Differential current source circuit and impedance measuring device

Abstract

The differential current source circuit and the impedance measuring device comprise a single-ended to differential circuit unit, two paths of Howland circuit units, namely, a Howland circuit unit A and a Howland circuit unit B, and a Howland compensating circuit unit; the single-ended to differential circuit unit is used for carrying out differential conversion on an input voltage signal and outputting two paths of voltage signals which are completely equal in voltage amplitude and frequency but opposite in polarity; the single-ended to differential circuit unit comprises two output ends which are electrically connected with the Howland circuit unit A and the Howland circuit unit B respectively; and the Howland compensating circuit unit is used for acquiring the direct current bias voltage of the Howland circuit unit A and the Howland circuit unit B. Through the additionally arranged Howland compensation circuit unit, the unbalance of the two Howland circuits is compensated, and the output impedance and the load capacity of the constant current source, namely the current source circuit, are increased.

Description

Technical Field

[0001] The present application relates to the field of bioelectronic technology, and in particular to a device and circuit for measuring human body impedance. Background Art

[0002] In the prior art, human body impedance is the total impedance including human skin, blood, muscle, cell tissue and their joints, including resistance and capacitance. It is generally believed that dry skin has a relatively high resistance of about 100,000 ohms at low voltage. When the voltage is between 500 and 1000 volts, the human body resistance drops to 1000 ohms.

[0003] There are two main ways to measure human body impedance: one is to use constant voltage stimulation and current collection; the other is to use constant current stimulation and voltage collection. In the constant voltage stimulation and current collection method, the magnitude of the current passing through the human body is not easy to control due to the different impedances of each person. When the human body impedance is small, the magnitude of the current flowing through the human body may exceed the safe current range that the human body can withstand, endangering personal safety, so the current flowing into the human body needs to be controlled. When the voltage changes, the human body impedance decreases, which further increases the risk.

[0004] From the perspective of detection technology, current detection is not easy to implement, and voltage detection has lower requirements for the detection module. Using constant current stimulation and voltage detection, the current source excitation is less affected by unknown contact impedance, and the current amplitude applied to the human body is constant and controllable, thus protecting the personal safety of the person being measured. Therefore, human body impedance measurement usually adopts the solution of constant current excitation and voltage detection.

[0005] In the prior art, the existing constant current source circuit has low output impedance, poor anti-interference ability, and does not consider compensating for the error of the device itself. When it is necessary to measure the change process of human body impedance, the dynamic range that can be adapted is small, the anti-interference ability is weak, and the measurement accuracy and stability are easily affected. Summary of the invention

[0006] The technical problem to be solved by the technical solution in this application is to propose a differential current source circuit, which compensates for the imbalance of the two Howland circuits by adding a Howland compensation circuit unit, thereby increasing the output impedance and load capacity of the constant current source, i.e., the current source circuit.

[0007] The technical solution of the present application for solving the above-mentioned problem is a differential current source circuit, including a single-ended to differential circuit unit, two Howland circuit units, namely Howland circuit unit A and Howland circuit unit B, and a DC feedback circuit unit, namely Howland compensation circuit unit; the single-ended to differential circuit unit is used to perform differential conversion on the input voltage signal, and output two voltage signals with completely equal voltage amplitudes and frequencies but opposite polarities, namely voltage signal A and voltage signal B; the single-ended to differential circuit unit includes two output ends, and the two output ends are electrically connected to Howland circuit unit A and Howland circuit unit B respectively; voltage signal A is used as the input signal of Howland circuit unit A; voltage signal B is used as the input signal of Howland circuit unit B; the input end of the Howland compensation circuit unit is electrically connected to the output ends of Howland circuit unit A and Howland circuit unit B; the Howland compensation circuit unit is used to obtain the common mode voltage of Howland circuit unit A and Howland circuit unit B, namely the DC bias voltage; the DC bias voltage is used to be input to the input ends of Howland circuit unit A and Howland circuit unit B.

[0008] The differential current source circuit also includes a signal preprocessing circuit, the output end of the signal preprocessing circuit is electrically connected to the input end of the single-ended to differential circuit unit, and outputs a voltage signal to the single-ended to differential circuit unit; the input end of the signal preprocessing circuit is used to connect to obtain an excitation signal.

