistance detection range under the same power consumption.

[0035] (4) The scheme uses a hardware module to subtract the periods of Q1 and Q2, which can eliminate the comparator delay error and common mode error, and improve the resistance detection accuracy.

[0036] (5) The voltage conversion frequency module provided by the scheme has a simple structure and principle, which is easy to control and can reduce the design complexity of the whole interface circuit.

[0037] (6) The scheme takes into account that the common mode voltage V CM generated by the switched capacitor amplifier C1 causes a common mode error tcm, so a counter module is used to count the periods of Q1 and Q2, which can be used for hardware module to subtract the periods to eliminate the comparator delay error and common mode error, and improve the resistance detection accuracy.

[0038] (7) The scheme takes into account that the detection accuracy of the traditional RTF is limited by the comparator delay error t d , and the use of the switched capacitor amplifier C1 causes the common mode error tcm of the common mode voltage V CM , and a hardware delay elimination module is used to eliminate the delay, thereby improving the resistance detection accuracy. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 is a circuit diagram of the interface circuit of the MEMS gas sensor provided by the embodiment 1 of the present application;

[0040] Figure 2 is a circuit diagram of the gear switching module in the interface circuit of the MEMS gas sensor provided by the embodiment 1 of the present application;

[0041] Figure 3 Figure 1 is a circuit diagram of a resistance-to-voltage conversion module in an interface circuit of a MEMS gas sensor according to an embodiment of the present application;

[0042] Figure 4 Figure 2 is a circuit diagram of a voltage-to-frequency conversion module in an interface circuit of a MEMS gas sensor according to an embodiment of the present application;

[0043] Figure 5 Figure 3 is a gear switching schematic of a state machine in an interface circuit of a MEMS gas sensor according to an embodiment of the present application;

[0044] Figure 6 Figure 4 is a timing diagram of an output voltage of a voltage-to-frequency conversion module in an interface circuit of a MEMS gas sensor according to an embodiment of the present application. DETAILED DESCRIPTION

[0045] In order to make the objectives, technical solutions, and advantages of the present application clearer, the present application will be described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely intended to explain the present application, and are not intended to limit the present application. In addition, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0046] Embodiment 1

[0047] As shown in Figure 1, the present embodiment provides an interface circuit of a MEMS gas sensor, which includes a reference voltage current generation module, a gear switching module, a resistance-to-voltage conversion module, a bias current generation module, and a post-processing module. Hereinafter, a MEMS gas sensor interface circuit based on RTF with twelve gears and a resistance detection range of 100Ω-150MΩ will be described as an example. Figure 1 The reference voltage current generation module is configured to convert an input voltage into a first reference voltage, a second reference voltage, a reference current, a common-mode reference voltage, a charging current, and a third reference voltage. For example, the reference voltage current generation module has an input terminal connected to a power supply voltage, and has six output terminals, which respectively output a 1.6V comparator reference voltage, a 1.2V comparator reference voltage, a 1uA reference current IREF, a 1.2V common-mode reference voltage, a 10uA charging current, and a 1.0V low-level reference voltage VL.

[0048]

[0049] ​The gear shifting module includes a first comparator, a second comparator, a state machine, and a lookup table. The high-level input of the first comparator is connected to a first reference voltage, and the low-level input of the second comparator is connected to a second reference voltage. The low-level inputs of the first comparator and the high-level inputs of the second comparator are connected to the outputs of the voltage conversion frequency module. The first and second comparators output 1 when the voltage at the high-level input is greater than that at the low-level input, and output 0 otherwise. The state machine is connected to the outputs of the first and second comparators and transitions to the next state when the input signal is 00. The output state remains unchanged when the input signal is 01 or 11. The input of the lookup table is connected to the output of the state machine and generates current mirror enable signals, capacitor switching array enable signals, and enable signal Buffer-EN for different gear positions based on the output state of the state machine.

