A variable gain chopper amplifier for a bio-signal sensing system

Abstract

The present application relates to a kind of variable gain chopper amplifier for biological signal sensing system, belong to the field of analog integrated circuit design, including two chopping switches S1 and S2, bias circuit, gain control module, cascode module and filter.First, power supply and bias circuit provide stable power supply voltage and static working point for the whole circuit, then modulate useful signal to chopping frequency through chopping switch S1, signal is converted into a certain amount of current after voltage through gain control module, and then current is converted into amplified ac voltage signal through cascode module, signal is modulated back to baseband through chopping switch S2, finally, high-frequency part is filtered out through low-pass filter, and the amplified low-frequency useful signal is obtained.The present application not only effectively reduces the low-frequency noise of operational amplifier by chopping principle, suppresses the imbalance of amplifier, and realizes the function of variable gain, is suitable for wide input dynamic range low-frequency biological signal processing.

Description

Technical Field

[0001] This invention belongs to the field of analog integrated circuit design, and specifically relates to a variable gain chopper amplifier for a biosignal sensing system. Background Technology

[0002] Human biosignal sensing technology is widely used in medical diagnosis, health monitoring, and intelligent treatment. However, biosignals have low frequencies, varying amplitudes, and are accompanied by extremely difficult-to-remove low-frequency noise. Furthermore, signal sensors need to be suitable for different application scenarios and overcome inherent offsets in the signal amplification circuit and various externally introduced noises. Therefore, the ability of a sensor system's front-end amplifier to suppress offset and low-frequency noise is particularly important. The design of the preamplifier directly affects the functionality and signal processing accuracy of the entire sensor system, making this area a hot research topic in both academia and industry.

[0003] Chopping is a signal processing technique. After the input signal passes through chopper switch S1, its spectrum is modulated onto odd harmonics. This is then amplified by an amplifier by a factor of A before being demodulated back to baseband (even harmonics) by S2. Simultaneously, DC offset components and low-frequency noise are modulated onto the odd harmonics. Finally, a low-pass filter removes the high-frequency components. Chopping technology reduces the output voltage offset of the operational amplifier and 1 / It also reduces low-frequency noise. Furthermore, variable gain (VGA) technology can amplify input signals of varying amplitudes, making it widely applicable in microelectronic devices and systems with wide input dynamic ranges, such as disk drive circuits and television tuners.

[0004] Traditional chopper amplifiers are only suitable for specific signal amplitudes and are difficult to apply to scenarios with a wide input dynamic range. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a variable gain chopper amplifier for a biosignal sensing system, which features low noise, low output offset, and a variable, wide-amplitude input signal dynamic range.

[0006] To achieve the above objectives, the specific solution adopted by the present invention is as follows:

[0007] The application discloses a variable gain chopper amplifier for a biological signal sensing system, which comprises a bias circuit and sequentially connected chopper switch S1, a gain control module, a cascode module, chopper switch S2 and a filter; first, the power supply is turned on, the bias circuit works, the bias current from the band gap reference is converted into the bias voltage of the cascode, and the circuit starts to work; then the input signal passes through the chopper switch S1, the useful signal is modulated to the odd harmonic wave of the chopper modulation frequency, and the DC offset component and the low frequency noise of the amplifier itself are modulated to the even harmonic wave; the signal passes through the gain control module to convert the voltage signal into the current signal, and then passes through the cascode module to amplify and convert the current signal into the voltage signal; the amplified signal passes through the chopper switch S2 to demodulate the useful signal back to the baseband, and the DC offset voltage and the noise component are demodulated to the even harmonic wave; finally, the low-pass filter is used to filter out the original DC offset component and the low frequency common mode noise signal and the like.

[0008] As further optimization of the above scheme, in the bias circuit, the gates of the NMOS transistors MB1, MB2, MB7 and MB8 are connected to each other, the sources thereof are connected to the drains of the NMOS transistors MB3, MB4, MB5 and MB6; the gate and the source of the MB1 are connected to the constant current source Ibias, the gate and the source of the MB3 are connected to the drain of the MB1, the sources of the MB3, MB4, MB5 and MB6 are connected to GND; the drains of the PMOS transistors MB9 and MB10 are connected to VB2, MB2, MB7 and VB1, the drains of the PMOS transistors MB11 and MB12 are connected to the sources of the PMOS transistors MB9 and MB10, the sources thereof are connected to VDD, the gate of the MB11 is connected to the gate of the MB9, and the gate of the MB12 is connected to the drain of the MB10; the gates of the PMOS transistors MB13 and MB14 are connected to each other, the sources thereof are connected to VDD, and the drains thereof are connected to the sources of the PMOS transistors MB15 and MB16, wherein the gate of the MB13 is connected to the drain thereof, the gate of the MB15 is connected to the drain thereof and the drain of the MB8; the drain of the MB16 is connected to the gates of the NMOS transistors VB3, MB17 and MB18, and is connected to the drain of the MB17, the source of the MB17 is connected to the drain of the MB18, and the source of the MB18 is connected to GND.