[0009] The signal preprocessing circuit unit includes resistors R1~R4, a sliding rheostat R5, a capacitor C1 and an operational amplifier U1; one end of the resistor R1 is connected to the external excitation signal, that is, the input AC voltage signal Vin, and the other end of the resistor R1 is connected together with the resistor R2 to the reverse input terminal of the operational amplifier U1, and the other end of the resistor R2 is connected to the input DC voltage signal Vdc; the two ends of the resistor R4 are respectively connected to the circuit ground and the in-phase input terminal of the operational amplifier U1; the other end of the resistor R4 is grounded; the capacitor C1 is connected between the reverse input terminal and the output terminal of the operational amplifier U1; the resistor R3 and the sliding rheostat R5 are connected in series and connected between the reverse input terminal and the output terminal of the operational amplifier U1.

[0010] The single-ended to differential circuit unit includes resistors R6, R7 and a single-ended to differential chip U2; the two ends of the resistor R6 are respectively connected to the circuit ground and pin 1 in the single-ended to differential chip U2, that is, the reverse input end of the internal operational amplifier; the two ends of the resistor R7 are bridged across pins 1 and 4 of the single-ended to differential chip U2, that is, the reverse input end and output end of the internal operational amplifier; the output end of the signal preprocessing circuit unit is connected to pin 8 of the single-ended to differential chip U2; pins 4 and 5 of the single-ended to differential chip U2, that is, the output ends of the internal operational amplifier are the differential voltage outputs Va and Vb of the single-ended to differential circuit unit.

[0011] Howland circuit unit A includes resistors R8, R9, R14, R17, R19, R22 and capacitors C2, C4, C7, C9, and operational amplifier U3; one end of resistor R8 is connected to the output Va of the single-to-differential circuit unit, and the other end is connected to the reverse input terminal of the operational amplifier U3; one end of resistor R9 and capacitor C2 are connected to the non-inverting input terminal of the operational amplifier U3, and the other end is connected to the output terminal of the operational amplifier U5; resistor R14 and capacitor C4 are connected across the reverse input terminal and output terminal of the operational amplifier U3; R17, C7 and R19 are connected in series between the non-inverting input terminal and output terminal of the operational amplifier U3.

[0012] Howland circuit unit B has a symmetrical structure with Howland circuit unit A; Howland circuit unit B is composed of resistors R10, R11, R13, R18, R21, R23 and capacitors C3, C5, C8, C10, and operational amplifier U4; one end of resistor R11 is connected to the output Vb of the single-to-differential circuit unit, and the other end is connected to the reverse input end of operational amplifier U4; resistor R13 and capacitor C5 are connected across the reverse input end and output end of operational amplifier U4; one end of resistor R10 and capacitor C3 is connected to the in-phase input end of operational amplifier U3, and the other end is connected to the output end of operational amplifier U5 ; R18, C8 and R21 are connected in series between the in-phase input terminal and the output terminal of the operational amplifier U4; the two ends of the resistor R20 are respectively connected to the resistors R19 and R21, serving as the common end of the two circuit units; the resistor R22 and the capacitor C9 are connected in parallel, one end of which is connected to one end of the resistor R20, and the other end is connected to one end of the external load resistor RL; the resistor R23 and the capacitor C10 are connected in parallel, one end of which is connected to the other end of the resistor R20, and the other end is connected to the other end of the external load resistor RL; the two ends of the capacitor C11 are respectively connected to the two ends of the resistor R20, and at the same time, the two ends of the capacitor C11 are respectively connected to the two ends of the external load resistor RL.

[0013] The Howland compensation circuit unit also includes a reverse operation circuit for reversely operating the common mode voltage output by the two Howland circuit units, and inputting the operation result to the in-phase input terminal of the two Howland circuit units to realize negative feedback regulation of the two Howland circuits.

[0014] The Howland compensation circuit unit is composed of resistors R12, R15, R16, capacitor C6 and operational amplifier U5; resistor R12 and capacitor C6 are connected in parallel between the reverse input terminal and the input terminal of the operational amplifier U5; one end of each resistor R15 and R16 is respectively connected to the output terminal Vao of the operational amplifier U3 and the output terminal Vbo of the operational amplifier U4, and the other ends of each resistor R15 and R16 are connected to the reverse input terminal of the operational amplifier U5; the output terminal of the operational amplifier U5 is connected to the common terminal Vcm of the resistors R9, R10 and the capacitors C2, C3.

[0015] The differential current source circuit further comprises a load, and the output end of the Howland circuit unit A and the output end of the Howland circuit unit B are electrically connected to the load.

[0016] An impedance measuring device includes the above-mentioned differential current source circuit; the impedance measuring device is used for human body impedance measurement. The human body impedance measuring device includes any one of a cardiac impedance measuring device and a thoracic impedance measuring device; the cardiac impedance measuring device is used for non-invasive cardiac output measurement; and the thoracic impedance measuring device is used for respiration measurement.