[0050] Among them, the gear shifting module, such as Figure 2 As shown, the system includes a first comparator, a second comparator, a state machine, and a lookup table. The high-level input of the first comparator is connected to the 1.6V reference voltage output from the reference voltage and current generation module, and the low-level input is connected to the output voltage V of the voltage conversion frequency module. H The high-level input of the second comparator is connected to the output voltage V of the voltage-to-frequency conversion module. H The low-level input is connected to the 1.2V reference voltage output from the reference voltage and current generation module. The two inputs of the state machine are connected to the outputs Shift0 and Shift1 of the first and second comparators, respectively. For example... Figure 5 As shown, due to the initial power-on, V H =0, the initial output of the state machine is S0 = 0000, Shift1Shift0 = 11, and the output of the state machine remains unchanged. During the charging process, the voltage V H Rise, when 1.2V <V H When the voltage is <1.6V, it indicates that the equivalent resistance value of the MEMS gas sensor is still within the current range, Shift1Shift0 = 01, and the state machine output remains unchanged. When the voltage is 1.6V... <V H When V indicates that the equivalent resistance of the MEMS gas sensor has switched to a larger range, Shift1Shift0 = 00, and the state machine output transitions to the next state S1 = 0001. H If the voltage is still greater than 1.6V, the state machine continues to shift gears until the output stabilizes. The output of the state machine is connected to the input of the lookup table. The output of the lookup table generates current mirror enable signals β1…β7, capacitor switching array enable signals α1…α4, and voltage buffer enable signal Buffer-EN for different gear levels. These signals are connected to the bias current generation module and the resistor-to-voltage conversion module.

[0051] a bias current generation module for generating bias current I according to the reference current IREF output by the reference voltage current generation module REF and the current mirror enable signal generates bias current I under different gears bais Specifically, the bias current generation module, the input end is connected with the 1uA reference current IREF output by the reference voltage current generation module. The enable end is connected with the current mirror enable signal β1…β7 output by the gear switching module, and the output end is connected with the current mirror tube gate of the resistance conversion voltage module, which provides charging current under different gears.

[0052] The resistance conversion voltage module is used to generate different gear voltage amplification ratios β by using the capacitor switching array enable signal and the enable signal Buffer-EN. Under the action of the third reference voltage and the voltage amplification ratio β, the equivalent resistance R of the MEMS gas sensor is converted into a stable level signal V S . H .

[0053] Specifically, the resistance conversion voltage module includes a mirror current source, a first transmission gate, a voltage buffer, a switched capacitor amplifier, a switched capacitor unit gain buffer, a second transmission gate, a third transmission gate, and a capacitor switching array. As shown in Figure 3 The capacitor switching array has four enable ends connected with the output ends α1…α4 of the lookup table. Only one enable end is effective at any time, selects a group of capacitor arrays, determines the capacitor C1 under the current gear, and thus determines the voltage amplification coefficient C1 / C2 under the current gear. The output end of the mirror current source is connected with the output end of the equivalent resistance corresponding to the MEMS gas sensor, which converts the equivalent resistance value of the MEMS gas sensor into a voltage value.