[0009] ​As a further optimization of the above scheme, in the gain control module, the input VINN, VINP is connected with the chopping switch S1, and the chopping switch S1 is connected with the gate of NMOS tubes M2 and M3; the source of M2 and M3 is connected with the drain of NMOS tube M1, and the source thereof is connected with the gate of PMOS tubes M5 and M6 and the source of PMOS tubes M7 and M8, and the gate of M7 and M8 is connected with the control voltage Vctrol; the source of PMOS tube M1 is connected with VDD, and the gate thereof is connected with VB1; the source of PMOS tube M4 is connected with VDD, and the drain thereof is connected with the source of M5 and M6, and the gate thereof is connected with VB1.

[0010] As a further optimization of the above scheme, in the cascode module, the gate of NMOS tubes M9, M10 and M17 is connected with each other; the source of M9 and M10 is connected with GND, and the drain thereof is connected with the source of NMOS tubes M11 and M12, and the drain of PMOS tubes M11 and M12 is connected with the output node of OUTN and OUTP and the gate of NMOS tubes M18 and M19 and the drain of PMOS tubes M13 and M14; the source of M13 and M14 is connected with the drain of PMOS tubes M15 and M16, and the source of M15 and M16 is connected with VDD; the resistance R1 is connected with the drain of M18 and M19, and the source thereof is connected with the drain of M17, and the source of M17 is connected with GND.

[0011] Beneficial effects: the variable gain chopping amplifier provided by the application provides stable power supply voltage and static working point for the whole circuit, and then modulates the useful signal to the chopping frequency through the chopping switch S1, and then converts the voltage into a certain amount of current through the gain control module, and then converts the current into the amplified voltage signal through the cascode module, and then modulates the signal back to the baseband through the chopping switch S2, and finally filters the direct current offset voltage and noise signal components previously modulated to the high frequency through the low-pass filter to obtain the amplified original low-frequency biological signal. The application effectively reduces the offset of the output voltage of the operational amplifier through the principle of chopping, suppresses the common-mode low-frequency noise through the chopping mechanism, realizes the function of variable gain, and is applicable to the biological signal sensing processing with wide input dynamic amplitude range. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 is the system structure diagram of the variable gain chopping amplifier;

[0013] Figure 2 is the overall circuit diagram of the variable gain amplifier;

[0014] Figure 3 is the low-frequency noise diagram of the operational amplifier without chopping technology;

[0015] Figure 4 is the low frequency noise figure of the op-amp without chopping technique;

[0016] Figure 5 is the input offset voltage figure without chopping technique;

[0017] Figure 6 is the offset voltage figure with chopping technique;

[0018] Figure 7 is the function realization figure of the variable gain. DETAILED DESCRIPTION

[0019] The technical solutions of the present application will be described in detail below with reference to the accompanying drawings. The embodiments described are only some of the embodiments of the present application, and modifications, changes or substitutions based on the present application should be covered within the protection scope of the present application.

[0020] The present application proposes a variable gain chopping amplifier for a biological signal sensing system, as shown in Figure 1 The variable gain chopping amplifier comprises two chopping switches S1 and S2, a bias circuit, a gain control module, a cascode module and a filter module. First, the power supply is turned on, the bias circuit works to convert the bias current from the band gap reference into the bias voltage of the cascode, and the circuit starts to work. Then the input signal passes through the chopping switch S1 to modulate the useful signal to the odd harmonics of the chopping modulation frequency, and the DC offset component and the low frequency noise signal are modulated to the even harmonics. The signal passes through the gain control module to convert the voltage form into the current form, and then passes through the cascode module to convert the current form into the voltage form to realize the programmable control of the variable gain. The amplified signal passes through the chopping switch S2 to demodulate the useful signal back to the baseband, and the DC offset component and the low frequency noise signal are demodulated to the even harmonics. Finally, the original DC offset component and the 1 / f and other low frequency common mode noise are filtered out through the low pass filter, and only the low frequency useful biological signal amplified by the amplifier is left.

