A modulation and demodulation circuit for detecting weak biopotential respiratory signals

Abstract

The present invention addresses the problems of "threshold effect", high power consumption, and large area in modulation and demodulation circuits for detecting weak biopotential respiratory signals. By modulating and demodulating the respiratory signal, the problem of "threshold effect" on the signal-to-noise ratio of small signals in traditional envelope detection is solved. In the subsequent data processing, a Sigma-Delta modulator based on second-order switched capacitors is adopted, eliminating the need for a series of filters after signal demodulation. At the same time, the complexity of the circuit structure is reduced, and the area and power consumption are reduced.

Description

Technical Field

[0001] The present invention belongs to the field of communication technology, and in particular relates to a modulation and demodulation circuit for detecting weak biopotential respiratory signals. Background Art

[0002] Currently, there are five main methods for respiratory detection: inductive plethysmography, thermistor method, pressure sensor method, acquisition through electrocardiogram (ECG) information, and impedance respiration detection. Inductive plethysmography requires a vest-like sensor device to be placed on the thorax and abdomen, which can cause the subject to feel restrained. Thermistor method has a long response time and a certain detection lag, which limits its application. Pressure sensors have high requirements for sensor placement and need to be fixed to the body, causing discomfort to the subject. They are rarely used in the field of dynamic monitoring. Acquiring respiratory signals through ECG information is a new respiratory signal detection technology, but there are currently no reports on this method in practical application. Impedance respiration detection is currently the most commonly used method for respiratory detection because it is non-invasive, safe, simple, inexpensive, and does not cause any side effects to the subject.

[0003] The human chest is equivalent to a volume conductor, and its impedance includes resistance Z, inductive reactance L and capacitive reactance C. Since the inductive reactance of the human body is very small, it can generally be ignored, and the capacitive reactance is also very small under the action of high-frequency current, so for high-frequency current, the chest impedance can be regarded as a change in resistance. There are three main impedance methods for measuring human body resistance: the bridge method, the constant current method and the constant voltage method. The bridge method has high requirements for skin treatment, and the bridge balance is difficult to adjust. It is rarely used in practice. The constant voltage method and the constant current method are essentially the same. The commonly used respiratory detection circuit uses the principle of respiratory impedance method. It borrows the chest monitoring electrode of the electrocardiogram and uses high-frequency excitation pulses to modulate the respiratory wave signal on it. The modulated circuit is then demodulated, amplified, and filtered to obtain a clear and stable respiratory curve. The circuit principle block diagram is shown below. Figure 1 As shown. The high-frequency excitation circuit usually uses a monostable / multivibrator to generate a square wave of about 60kHz. After a constant current, it is applied to the human chest through the ECG electrodes to obtain a high-frequency signal. The high-frequency signal modulated by the respiratory signal is sent to the envelope detection circuit after passing through a low-noise, low-drift, high common-mode rejection ratio preamplifier for detection and demodulation. Finally, the demodulated signal is subjected to high-pass filtering, low-pass filtering and amplification respectively, and sent to the A / D module for data processing. Figure 1The respiration detection circuit shown uses an envelope detector circuit in its demodulation stage to detect the respiration signal modulated on a high-frequency carrier. This circuit consists of a detector diode, a low-pass filter, and a voltage follower. However, due to the limitations of the diode, this circuit suffers from a narrow dynamic range and low sensitivity. Furthermore, envelope detection is only suitable for high signal-to-noise ratios. When the input signal-to-noise ratio drops below a certain value, the envelope detector interprets the useful signal as noise, resulting in a "threshold effect." Furthermore, the overall circuit structure is complex, consuming significant hardware resources and, consequently, a large area. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the defects of "threshold effect", high power consumption and large area in the modulation and demodulation circuit for detecting weak biopotential respiratory signals.