[0054] The equivalent resistance output end corresponding to the MEMS gas sensor is connected with the input end of the first transmission gate and the voltage buffer. The PMOS gate of the first transmission gate and the enable signal of the voltage buffer are both connected with the enable signal Buffer-EN. Therefore, in the large resistance gear, the first transmission gate works to transmit the input voltage (VS). In the small resistance gear, the voltage buffer works to realize voltage buffering and transmit the input voltage (VS). The output ends of the first transmission gate and the voltage buffer are both connected with the switch Sw1. When the clock clk+ is high, the switches Sw1 and Sw2 are closed, Sw3 is disconnected, C1 is pre-charged to Vout1-Vcm, and when the open-loop gain of A1 is large, the output of the switched capacitor amplifier is Vcm due to virtual short. When the clock clk+ is low, the switch Sw3 is closed, the switches Sw1 and Sw2 are disconnected, and the charge is redistributed between C1 and C2. The output of the switched capacitor amplifier is Vout2=(C1 / C2)*Vout1. When the clock clk- is high, the switches Sw4 and Sw6 are closed, Sw5 is disconnected, C3 is pre-charged to Vout2-Vcm, and when the open-loop gain of A2 is large, the output of the switched capacitor unit gain buffer is Vcm due to virtual short. When clk- is low, the switch Sw5 is closed, Sw4 and Sw6 are disconnected, and the output of the switched capacitor unit gain buffer is Vout2 due to the conservation of the total amount of charge on the capacitor C3. Since the clock clk+ and the clock clk- are mutually inverted, the output waveforms of the switched capacitor amplifier and the switched capacitor unit gain buffer are as shown in Figure 6 The second transmission gate transmits in the half cycle controlled by the clock clk+ and only transmits Vout2 during the low level of clk+. Similarly, the third transmission gate transmits in the half cycle controlled by the clock clk- and only transmits Vout2 during the low level of clk-. Therefore, the stable level V H .

[0055] The post-processing module is connected with the reference voltage and current generation module and the resistance conversion voltage module, and is used to convert the stable level signal V H and the common-mode reference voltage into oscillation waveforms Q1 and Q2. The two groups of oscillation waveforms Q1 and Q2 are periodically subtracted to obtain the equivalent resistance R S of the MEMS gas sensor corresponding to the period T. The post-processing module comprises, in sequence, a voltage conversion frequency module, a counter module, and a hardware delay elimination module.

[0056] As an optional embodiment, the resistance conversion voltage module comprises: a mirror current source, a first transmission gate, a voltage buffer, a switched capacitor amplifier C1, C2, C3, a switched capacitor unit gain buffer, a second transmission gate, a third transmission gate, a capacitor switching array, and switches Sw1, Sw2, Sw3, Sw4, Sw5, Sw6. The output end of the mirror current source is connected to one end of an equivalent resistance, the other end of the equivalent resistance is grounded, and the resistance value of the equivalent resistance is converted into a voltage value. The output end of the equivalent resistance is connected to the input end of the first transmission gate and the voltage buffer. In a small resistance gear position, the enable end of the first transmission gate is effective, and the output end of the first transmission gate is connected to the input end of the switched capacitor amplifier C1. In a large resistance gear position, the enable end of the voltage buffer is effective, and voltage buffering is realized. The output end of the voltage buffer is connected to the input end of the switched capacitor amplifier C1. The output end of the switched capacitor amplifier C1 is connected to the input end of the second transmission gate, and the output end of the switched capacitor unit gain buffer is connected to the input end of the third transmission gate. The capacitor switching array is connected in parallel across the switched capacitor amplifier C1. One end of the capacitor C2 of the switched capacitor amplifier is connected to the capacitor C1, and the other end is connected to the output end of the switched capacitor amplifier A1. One end of the capacitor C3 is connected to the negative input end of the switched capacitor unit gain buffer, and the other end is connected to the switch Sw4. One end of the switch Sw1 is connected to the output end of the voltage buffer, and the other end is connected to the switch Sw3. One end of the switch Sw3 is connected to the switch Sw1, and the other end is grounded. The switch Sw2 is connected in parallel across the capacitor C2. One end of the switch Sw4 is connected to the output end of the switched capacitor amplifier A1, and the other end is connected to the capacitor C3. One end of the switch Sw5 is connected to the capacitor C3, and the other end is connected to the output end of the switched capacitor unit gain buffer A2. One end of the switch Sw6 is connected to the capacitor C3, and the other end is connected to the output end of the switched capacitor unit gain buffer A2. The enable ends of the capacitor switching array input capacitor switching array enable signals α1…αj, only one enable end is effective at any time to select a group of capacitor arrays, determine the capacitor C1 under the current gear position, and thus determine the voltage amplification mirror current source output end connected to the output end of the equivalent resistance, the resistance value of the equivalent resistance is converted into a voltage value coefficient C1 / C2.