[0017] An impedance measurement device method is based on the above-mentioned differential current source circuit; the excitation signal is input to a single-ended to differential circuit unit, and converted into two voltage signals with completely equal voltage amplitudes and frequencies but opposite polarities; the voltage signals with opposite polarities are used as input signals of two Howland circuit units respectively; the sum of the output signals of the two Howland circuit units is reversely calculated by a Howland compensation circuit unit, and then fed back to the output ends of the two Howland circuit units; the output ends of the two Howland circuit units are respectively connected to the two input ends of the external human body impedance; the excitation signal is provided to the impedance measurement method through a differential current source circuit; and the human body impedance is measured by measuring the output under the excitation signal. A human body impedance measurement method is based on the above-mentioned human body impedance measurement device. The human body impedance measurement device includes any one of a cardiac impedance measurement device and a thoracic impedance measurement device; the cardiac impedance measurement device is used for non-invasive cardiac output measurement; and the thoracic impedance measurement device is used for respiration measurement.

[0018] One of the beneficial effects of the present application is to use the classic Howland circuit topology as the basis to implement the design of the differential constant current source circuit structure, and introduce a Howland compensation circuit unit to compensate for the imbalance of the two Howland circuits, so that the actual constant current source characteristics tend to the ideal value.

[0019] One of the beneficial effects of the present application is that in the prior art, the voltage input cannot contain a DC component, the circuit cannot change the input voltage through hardware, the system will be affected by the high-frequency interference of the input signal, and the symmetry of the circuit must be ensured to achieve the best performance of the circuit. The present application does not have high requirements for the voltage input. If it contains a DC component, the DC component can be automatically eliminated through the DC feedback circuit unit in the present application, which greatly expands the dynamic range of the constant current source output and improves the load capacity of the constant current source. It can eliminate the average voltage introduced back to the input end caused by the offset of the two Howland op amps, and eliminate the influence of the offset output caused by the offset voltage, bias current and offset current of the op amp and other components themselves.

[0020] One of the beneficial effects of the present application is that the output impedance is infinite and the load capacity is greatly improved.

[0021] One of the beneficial effects of the present application is that the Howland compensation circuit performs a reverse operation on the sum of the voltages output by the two Howland circuit units and superimposes it on the non-inverting input terminal of the operational amplifier, thereby eliminating the DC offset signal caused by the performance difference between the two operational amplifiers and further increasing the load capacity of the constant current source.

[0022] One of the beneficial effects of the present application is that the two Howland circuits are symmetrical structures, and the sum of the input AC signals Va and Vb is 0, so the Howland compensation circuit has no effect on the AC signal, which is particularly suitable for measuring changes in human body resistance and is very applicable to such weak signal application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic block diagram of a constant current source circuit;

[0024] Figure 2 is a schematic diagram of a constant current source circuit;

[0025] Figure 3 This is a typical Howland circuit topology diagram;

[0026] Figure 4 is a schematic diagram of a typical Howland circuit embodiment;

[0027] Figure 5 is a schematic diagram of a specific constant current source circuit embodiment;

[0028] Figure 6 It is a schematic diagram comparing the output impedance of the constant current source circuit of the present application and the typical Howland circuit;

[0029] Figure 7 It is a comparative diagram of the relationship between the output current and the load resistance of the constant current source circuit of the present application and the typical Howland circuit, that is, a comparative diagram of the load capacity;

[0030] Figure 8 It is a block diagram of the non-invasive cardiac output monitoring system. DETAILED DESCRIPTION

[0031] The content of this application is further described in detail below in conjunction with the accompanying drawings.

[0032] It should be noted that the following is a description of the preferred embodiments of the present application, which does not constitute any limitation to the present application. The description of the preferred embodiments of the present application is only an explanation of the general principles of the present application. The embodiments described in this application are only a part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0033] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second", and technical features numbered with Arabic numerals 1, 2, 3, etc., and numbers such as "A" and "B", are only used for descriptive purposes and are only for the convenience of explanation, and do not represent a sequential relationship in time or space; they cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second", and numbered with Arabic numerals 1, 2, 3, etc. may explicitly or implicitly include one or more of the features. In the description of the present application, “several” means two or more than two, unless otherwise clearly and specifically defined.

[0034] Howland Circuit Simulation of Howland Current Pump The Howland circuit is an ideal current source circuit that keeps the current constant with respect to changes in load resistance or voltage.