[0005] The present invention provides a modulation and demodulation circuit for detecting weak biopotential respiratory signals, comprising a first transistor circuit,

[0006] The first transistor circuit includes a first transmission gate, a second transmission gate, a third transmission gate, a fourth transmission gate, a first resistor, a second resistor, a third resistor, and a fourth resistor;

[0007] The first end of the first transmission gate is connected to the IO1 interface and the first end of the third transmission gate respectively;

[0008] The second end of the first transmission gate is connected to the first end of the first resistor and the second end of the second transmission gate respectively;

[0009] The first end of the second transmission gate is connected to the IO3 interface and the first interface of the fourth transmission gate;

[0010] The second end of the third transmission gate is respectively connected to the second end of the fourth transmission gate and the first end of the third resistor;

[0011] The second end of the first resistor is respectively connected to the first end of the second resistor, the second end of the second resistor, and the IO2 interface;

[0012] The second end of the third resistor is respectively connected to the first end of the fourth resistor, the second end of the fourth resistor, and the IO4 interface;

[0013] The third terminal of the first transmission gate, the fourth terminal of the second transmission gate, the fourth terminal of the third transmission gate, and the third terminal of the fourth transmission gate are respectively connected to the clock signal S1 port;

[0014] The fourth terminal of the first transmission gate, the third terminal of the second transmission gate, the third terminal of the third transmission gate, and the fourth terminal of the fourth transmission gate are respectively connected to the clock signal_S1 port.

[0015] Furthermore, the first transistor circuit is composed of switches driven by four clock signals S1, _S1, S2, and _S2, and the clock signal has a frequency of fchop In the lower phase complementarity, IO1 and IO3 are a set of differential input signals, IO2 and IO4 are a set of differential output signals, and the switches in the first transistor circuit are implemented by four transmission gates. The four transmission gate switches are rotated to open and close under the control of non-overlapping clocks S1, _S1, S2, and _S2.

[0016] Furthermore, the system inputs the human breathing signal into the first transistor circuit to modulate it into a differential signal INn-INp. The differential signal INn-INp is amplified by the PGA and then input into the first transistor circuit to demodulate the signal back to the original signal to obtain a differential signal.

[0017] Furthermore, the present invention further includes a human breathing signal simulation circuit, wherein the human breathing signal simulation circuit includes a fifth resistor, a sixth resistor, a third resistor, an eighth resistor, a ninth resistor, a tenth resistor, an eleventh resistor, a twelfth variable resistor, a first capacitor, a second capacitor, a third capacitor, a fourth capacitor, a fifth capacitor, a sixth capacitor, a seventh capacitor, and an eighth capacitor;

[0018] The first end of the fifth resistor is connected to the second end of the first capacitor and the first end of the second capacitor respectively;

[0019] A first terminal of the first capacitor is grounded;

[0020] The second end of the second capacitor is respectively connected to the first end of the seventh resistor, the first end of the seventh capacitor, and the second end of the fifth capacitor;

[0021] The second end of the sixth resistor is connected to the first end of the fourth capacitor and the first end of the third capacitor respectively;

[0022] The second terminal of the fourth capacitor is grounded;

[0023] The second end of the third capacitor is respectively connected to the first end of the eighth resistor, the first end of the eighth capacitor, and the second end of the sixth capacitor;

[0024] The second end of the eighth resistor is connected to the second end of the twelfth resistor;

[0025] The second end of the eleventh resistor is connected to the first end of the twelfth resistor;

[0026] The second end of the seventh capacitor is connected to the second end of the seventh resistor and the first end of the eleventh resistor respectively;

[0027] The first end of the fifth capacitor is connected to the first end of the ninth resistor and the first end of the variable gain amplifier respectively;

[0028] The first end of the sixth capacitor is connected to the second end of the first resistor and the second end of the variable gain amplifier respectively;

[0029] The second end of the first resistor is connected to the first end of the tenth resistor.

[0030] The present invention addresses the problems of "threshold effect", high power consumption, large area, etc. existing in modulation and demodulation circuits for detecting weak biopotential respiratory signals. By modulating and demodulating the respiratory signal, the problem of "threshold effect" on the signal-to-noise ratio of small signals in traditional envelope detection is solved. In the subsequent data processing, a Sigma-Delta modulator based on second-order switched capacitors is adopted, which eliminates the need for a series of filters after signal demodulation, and also reduces the complexity of the circuit structure, area and power consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Block diagram of the traditional respiratory impedance method respiration detection circuit.