[0057] As an optional embodiment, when the clock clk+ is high, the switches Sw1, Sw2 are closed, Sw3 is open, C1 is pre-charged to Vout1-Vcm, when the open loop gain of A1 reaches the first threshold, the switched capacitor amplifier C1 output is Vcm due to virtual short, when the clock clk+ is low, the switch Sw3 is closed, the switches Sw1, Sw2 are open, the charge is redistributed between C1, C2, the switched capacitor amplifier C1 output is Vout2=(C1 / C2)*Vout1. When the clock clk- is high, the switches Sw4, Sw6 are closed, Sw5 is open, C3 is pre-charged to Vout2-Vcm, when the open loop gain of A2 reaches the second threshold, the switched capacitor unit gain buffer output is Vcm due to virtual short. When clk- is low, the switch Sw5 is closed, Sw4, Sw6 are open, due to the conservation of the total amount of charge on the capacitor C3, the switched capacitor unit gain buffer output is Vout2. The second transmission gate transmits Vout2 during clk+ is low, and the third transmission gate transmits Vout2 during clk- is low. The superposition of the second and third transmission gate outputs generates a stable level V H , V H = V CM + βI REF · R S , where R S is the equivalent resistance of the sensor, β=k*C1 / C2 is the product of the current amplification coefficient k and the voltage amplification coefficient C1 / C2, k is the width-length ratio of the current mirror tube of the resistance conversion voltage module and the current source tube of the bias current module, V CM is the third reference voltage.

[0058] As an optional embodiment, the post-processing module includes, connected in sequence: a voltage conversion frequency module, a counter module, and a hardware delay elimination module. The voltage conversion frequency module is configured to convert the stable level signal V H into an oscillation waveform Q1 and convert the third reference voltage into an oscillation waveform Q2. The counter module is configured to record the periods T1 and T2 of the oscillation waveforms Q1 and Q2, respectively. The hardware delay elimination module is configured to subtract the periods T1 and T2 of the two sets of oscillation waveforms Q1 and Q2 to obtain the equivalent resistance R S of the MEMS gas sensor corresponding to the period T; and the hardware delay elimination module is connected to the SPI bus and uses the MCU unit to perform an arithmetic operation of subtracting the periods of the two sets of oscillation waveforms, thereby achieving delay elimination.

[0059] As an optional embodiment, as Figure 4As shown, the voltage conversion frequency module comprises: a first oscillating waveform generation group and a second oscillating waveform generation group with the same structure, each comprising: a PMOS current mirror, an NMOS current mirror, a first switch tube, a second switch tube, an integration capacitor, a high-level comparator, a low-level comparator, and an RS flip-flop. H The current mirror mirrors the charging current output by the reference current voltage module to charge and discharge the integration capacitor, the output end of the integration capacitor is connected to the negative input end V1- and the positive input end V2+ of the high-level comparator and the low-level comparator, and the output voltage V CM of the resistance conversion voltage module at the V1+ end and the output voltage VL of the reference voltage current generation module at the V2- end are compared to perform level inversion. The output ends of the high-level comparator and the low-level comparator are connected to the reset end R and the set end S of the RS flip-flop, respectively, to output two non-overlapping oscillating waveforms Q and QN, the Q end is connected to the gate of the first switch tube for controlling the charging of the integration capacitor, and the QN end is connected to the gate of the second switch tube for controlling the discharging of the integration capacitor to generate a final stable oscillating square wave.

[0060] As an optional implementation, the counter module is provided with three input ends, the first input end is connected to the output end of the first waveform generation group to record the period T1 of the oscillating waveform Q1, the second input end is connected to the output end of the second waveform generation group to record the period T2 of the oscillating waveform Q2, and T2 is the error term oscillating period generated by V CM , tcm is the common-mode level error term generated by V CM , and td is the delay error term generated by the comparator.