[0035] The load in the classic Howland circuit is single-ended grounded. In the prior art, the first circuit module and the second circuit module of the differential structure in patent CN202210498161 are classic Howland circuit structures. The load of the Howland circuit is connected to the output of the first circuit module and the second circuit module, respectively, and the load is in a floating state. In the differential structure, since the load is not grounded, the current can only flow through the resistor at the in-phase end or the reverse end of the op amp to the ground, which causes a large DC voltage to exist between the in-phase end and the reverse end of the Howland op amp, which eventually causes the constant current source circuit to output a large DC bias voltage, seriously reducing the load-carrying capacity of the constant current source. At the same time, the two op amp reverse input terminals in the first circuit module and the second circuit module are directly grounded, just like the classic Howland circuit, without considering compensating for the large DC bias voltage output by the circuit caused by the floating ground of the load in the differential structure. At the same time, the patent also does not consider the problem that the characteristics of the two-way differential Howland op amp cannot be fully matched. The mismatch between the two op amps will cause the op amp to output a DC offset signal, which will reduce the load-carrying capacity of the circuit as a high-frequency AC constant current source.

[0036] This application aims to address the phenomenon that the floating load in the differential structure will cause the circuit to output a large DC bias voltage. The application introduces a Howland compensation circuit unit, establishes DC negative feedback, and introduces the common-mode voltage output by the two Howland op amps back to the input terminal, eliminating the large DC bias voltage output by the circuit caused by the floating load, reducing the DC offset voltage output by the circuit caused by the offset voltage, bias current and offset current of the op amp itself, and greatly improving the load carrying capacity of the constant current source. The differential structure constant current source circuit can double the output impedance, has better anti-interference ability than the single-ended structure constant current source circuit, and theoretically has infinite common-mode suppression ability.

[0037] like Figure 1In an embodiment of a differential current source circuit, the circuit comprises a single-ended to differential circuit unit, two Howland circuit units, namely, a Howland circuit unit A and a Howland circuit unit B, and a DC feedback circuit unit, namely, a Howland compensation circuit unit; the single-ended to differential circuit unit is used to perform differential conversion on an input voltage signal, and output two voltage signals with completely equal voltage amplitudes and frequencies but opposite polarities, namely, voltage signal A and voltage signal B; the single-ended to differential circuit unit comprises two output terminals, and the two output terminals are electrically connected to the Howland circuit unit A and the Howland circuit unit B respectively; the voltage signal A is used as an input signal of the Howland circuit unit A; the voltage signal B is used as an input signal of the Howland circuit unit B; the input terminal of the Howland compensation circuit unit is electrically connected to the output terminals of the Howland circuit unit A and the Howland circuit unit B; the Howland compensation circuit unit is used to obtain the common mode voltage of the Howland circuit unit A and the Howland circuit unit B, namely, the DC bias voltage; the DC bias voltage is used to be input to the input terminals of the Howland circuit unit A and the Howland circuit unit B.

[0038] like Figure 1 In an embodiment of a differential current source circuit, it also includes a signal preprocessing circuit, the output end of the signal preprocessing circuit is electrically connected to the input end of the single-ended to differential circuit unit, and a voltage signal is output to the single-ended to differential circuit unit; the input end of the signal preprocessing circuit is used to connect to obtain an excitation signal.

[0039] like Figure 2 As shown, the signal preprocessing circuit unit includes resistors R1~R4, a sliding rheostat R5, a capacitor C1 and an operational amplifier U1; one end of the resistor R1 is connected to the external excitation signal, that is, the input AC voltage signal Vin, the other end of the resistor R1 is connected together with the resistor R2 to the reverse input terminal of the operational amplifier U1, and the other end of the resistor R2 is connected to the input DC voltage signal Vdc; the two ends of the resistor R4 are respectively connected to the circuit ground and the in-phase input terminal of the operational amplifier U1; the other end of the resistor R4 is grounded; the capacitor C1 is connected between the reverse input terminal and the output terminal of the operational amplifier U1; the resistor R3 and the sliding rheostat R5 are connected in series and connected between the reverse input terminal and the output terminal of the operational amplifier U1.

[0040] The output of the signal preprocessing circuit unit can be expressed by formula (1).

[0041]

[0042] The single-ended to differential circuit unit is composed of resistors R6, R7 and a single-ended to differential chip U2. Resistor R6 is connected to the circuit ground and pin 1 of the single-ended to differential chip U2, i.e., the reverse input terminal of the internal operational amplifier. Resistor R7 is connected across pins 1 and 4 of the single-ended to differential chip U2, i.e., the reverse input terminal and output terminal of the internal operational amplifier.