[0021] The overall circuit diagram of the variable gain amplifier is shown in Figure 2 .

[0022] The bias circuit, the gates of NMOS transistors MB1, MB2, MB7 and MB8 are connected to each other, the sources thereof are connected to the drains of NMOS transistors MB3, MB4, MB5 and MB6; the gate and the source of MB1 are connected to a constant current source Ibias, the gate and the source of MB3 are connected to the drain of MB1, the sources of MB3, MB4, MB5 and MB6 are connected to GND; the drains of PMOS transistors MB9 and MB10 are connected to VB2, MB2, MB7 and VB1, the drains of PMOS transistors MB11 and MB12 are connected to the sources of MB9 and MB10, the sources thereof are connected to VDD, the gate of MB11 is connected to the gate of MB9, the gate of MB12 is connected to the drain of MB10; the gates of PMOS transistors MB13 and MB14 are connected to each other, the sources thereof are connected to VDD, the drains thereof are connected to the sources of PMOS transistors MB15 and MB16, wherein the gate of MB13 is connected to the drain of MB13, the gate of MB15 is connected to the drain of MB15 and the drain of MB8; the drain of MB16 is connected to the gates of NMOS transistors VB3, MB17 and MB18, and is connected to the drain of MB17, the source of MB17 is connected to the drain of MB18, the source of MB18 is connected to GND.

[0023] The gain control module, input VINN and VINP are connected to chopper switch S1, the chopper switch S1 is connected to the gates of NMOS transistors M2 and M3; the sources of M2 and M3 are connected to the drain of NMOS transistor M1, the sources thereof are connected to the gates of PMOS transistors M5 and M6 and the sources of PMOS transistors M7 and M8, the gates of M7 and M8 are connected to control voltage Vctrol; the source of PMOS transistor M1 is connected to VDD, the gate thereof is connected to VB1; the source of PMOS transistor M4 is connected to VDD, the drain thereof is connected to the sources of M5 and M6, the gate thereof is connected to VB1.

[0024] The cascode module, characterized in that the gates of NMOS transistors M9, M10 and M17 are connected to each other, the sources of M9 and M10 are connected to GND, the drains thereof are connected to the sources of NMOS transistors M11 and M12, the drains of PMOS transistors M11 and M12 are connected to OUTN and OUTP output nodes and the gates of NMOS transistors M18 and M19 and the drains of PMOS transistors M13 and M14; the sources of M13 and M14 are connected to the drains of PMOS transistors M15 and M16, the sources of M15 and M16 are connected to VDD; a resistor R1 is connected to the drains of M18 and M19, the sources thereof are connected to the drain of M17, the source of M17 is connected to GND.

[0025] The cascode current mirror precisely copies the current from the bandgap reference to each branch of the bias circuit. In the case of the transistor being on, the current in the circuit remains unchanged. According to the saturation current formula, it can be known that only the width-length ratio of the transistor needs to be adjusted to obtain different gate voltages. At this time, the gate voltage is the bias voltage supplied to the gain control module and the cascode module.

[0026] The input signal is modulated to the odd harmonic of the chopping signal through the chopping switch S1, and then the signal enters a two-stage direct-coupled amplifier circuit. The load transistors M7 and M8 of M2 and M3 can change their equivalent output resistance by changing Vcontrol , so as to change the gate voltage of M5 and M6, and further change the drain current of M5 and M6. The following formula describes this:

[0027]

[0028] Among them is the drain current of PMOS M5, which is half of the current flowing through the current source M4 when in the saturation state, and is a constant, is controlled by Vcontrol, and is a constant, so changes, also changes, ultimately leading to controlled by Vcontrol.

[0029] The drain currents of M5 and M6 flow through the cascode module to convert the current into voltage, and ultimately realize the voltage amplification. Through half-circuit analysis, the voltage gain can be obtained as described in the following formula:

[0030]

[0031] Replace Finally get:

[0032]

[0033] Because Vcontrol controls the equivalent output resistance , the gain of the entire circuit is variable under the control of Vcontrol.

[0034] For the full differential operational amplifier, the high gain amplifier output common mode level is quite sensitive to the characteristics and mismatch of the device, and cannot be stabilized by differential feedback, so a common mode feedback network must be added to detect the common mode level of the two output terminals and adjust the bias current of the amplifier accordingly. Transistors M17, M18, 19 and R1 form a simple common mode feedback circuit for detecting the output common mode voltage.

[0035] The output voltage is demodulated to the base band by the second chopping switch, while the offset voltage and low frequency noise are modulated to the even harmonics. At this time, the offset voltage and low frequency noise will also be amplified in the high frequency band, and then filtered out by the low pass filter.