[0061] As an optional implementation, the period T1 of the oscillating waveform Q1 is represented as: T1=2C L βαR S +t cm +t d , β=k*C1 / C2 is the product of the current amplification coefficient k and the voltage amplification coefficient C1 / C2, k is the width-length ratio of the current mirror tube of the resistance conversion voltage module and the current source tube of the bias current module, V CM is the third reference voltage, and α is the voltage amplification coefficient.

[0062] The interface circuit of the MEMS gas sensor provided by the embodiment of the application adopts an R-V conversion structure, fixes a charging current, changes a voltage amplification coefficient and a current mirroring ratio through state switching to realize step detection, so that a wider resistance detection range is realized under the same power consumption. Meanwhile, a hardware delay elimination module is adopted to eliminate delay, so as to improve the resistance detection precision. The problems that the resistance measurement range and the measurement precision are limited in the traditional RTF detection method are improved.

[0063] Embodiment 2

[0064] The embodiment provides a control method of an interface circuit of a MEMS gas sensor, which comprises the following steps:

[0065] S1: the initial value of the state machine of the gear switching module is output as S0=0000, and S2 is entered.

[0066] S2: the corresponding current mirror enable signals β1...βi, the enable signals α1...αj of the capacitor switching array and the voltage follower enable signal Buffer-EN are determined according to the current output of the state machine by looking up a table.

[0067] S4: the bias current voltage module provides corresponding bias currents to the resistance conversion voltage module according to the current mirror enable signals. The capacitor switching array of the resistance conversion voltage module selects corresponding capacitor arrays to obtain a voltage amplification coefficient according to the corresponding enable signals.

[0068] S5: the resistance conversion voltage module outputs a voltage V bias The MEMS gas sensor is charged with an equivalent resistance. Since the voltage follower enable signal is zero, a voltage V H is superimposed at the output end under the action of the voltage amplification coefficient.

[0069] S6: V H is greater than 1.6V and 1.3V, Shift1Shift0=00, the state machine of the gear switching module is switched to the next state.

[0070] S7: if the output of the state machine of the gear switching module is 1011, the resistance conversion voltage module outputs a voltage V H under the action of the voltage amplification coefficient, otherwise, S2 to S6 are repeated.

[0071] S8: V H obtained in S7 is input to a post-processing module to generate stable oscillation waveforms Q1 and Q2. Under the control of a reference clock, the period count of Q1 and Q2 is performed, the period is subtracted to eliminate the delay error term, and the period T corresponding to the equivalent resistance of the MEMS gas sensor is generated.

[0072] In one example, the initial resistance value of the MEMS gas sensor corresponding to the equivalent resistance of the MEMS gas sensor jumps to 1kΩ at 30ms, and the specific implementation steps are as follows:

[0073] Step one: the initial value of the state machine of the gear switching module is output as S0=0000, the corresponding current mirror enable signals (β1...β7=1000000) of the lookup table, the enable signals (α1...α4=1000) of the capacitor switching array and the voltage follower enable signal Buffer-EN=0 are output.

[0074] Step two: the bias current voltage module provides the corresponding 100uA bias current to the resistance conversion voltage module according to the current mirror enable signal. At the same time, the capacitor switching array of the resistance conversion voltage module selects the corresponding capacitor C1 array according to the corresponding enable signal, so that the voltage amplification coefficient C1 / C2=20.

[0075] Step three: the resistance conversion voltage module amplifies the voltage Vout1 to Vout2=(C1 / C2)*Vout1 through the 100uA current source I bias Charges the equivalent resistance Rsens corresponding to the MEMS gas sensor. Since the voltage follower enable signal is zero, the first transmission gate enables buffer-nen at this time, and the transmission voltage Vout1 is approximately equal to the power supply voltage 3.3V. The switched capacitor amplifier amplifies Vout1 to Vout2=(C1 / C2)*Vout1 under the control of the clock clk+, and transmits Vout2 through the second transmission gate half cycle. The switched capacitor unit gain buffer samples Vout2 under the control of the clock clk-, and transmits Vout2 through the third transmission gate half cycle. The output end superimposes V H ≈3.3V.