[0043] The output V1 of the signal preprocessing circuit unit is connected to the 8th pin of the single-ended to differential chip U2. Pin 4 and pin 8 are the differential voltage outputs Va and Vb of the single-ended to differential circuit unit, respectively. The magnitude relationship is shown in formula (2).

[0044] V a =-V b =V 1 (2)

[0045] like Figure 2 Howland circuit unit A and Howland circuit unit B are obtained by deforming the classic Howland circuit topology. The classic Howland circuit topology is as follows: Figure 3 As shown. Howland circuit unit A is composed of resistors R8, R9, R14, R17, R19, R22 and capacitors C2, C4, C7, C9, and operational amplifier U3. Howland circuit unit b is composed of resistors R10, R11, R13, R18, R21, R23 and capacitors C3, C5, C8, C10, and operational amplifier U4. One end of resistor R8 is connected to the output Va of the single-to-differential circuit unit, and the other end is connected to the reverse input end of operational amplifier U3. One end of resistor R9 and capacitor C2 are connected to the non-inverting input end of operational amplifier U3, and the other end is connected to the output end of operational amplifier U5. Resistor R14 and capacitor C4 are connected across the reverse input end and output end of operational amplifier U3. R17, C7 and R19 are connected across the non-inverting input end of operational amplifier U3. Similarly, Howland circuit unit B has a symmetrical structure with Howland circuit unit A. One end of the resistor R11 is connected to the output Vb of the single-to-differential circuit unit, and the other end is connected to the reverse input end of the operational amplifier U4. The resistor R13 and the capacitor C5 are connected across the reverse input end and the output end of the operational amplifier U4. The resistor R10 and the capacitor C3 are connected to the non-inverting input end of the operational amplifier U3 at one end, and the other end is connected to the output end of the operational amplifier U5. R18, C8 and R21 are connected across the non-inverting input end of the operational amplifier U4. The resistor R20 is connected to the resistors R19 and R21 as the common end of the two circuit units. The resistor R22, the capacitor C9 and the resistor R23, the capacitor C10 are connected to the two ends of the resistor R20 respectively, and the other end is connected to the two ends of the load resistor RL, and the capacitor C11 is also connected to the two ends of the load resistor RL.

[0046] The output currents Ia and Ib of Howland circuit unit A and Howland circuit unit B are calculated by formula (3):

[0047]

[0048] The output impedances Rao and Rbo of Howland circuit unit A and Howland circuit unit B are respectively expressed by (4)

[0049]

[0050] The voltage-controlled constant current function can be realized only when the Howland circuit unit A and the Howland circuit unit B respectively satisfy the conditions of formula (5).

[0051]

[0052] When the capacitance values ​​of capacitors C2 to C5, C7, and C8 are appropriately selected, the condition of formula (5) can be simplified to formula (6)

[0053]

[0054] At this time, the output current of the constant current source circuit is simplified from formula (3) to formula (7), and the output impedance is simplified from formula (4) to formula (8). The theoretical output impedance is infinite.

[0055]

[0056] R a =R b =∞ (8)

[0057] The Howland compensation circuit unit is composed of resistors R12, R15, R16, capacitor C2 and operational amplifier U5. One end of resistors R15 and R16 are connected to the output terminal Vao of operational amplifier U3 and the output terminal Vbo of operational amplifier U4 respectively, and the other ends are connected to the reverse input terminal of operational amplifier U5. The output terminal of operational amplifier U5 is connected to the common terminal Vcm of resistors R9, R10 and capacitors C2 and C3.

[0058] Howland compensation circuit calculates the common mode voltage output by two Howland circuit units and inputs it to the in-phase input terminal of the operational amplifier. It can eliminate the large DC bias voltage output by the circuit caused by the load floating, reduce the DC offset voltage caused by the performance difference between the two operational amplifiers, and improve the load capacity of the constant current source circuit. Since the two Howland circuits are symmetrical structures and the sum of the input AC signals Va and Vb is 0, the Howland compensation circuit has no effect on the AC signal. The voltage of the common terminal Vcm is obtained by formula (7).

[0059]

[0060] Finally, the current IL flowing through the load is:

[0061] I L =I a =-I b (8)

[0062] In this application, two Howland circuit units are responsible for converting two voltage signals into corresponding current signals, and finally passing them into the load. The Howland compensation circuit unit establishes DC negative feedback, introduces the common mode voltage output by the two Howland op amps back to the input terminal, eliminates the large DC bias voltage output by the circuit caused by the load floating, reduces the DC offset voltage output by the circuit caused by the offset voltage, bias current and offset current of the op amp and other devices themselves, and greatly improves the load carrying capacity of the constant current source.