[0032] Figure 2 Technical schematic diagram.

[0033] Figure 3 Circuit diagram of the first transistor.

[0034] Figure 4 Circuit block diagram of a new modulation and demodulation circuit for detecting respiratory signals.

[0035] Figure 5 Test waveform. DETAILED DESCRIPTION

[0036] The inventive concept of the present invention is to modulate and demodulate the respiratory signal through the first transistor circuit, thereby reducing the complexity of the entire analog front-end circuit structure, and reducing the area and power consumption. The specific working principle is as follows:

[0037] First, use a square wave signal to modulate the input signal Vin to a high frequency. In order to prevent signal aliasing, the frequency of the square wave signal should be the input signal cutoff frequency f T More than twice. S(t) is the frequency f chop , a square wave signal with a duty cycle of 50%, its Fourier expression is: in, Its spectrum is in, n represents the number of transformation points, odd represents an odd number, and even represents an even number. After the high-frequency modulated input signal is amplified by the operational amplifier, the demodulation signal is demodulated. The principle diagram is as follows Figure 2 As shown, combined with the principle diagram, the entire modulation and demodulation process can be decomposed as follows:

[0038] 1. The square wave signal S(t) modulates the input signal Vin, and its spectrum is: This is equivalent to moving the spectrum of the input signal to odd-numbered harmonic frequencies, and the amplitude gradually decreases as the odd value increases.

[0039] Second, the modulated signal is input to a variable gain amplifier (PGA), amplified by the PGA and then output. The gain of the PGA is set to A, and its spectrum is:

[0040] 3. The demodulation signal S(t) demodulates the output signal of the amplifier and obtains the spectrum of the output signal as follows:

[0041] From the above process, it can be seen that the input signal is modulated and demodulated, so that the signal can be restored.

[0042] The first transistor circuit diagram is as follows Figure 3 As shown, the first transistor circuit is composed of switches driven by four clock signals S1, _S1, S2, and _S2. The clock signal has a frequency of f chop The lower phases are complementary. IO1 and IO3 are a set of differential input signals, and IO2 and IO4 are a set of differential output signals. The switches in the first transistor circuit are implemented by four transmission gates. Compared with traditional NMOS switches, transmission gate switches have smaller and more stable on-resistance, usually several hundred Ω; and transmission gate switches can offset some injection and feedthrough effects. These four transmission gate switches are rotated to open and close under the control of non-overlapping clocks S1, _S1, S2, and _S2. For example, when S1=1, _S1=0, S2=0, and _S2=1, the output signal is in phase with the input signal; when S1=0, _S1=1, S2=1, and _S2=0, the output signal is in phase with the input signal, thus completing the signal modulation.

[0043] The structure of the modulation and demodulation circuit designed by the present invention for detecting weak biopotential respiratory signals is as follows: Figure 4 As shown in FIG, it includes a breathing modulation circuit, a variable gain amplifier (PGA), a breathing demodulation circuit, and a Sigma-Delta modulator circuit. The specific implementation process is as follows:

[0044] The respiratory modulation circuit is controlled by the MOD_EN bit. When MOD_EN=0, the respiratory modulation circuit is turned off; when MOD_EN=1, the respiratory modulation circuit is turned on. The respiratory demodulation circuit is controlled by the DEMOD_EN bit. When DEMOD_EN=0, the respiratory demodulation circuit is turned off; when DEMOD_EN=1, the respiratory demodulation circuit is turned on. After the modulation circuit is turned on, first, the 64kHz high-frequency signal RESP_CLK passes through a constant current and is applied to the human chest through the ECG electrodes. At this time, the change in chest impedance under the action of high-frequency current is basically equal to the change in resistance, and a high-frequency signal modulated by the respiratory signal is obtained. This signal is proportional to the human respiratory frequency. Since the chest impedance does not change much during breathing, only a few tens of Ω, using a large resistor of tens of kΩ can make the current reach a relatively stable state, so Figure 4 In the circuit, a large resistor R11 is connected in series with a small variable resistor R12 to simulate the changes in human respiratory impedance. The frequency of R12 changes according to the human respiratory frequency. Since medical product standards have leakage current limits, R7 and R8 are primarily used to limit the output current, while a series of capacitors are used to prevent DC from reaching the human body and causing harm. The modulated differential signal INn-INp is then amplified by a variable gain amplifier (PGA) with low noise, low drift, low power consumption, and high common-mode rejection ratio. Vbias is used to apply a DC bias to the PGA, producing the amplified signals PGA_ON-PGA_OP. Finally, the amplified signal is sent to a respiration demodulation circuit for demodulation, generating the amplified respiration signals IO1-IO2. This signal is then fed into a second-order switched-capacitor Sigma-Delta modulator for data processing. Because this modulator has built-in noise shaping and low-pass filtering capabilities, no additional filters are required after the respiration signal is demodulated.

[0045] right Figure 4 The overall circuit block diagram shown in the figure is used for specture simulation. R11 is set to 6KΩ, and R12 is controlled by a sine wave voltage source with an amplitude of 200mV and a frequency simulating the human respiratory frequency of 3Hz, which is set to a 64kHz square wave. The first transistor circuit modulates the simulated human respiratory signal (sinusoidal signal) into a differential signal INn-INp. After amplification by the PGA, the first transistor circuit demodulates the signal back to the original signal to obtain the differential signal IO1-IO2. The waveform after modulation and demodulation is shown as follows: Figure 5 shown.

[0046] This structure overcomes the "threshold effect" problem of traditional envelope detection circuits. Moreover, since the subsequent modulator has its own noise shaping and low-pass filtering functions, there is no need to add additional filtering circuits, thereby reducing the circuit complexity of the entire circuit and saving area and power consumption.

Claims

1. A modulation and demodulation circuit for detecting weak biopotential respiratory signals, characterized in that: comprising a first transistor circuit, The first transistor circuit includes a first transmission gate, a second transmission gate, a third transmission gate, a fourth transmission gate, a first resistor, a second resistor, a third resistor, and a fourth resistor; The first end of the first transmission gate is connected to the IO1 interface and the first end of the third transmission gate respectively; The second end of the first transmission gate is connected to the first end of the first resistor and the second end of the second transmission gate respectively; The first end of the second transmission gate is connected to the IO3 interface and the first interface of the fourth transmission gate; The second end of the third transmission gate is respectively connected to the second end of the fourth transmission gate and the first end of the third resistor; The second end of the first resistor is respectively connected to the first end of the second resistor, the second end of the second resistor, and the IO2 interface; The second end of the third resistor is respectively connected to the first end of the fourth resistor, the second end of the fourth resistor, and the IO4 interface; The third terminal of the first transmission gate, the fourth terminal of the second transmission gate, the fourth terminal of the third transmission gate, and the third terminal of the fourth transmission gate are respectively connected to the clock signal S1 port; The fourth terminal of the first transmission gate, the third terminal of the second transmission gate, the third terminal of the third transmission gate, and the fourth terminal of the fourth transmission gate are respectively connected to the clock signal_S1 port; The first transistor circuit is composed of four switches driven by clock signals S1, _S1, S2, and _S2. The clock signal has a frequency of f chop In the lower phase complementarity, IO1 and IO3 are a set of differential input signals, IO2 and IO4 are a set of differential output signals, and the switches in the first transistor circuit are implemented by four transmission gates. The four transmission gate switches are alternately opened and closed under the control of non-overlapping clocks S1, _S1, S2, and _S2; The circuit inputs the human breathing signal into the first transistor circuit to modulate it into a differential signal INn-INp. The differential signal INn-INp is amplified by the PGA and then input into the first transistor circuit to demodulate the signal back to the original signal.

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