[0076] Step four: since V H is greater than 1.6V and 1.3V, Shift1Shift0=00, the gear is switched to S1=0001, and the above steps one to four are repeated until the gear is stable to S11=1011. According to Table 1, the bias current is 10nA at this time, the voltage amplification coefficient is 5 / 16, and V H is stable to about 1.4V.

[0077] Table 1

[0078]

[0079] Step five: V H The first oscillation waveform generation group input to the voltage conversion frequency module generates a stable oscillation waveform Q1, and the second oscillation waveform generation group generates an oscillation waveform Q2.

[0080] Step six: the oscillation waveforms Q1, Q2 are input to the counter module, and the periods of Q1 and Q2 are counted respectively under the control of the reference clock clkref. The period difference is calculated by the MCU arithmetic unit to eliminate the delay error term, and the equivalent resistance value of the MEMS gas sensor corresponding to 100MΩ is generated.

[0081] Those skilled in the art will readily understand that the above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An interface circuit for a MEMS gas sensor, characterized by The application relates to a voltage-current reference module, and belongs to the technical field of integrated circuits. The reference voltage current generating module is used for converting an input voltage into a first reference voltage, a second reference voltage, a reference current, a common-mode reference voltage, a charging current and a third reference voltage. The gear switching module comprises a first comparator, a second comparator, a state machine and a lookup table; the high-level input end of the first comparator is connected with the first reference voltage, the high-level input end of the second comparator is connected with the second reference voltage, and the low-level input end of the first comparator and the low-level input end of the second comparator are connected with the output end of the voltage conversion frequency module; the first comparator and the second comparator are used for outputting 1 when the voltage of the high-level input end is greater than the voltage of the low-level input end, and outputting 0 when the voltage of the high-level input end is less than the voltage of the low-level input end; the state machine is connected with the output end of the first comparator and the output end of the second comparator, and is used for outputting a jump to the next state when the input end signal is 00; the state remains unchanged when the input end signal is 01 or 11; the input end of the lookup table is connected with the output end of the state machine, and is used for generating a current mirror enable signal, a capacitor switching array enable signal and an enable signal Buffer-EN under different gears according to the output state of the state machine; The resistance conversion voltage module comprises: The bias current generation module is configured to generate a bias current I according to a reference current I output by a reference voltage current generation module REF And the current mirror enables signal generates bias current I under different gears bais ; The resistance conversion voltage module is used for generating voltage amplification ratios at different gears by using a capacitor switching array enable signal and an enable signal Buffer-EN , under the action of a third reference voltage and voltage amplification ratio , the equivalent resistance of the MEMS gas sensor is converted into a stable level signal ;​ The post-processing module is connected with the reference voltage and current generating module and the resistance conversion voltage module, and is used for converting the stable level signal V H And common mode reference voltage into oscillating waveforms Q1 and Q2; and the period of two groups of oscillating waveforms Q1, Q2 is subtracted to obtain the equivalent resistance of the MEMS gas sensor Corresponding period T.