[0063] In the present application, the Howland compensation circuit operates on the common-mode voltage output by the two Howland circuit units and inputs it to the in-phase input terminal of the operational amplifier. This can eliminate the large DC bias voltage output by the circuit caused by the load floating, reduce the DC offset voltage caused by the performance difference between the two operational amplifiers, and improve the load-carrying capacity of the constant current source circuit.

[0064] Figure 5 An application example of the present application is given. The constant current source circuit is used as a high-frequency current stimulation circuit of an ICG measurement system. The device models of U1, U3, and U4 are LT1469CS8. The model of U2 is ADA4941-1YRZ-R7, and the model of U5 is OPA827AIDGKR. The devices U1 to U5 are powered by ±9V. The values ​​of resistors R1, R15, and R16 are 10KΩ, R2 is 20KΩ, R3 is 1.5KΩ, R4 is 2KΩ, the sliding rheostat is adjustable from 0 to 1KΩ, R6 to R11 is 1KΩ, R12 is 2.49KΩ, R13 to R14 is 20Ω, the values ​​of R17 to R19 and R21 are 10Ω, R20 is 2MΩ, and the size of R22 to R23 is 560KΩ. RL represents the chest impedance of the human body, which is approximately 5 to 100Ω; the value of C1 to C5 is 30pF, C6 and C11 are 120pF, C7 to C8 is 12.6uF, and C9 to C10 is 1.2uF.

[0065] Circuit input signal Vin = -1.25-2.5Sin (2π · 5 × 10 4 t)(V), Vdc=2.5(V). The signal V1 output by the signal preprocessing unit has no DC component and is a high-frequency signal. Figure 5 and Figure 2By comparison, substituting the specific value into formula (1), the calculation result is shown in (9):

[0066]

[0067] The input relationship of the two Howland circuit units is shown in (10):

[0068] V a =-V b =V 1 =Sin(2π·5×10 4 t)(V) (10)

[0069] The values ​​of the components in the Howland circuit unit A and the Howland circuit unit B in the embodiment respectively satisfy the conditions of formula (5), and the circuit meets the constant current condition, as shown in formula (11).

[0070]

[0071] According to formula (7), the outputs of Howland circuit unit A and Howland circuit unit B are shown in formula (12):

[0072]

[0073] Finally, the current flowing into the load is as shown in formula (13):

[0074] I L =I a =-I b =2Sin(2π·5×10 4 t)(mA) (13)

[0075] Figure 4 This paper gives an example of a classic Howland constant current source circuit used as a high-frequency current stimulation circuit for an ICG measurement system. The op amp U1 is LT1469CS8, the resistors R1-R2 are 1KΩ, R3 is 20Ω, and R4-R5 are 10Ω. ICG is non-invasive cardiac output (Impedance Cardiography), also known as impedance cardiography.

[0076] The circuit input is shown in formula (14). When the Howland circuit is used as a high-frequency constant current source circuit, the input signal is required to have no DC component. However, due to the design of the signal preprocessing circuit unit, the present application has no external conditions on the input signal.

[0077] Vin=Sin(2π·5×10 4 t)(V) (14)

[0078] The values ​​of the components in this classic Howland circuit embodiment satisfy equation (15), and the circuit meets the constant current condition.

[0079]

[0080] Finally, the current flowing through the load can be seen from equation (16):

[0081]

[0082] It can be seen from the results of (10)~(11), (13)(14)~(16) that when the input signal and the circuit balance conditions are the same, the high-frequency differential constant current source circuit of the present application and the classic Howland constant current source circuit have the same current output.

[0083] use Figure 5 and Figure 4 The application examples in the figure are used to compare and verify the performance of the two circuits. Figure 6 and Figure 7 It is a schematic diagram comparing the output impedance and load capacity of the high-frequency differential constant current source circuit in the embodiment of the present application and the classic Howland circuit.

[0084] Comparison of output impedance values: First, select 11 excitation signals of different frequencies from 1kHz to 100kHz, and compare the output impedance. At this time, the peak-to-peak value of the input voltage signal is selected to be 1V, and the op amp power supply is ±9V to ensure that the op amp voltage output is not saturated. As the input signal frequency increases, the output impedance of both current sources decreases. When the input signal frequency is 50Khz, the output impedance of the high-frequency differential constant current source circuit in this application is 554.97KΩ, and the output impedance of the classic Howland current source is 278.67KΩ. The output impedance of this application is 199.14% of the classic Howland circuit, and the output impedance is increased by 99.14%. On average of 11 sets of test data, the output impedance of this application is 98.67% higher than that of the classic Howland circuit. The specific situation is as follows Figure 6 shown.