[0036] Through experiments, the above implementation method can achieve the following beneficial effects as follows: Figure 3 The low frequency noise at 200Hz is 170.2nV / sqr(Hz) without chopping, Figure 4 The low frequency noise at 200Hz is 37.6nV / sqr(Hz) after chopping, which is reduced by 132.6nV / sqr(Hz); the offset voltage is the difference between the input when the output is equal, Figure 5 The input offset voltage without chopping is 1.8mv, Figure 6 The input offset voltage after chopping is 1.1mv, which is reduced by 0.7mv; Figure 7 The variable gain is realized, and the gain of the operational amplifier changes regularly with the change of the voltage Vcontrol, realizing a wide input dynamic range.

[0037] It should be noted that the above-mentioned implementation scheme should be understood as illustrative, rather than limiting the protection scope of the present application, and the protection scope of the present application is subject to the claims. For those skilled in the art, some non-essential improvements and adjustments of the present application without departing from the essence and scope of the present application still belong to the protection scope of the present application.

Claims

1. A variable gain chopper amplifier for a biosignal sensing system, characterized in that: The circuit includes a bias circuit and a chopper switch S1, a gain control module, a cascode module, another chopper switch S2, and a filter connected in sequence. First, the power is turned on, and the bias circuit begins operation, converting the bias current flowing from the bandgap reference into a cascode bias voltage. The input signal then passes through chopper switch S1, modulating the useful signal onto the odd harmonics of the chopper modulation frequency, while the amplifier's DC offset and low-frequency noise are modulated onto the even harmonics. The gain control module converts the voltage signal into a current signal, which is then amplified and converted back into a voltage signal by the cascode module. The amplified signal then passes through chopper switch S2 to demodulate the useful signal back to baseband, while the DC offset and noise are demodulated to even harmonics. Finally, a low-pass filter removes the original DC offset and 1 / f low-frequency common-mode noise. In the bias circuit, the gates of NMOS transistors MB1, MB2, MB7, and MB8 are interconnected, and their sources are connected to the drains of NMOS transistors MB3, MB4, MB5, and MB6. The gate and source of MB1 are connected to the constant current source Ibias. The gate and source of MB3 are connected to the drain of MB1. The sources of MB3, MB4, MB5, and MB6 are connected to GND. The drains of PMOS transistors MB9 and MB10 are connected to the drains of VB2, MB2, MB7, and VB1. The drains of PMOS transistors MB11 and MB12 are connected to the sources of MB9 and MB10, and their sources are connected to VND. The D phase is connected, the gate of MB11 is connected to the gate of MB9, and the gate of MB12 is connected to the drain of MB10; the gates of PMOS transistors MB13 and MB14 are connected, their sources are connected to VDD, and their drains are connected to the sources of PMOS transistors MB15 and MB16, where the gate of MB13 is connected to its drain, the gate of MB15 is connected to its drain and the drain of MB8; the drain of MB16 is connected to the gates of NMOS transistors VB3, MB17, and MB18, and is also connected to the drain of MB17, the source of MB17 is connected to the drain of MB18, and the source of MB18 is connected to GND; In the gain control module, inputs VINN and VINP are connected to chopper switch S1, which in turn is connected to the gates of NMOS transistors M2 and M3. The sources of M2 and M3 are connected to the drain of NMOS transistor M1, and their sources are connected to the gates of PMOS transistors M5 and M6 and the sources of PMOS transistors M7 and M8. The gates of M7 and M8 are connected to the control voltage Vctrol. The source of PMOS transistor M1 is connected to VDD, and its gate is connected to VB1. The source of PMOS transistor M4 is connected to VDD, its drain is connected to the sources of M5 and M6, and its gate is connected to VB1. In the cascode module, the gates of NMOS transistors M9, M10, and M17 are interconnected; the sources of M9 and M10 are connected to GND, and their drains are connected to the sources of NMOS transistors M11 and M12; the drains of PMOS transistors M11 and M12 are connected to the OUTN and OUTP output nodes, as well as the gates of NMOS transistors M18 and M19 and the drains of PMOS transistors M13 and M14; the sources of M13 and M14 are connected to the drains of PMOS transistors M15 and M16, and the sources of M15 and M16 are connected to VDD; resistor R1 is connected to the drains of M18 and M19, and its source is connected to the drain of M17, with the source of M17 connected to GND.

2. The application of the variable gain chopper amplifier according to claim 1 in a biosignal sensing system.

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