2. The interface circuit for a MEMS gas sensor of claim 1, wherein, a mirror current source, a first transmission gate, a voltage buffer, a switched capacitor amplifier C1, a switched capacitor amplifier C2, a switched capacitor amplifier C3, a switched capacitor unit gain buffer, a second transmission gate, a third transmission gate, a capacitor switching array and switches Sw1, Sw2, Sw3, Sw4, Sw5 and Sw6. ​ The output end of the mirror current source is connected with one end of the equivalent resistance, the other end of the equivalent resistance is grounded, and the resistance value of the equivalent resistance is converted into a voltage value; the output end of the equivalent resistance is connected with the input end of the first transmission gate and the voltage buffer; under the small resistance gear, the first transmission gate enables the output end to be connected with the input end of the switched capacitor amplifier C1, under the large resistance gear, the voltage buffer enables the output end to be connected with the input end of the switched capacitor amplifier C1, and voltage buffering is realized; the output end of the voltage buffer is connected with the input end of the switched capacitor amplifier C1; the output end of the switched capacitor amplifier C1 is connected with the input end of the second transmission gate, and the output end of the switched capacitor unit gain buffer is connected with the input end of the third transmission gate; the capacitor switching array is connected in parallel between the two ends of the switched capacitor amplifier C1; one end of the capacitor C2 is connected with the capacitor C1, and the other end is connected with the output end of the switched capacitor amplifier A1; one end of the capacitor C3 is connected with the negative input end of the switched capacitor unit gain buffer, and the other end is connected with the switch Sw4; one end of the switch Sw1 is connected with the output end of the voltage buffer, and the other end is connected with the switch Sw3; one end of the switch Sw3 is connected with the switch Sw1, and the other end is grounded; the switch Sw2 is connected in parallel on the capacitor C2; one end of the switch Sw4 is connected with the output end of the switched capacitor amplifier A1, and the other end is connected with the capacitor C3; one end of the switch Sw5 is connected with the capacitor C3, and the other end is connected with the output end of the switched capacitor unit gain buffer A2; one end of the switch Sw6 is connected with the capacitor C3, and the other end is connected with the output end of the switched capacitor unit gain buffer A2; The enable terminal of the capacitor switching array inputs a capacitor switching array enable signal At any moment, only one enable terminal is effective to select a group of capacitor arrays, determine the capacitor C1 under the current gear, and determine the output terminal of the voltage amplifier mirror current source under the current gear and the output terminal of the equivalent resistance. The resistance value of the equivalent resistance is converted into a voltage value coefficient C1 / C2.

3. The interface circuit of a MEMS gas sensor of claim 2, wherein, When clock clk+ is high, switches Sw1, Sw2 are closed, Sw3 is open, C1 is pre-charged to Vout1-Vcm, when A1 open loop gain reaches the first threshold, the output of the switched capacitor amplifier C1 is Vcm, when clock clk+ is low, switch Sw3 is closed, switches Sw1, Sw2 are open, the charge is redistributed between C1, C2, the output of the switched capacitor amplifier C1 is Vout2=(C1 / C2)*Vout1; when clock clk- is high, switches Sw4, Sw6 are closed, Sw5 is open, C3 is pre-charged to Vout2-Vcm, when A2 open loop gain reaches the second threshold, the output of the switched capacitor unit gain buffer is Vcm; when clk- is low, switch Sw5 is closed, Sw4, Sw6 are open, the output of the switched capacitor unit gain buffer is Vout2; the second transmission gate transmits Vout2 during clk+ is low, the third transmission gate transmits Vout2 during clk- is low; the stable level is generated by superimposing the outputs of the second and third transmission gates , , wherein R S is the equivalent resistance of the sensor, =k*C1 / C2, is the product of the current amplification factor k and the voltage amplification factor C1 / C2, k is the width-length ratio of the current mirror tube of the resistance conversion voltage module and the current source tube of the bias current module, is the third reference voltage.

4. The interface circuit for a MEMS gas sensor of claim 1, wherein, The post-processing module comprises, connected in sequence: a voltage conversion frequency module, a counter module, and a hardware delay elimination module; The voltage conversion frequency module is configured to convert the stable level signal The third reference voltage is converted into an oscillating waveform Q2 by converting the oscillating waveform Q1. The counter module is configured to record the period T1 of the oscillation waveform Q1 and the period T2 of the oscillation waveform Q2, respectively. The hardware delay elimination module is configured to subtract the period T1 of the two groups of oscillation waveforms Q1 and the period T2 of the two groups of oscillation waveforms Q2 to obtain an equivalent resistance of the MEMS gas sensor The corresponding period T.