[0085] Comparison of load capacity: The constant current characteristics of the constant current circuit based on the Howland circuit topology are not only affected by the output impedance, but also by the maximum output voltage of the op amp. When the output voltage of the op amp generated by the designed constant current source circuit parameters exceeds the maximum output voltage of the op amp, the Howland circuit will lose its constant current characteristics. We call the ability of the constant current source circuit to carry the maximum load when the output current is constant the load capacity of the constant current source.

[0086] like Figure 7As shown, the schematic diagram comparing the load capacity of the high-frequency differential constant current source circuit and the classic Howland circuit shows that in the embodiment of the present application, the peak-to-peak value of the input voltage signal is 1V, the frequency is 50KHz, and the op amp power supply is ±9V. The test load changes from 20 in 1KΩ to 20KΩ to compare the output current of the two constant current source circuits. When the load is small, the circuit outputs of the high-frequency differential constant current source circuit and the classic Howland circuit are both stable at 2.00mA. As the load continues to increase, the current output by the two constant current source circuits slowly decreases.

[0087] The output current of the classic Howland circuit is reduced to 1.98mA when the load is 8KΩ, while the output current of the high-frequency differential constant current source circuit of the present application is reduced to 1.98mA when the load is 15KΩ. If compared with the evaluation standard of the current change rate of 1%, it is believed that the load capacity of the classic Howland circuit is 8KΩ, while the load capacity of the high-frequency differential constant current source circuit of the present application is 15K ohms, which is almost doubled. In fact, as the supply voltage of the op amp increases, the load capacity of the high-frequency differential constant current source circuit of the present application can be further improved.

[0088] like Figure 8 The figure shows a schematic diagram of a human body impedance measurement device, which provides an excitation signal for the impedance measurement method through a differential current source circuit; and measures the human body impedance by measuring the output under the excitation signal. The human body impedance measurement method is based on the human body impedance measurement device. The human body impedance measurement device includes any one of a cardiac impedance measurement device and a thoracic impedance measurement device; the cardiac impedance measurement device is used for non-invasive cardiac output measurement; and the thoracic impedance measurement device is used for respiration measurement.

[0089] The above description is only an embodiment of the present application and does not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the application specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A differential current source circuit, characterized in that: It includes a single-ended to differential circuit unit, two Howland circuit units, namely, a Howland circuit unit A and a Howland circuit unit B, and a DC feedback circuit unit, namely, a Howland compensation circuit unit; The single-ended to differential circuit unit is used to perform differential conversion on the input voltage signal, and output two voltage signals with completely equal voltage amplitude and frequency but opposite polarity, namely, voltage signal A and voltage signal B; The single-ended to differential circuit unit includes two output terminals, which are electrically connected to a Howland circuit unit A and a Howland circuit unit B respectively; the voltage signal A is used as an input signal of the Howland circuit unit A; and the voltage signal B is used as an input signal of the Howland circuit unit B; The input end of the Howland compensation circuit unit is used to electrically connect the output ends of the Howland circuit unit A and the Howland circuit unit B; the Howland compensation circuit unit is used to obtain the common mode voltage of the Howland circuit unit A and the Howland circuit unit B, that is, the DC bias voltage; the DC bias voltage is used to input into the input ends of the Howland circuit unit A and the Howland circuit unit B.

2. The differential current source circuit according to claim 1, characterized in that: It also includes a signal preprocessing circuit, the output end of the signal preprocessing circuit is electrically connected to the input end of the single-ended to differential circuit unit, and outputs a voltage signal to the single-ended to differential circuit unit; The input end of the signal preprocessing circuit is used to connect and obtain the excitation signal.

3. The differential current source circuit according to claim 2, characterized in that: The signal preprocessing circuit unit includes resistors R1~R4, a sliding rheostat R5, a capacitor C1 and an operational amplifier U1; one end of the resistor R1 is connected to the external excitation signal, that is, the input AC voltage signal Vin, and the other end of the resistor R1 is connected together with the resistor R2 to the reverse input terminal of the operational amplifier U1, while the other end of the resistor R2 is connected to the input DC voltage signal Vdc; the two ends of the resistor R4 are respectively connected to the circuit ground and the in-phase input terminal of the operational amplifier U1; the other end of the resistor R4 is grounded; the capacitor C1 is connected between the reverse input terminal and the output terminal of the operational amplifier U1; the resistor R3 and the sliding rheostat R5 are connected in series and connected between the reverse input terminal and the output terminal of the operational amplifier U1.

4. The differential current source circuit according to claim 1, characterized in that: The single-ended to differential circuit unit includes resistors R6, R7 and a single-ended to differential chip U2; The two ends of the resistor R6 are respectively connected to the circuit ground and the 1 pin of the single-ended to differential chip U2, i.e., the reverse input terminal of the internal operational amplifier; The two ends of the resistor R7 are connected across the 1st and 4th pins of the single-ended to differential chip U2, that is, the inverting input and output of the internal operational amplifier; The output end of the signal preprocessing circuit unit is connected to pin 8 of the single-ended to differential chip U2; pins 4 and 5 of the single-ended to differential chip U2, that is, the output ends of the internal operational amplifier are the differential voltage outputs Va and Vb of the single-ended to differential circuit unit respectively.

5. The differential current source circuit according to claim 1, characterized in that: Howland circuit unit A includes resistors R8, R9, R14, R17, R19, R22 and capacitors C2, C4, C7, C9, and operational amplifier U3; one end of resistor R8 is connected to the output Va of the single-to-differential circuit unit, and the other end is connected to the reverse input terminal of the operational amplifier U3; one end of resistor R9 and capacitor C2 are connected to the non-inverting input terminal of the operational amplifier U3, and the other end is connected to the output terminal of the operational amplifier U5; resistor R14 and capacitor C4 are connected across the reverse input terminal and output terminal of the operational amplifier U3; R17, C7 and R19 are connected in series between the non-inverting input terminal and output terminal of the operational amplifier U3.

6. The differential current source circuit according to claim 5, characterized in that: Howland circuit unit B has a symmetrical structure with Howland circuit unit A; Howland circuit unit B is composed of resistors R10, R11, R13, R18, R21, R23 and capacitors C3, C5, C8, C10, and op amp U4; One end of the resistor R11 is connected to the output Vb of the single-to-differential circuit unit, and the other end is connected to the reverse input end of the operational amplifier U4; Resistor R13 and capacitor C5 are connected across the inverting input terminal and output terminal of operational amplifier U4; One end of the resistor R10 and the capacitor C3 is connected to the non-inverting input terminal of the operational amplifier U3, and the other end is connected to the output terminal of the operational amplifier U5; R18, C8 and R21 are connected in series between the non-inverting input and output of operational amplifier U4; The two ends of the resistor R20 are connected to the resistors R19 and R21 respectively, serving as the common end of the two circuit units; the resistor R22 and the capacitor C9 are connected in parallel, one end of which is connected to one end of the resistor R20, and the other end is connected to one end of the external load resistor RL; The resistor R23 and the capacitor C10 are connected in parallel, one end of the resistor R23 is connected to the other end of the resistor R20, and the other end of the capacitor C10 is connected to the other end of the external load resistor RL; Both ends of the capacitor C11 are connected to both ends of the resistor R20 , and both ends of the capacitor C11 are connected to both ends of the external load resistor RL , respectively.

7. The differential current source circuit according to claim 1, characterized in that: The Howland compensation circuit unit also includes a reverse operation circuit for reversely operating the common mode voltage output by the two Howland circuit units, and inputting the operation result to the in-phase input terminal of the two Howland circuit units to realize negative feedback regulation of the two Howland circuits.

8. The differential current source circuit according to claim 7, characterized in that: The Howland compensation circuit unit is composed of resistors R12, R15, R16, capacitor C6 and operational amplifier U5; The resistor R12 and the capacitor C6 are connected in parallel between the inverting input terminal and the input terminal of the operational amplifier U5; One end of each resistor R15 and R16 is connected to the output terminal Vao of the operational amplifier U3 and the output terminal Vbo of the operational amplifier U4, respectively, and the other end of each resistor R15 and R16 is connected to the reverse input terminal of the operational amplifier U5; The output end of the operational amplifier U5 is connected to the common end Vcm of the resistors R9 and R10 and the capacitors C2 and C3.

9. The differential current source circuit according to claim 1, characterized in that: A load is also included, and the output end of the Howland circuit unit A and the output end of the Howland circuit unit B are electrically connected to the load.

10. An impedance measuring device, characterized in that: A differential current source circuit comprising any one of claims 1 to 9; The impedance measuring device is used for measuring human body impedance.

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