5. The interface circuit for a MEMS gas sensor of claim 4, wherein, The voltage conversion frequency module comprises a first oscillating waveform generating group and a second oscillating waveform generating group which are identical in structure and each comprises a PMOS current mirror, an NMOS current mirror, a first switch tube, a second switch tube, an integration capacitor, a high-level comparator, a low-level comparator and an RS flip-flop; the current mirror mirrors the charging current output by a reference current voltage module to charge and discharge the integration capacitor; the output end of the integration capacitor is connected with the negative input end V1- and the positive input end V2+ of the high-level comparator and the low-level comparator; the high-level comparator and the low-level comparator compare the output voltage V H of a resistance conversion voltage module at the V1+ end with the output voltage VL of a reference voltage current generating module at the V2- end, and perform level inversion; the output ends of the high-level comparator and the low-level comparator are connected with the reset end R and the set end S of the RS flip-flop respectively, and output two non-overlapping oscillating waveforms Q and QN; the Q end is connected with the gate of the first switch tube for controlling the charging of the integration capacitor; the QN end is connected with the gate of the second switch tube for controlling the discharging of the integration capacitor, and a final stable oscillating square wave is generated.

6. The interface circuit for a MEMS gas sensor of claim 5, wherein, The counter module has three input ends, the first input end is connected with the output end of the first waveform generating group to record the period T1 of the oscillation waveform Q1; the second input end is connected with the output end of the second waveform generating group to record the period T2 of the oscillation waveform Q2, T2 is V CM The generated error term oscillation period, T2=tcm+td; the third input end is connected with the reference clock; tcm is the common mode level V CM The generated common mode delay error term, td is the delay error term generated by the comparator.

7. The interface circuit for a MEMS gas sensor of claim 6, wherein, The period T1 of the oscillation waveform Q1 is expressed as: , =k*C1 / C2, where k is the product of the current amplification factor k and the voltage amplification factor C1 / C2, k is the width-length ratio of the current mirror tube of the resistance conversion voltage module and the current source tube of the bias current module, is a third reference voltage, is a voltage amplification factor.

8. A method of controlling an interface circuit of a MEMS gas sensor according to any one of claims 1 to 7, characterized in that, Comprise: S1: The state machine of the gear switching module outputs an initial value S0=0000 and enters S2; S2: look-up table determines corresponding current mirror enable signals β1...βi, capacitor switching array enable signals, and voltage follower enable signal Buffer-EN according to current state machine output and voltage follower enable signal Buffer-EN; S4: The bias current generation module provides corresponding bias current to the resistance-to-voltage conversion module according to the current mirror enable signal; the capacitor switching array of the resistance-to-voltage conversion module selects the corresponding capacitor array according to the corresponding enable signal to obtain a voltage amplification coefficient. S5: the resistance conversion voltage module passes through the current source I bias The equivalent resistance of the MEMS gas sensor is charged, and the output end is superimposed to generate V H ; S6: V H If the voltage is greater than 1.6V, the output Shift0 of the first comparator and the output Shift1 of the second comparator of the gear shifting module are represented as Shift1Shift0=00, and the state machine of the gear shifting module switches to the next state. S7: If the state machine of the gear shifting module outputs 1011, the resistance conversion voltage module outputs voltage V under the action of the voltage amplification coefficient H ; otherwise, repeat S2 to S6; S8: obtaining V from S7 H The input to the post-processing module generates stable oscillation waveform Q1 and oscillation waveform Q2; under the control of the reference clock, the period of Q1 and Q2 is counted respectively, the period is subtracted to eliminate the delay error term, and the period T corresponding to the equivalent resistance of the MEMS gas sensor is generated.

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