Signal acquisition circuit for brain-computer interface

Abstract

The invention discloses a signal acquisition circuit for a brain-computer interface, and relates to the technical field of integrated circuits. The acquisition circuit comprises a sensing amplifier, wherein the sensing amplifier comprises a chopping modulation capacitance coupling instrument amplifier and a band-pass filter which are connected in series; the chopping modulation capacitance coupling instrument amplifier comprises an input main chopping modulator, an input capacitor, a negative feedback circuit, a first amplification circuit and a second amplification circuit which are sequentially connected in series; a pre-charging circuit, an input capacitor and a first amplifying circuit are connected to the input main chopping modulator in parallel, a positive feedback enhancing loop is connected to a second amplifying circuit in parallel, the first amplifying circuit and the second amplifying circuit are connected with a first direct-current servo loop and a negative feedback circuit in parallel, and a ripple suppression loop is connected to the second amplifying circuit in parallel. And the second amplification circuit is connected with the signal output end through the band-pass filter. The sensing amplifier in the acquisition circuit can output the electroencephalogram signals with higher quality, so that the quality of the acquired brain-computer interface signals is higher.

Description

Technical Field

[0001] The present invention relates to the technical field of integrated circuits, and in particular to a signal acquisition circuit for a brain-computer interface. Background Art

[0002] At present, according to the existing data, diseases of organs in important parts of the human body, such as the heart and brain, pose a great threat to human life, and these serious physiological diseases are traceable and preventable to a certain extent. Therefore, real-time and long-term monitoring of various physiological signals of the human body (such as ECG signals, EEG signals, etc.) plays a very important role in the prevention and treatment of chronic diseases, rehabilitation management, sports health and scientific elderly care. Brain-computer interface (BCI) refers to a technology that establishes a channel of communication and control between the human brain and the external environment without relying on the conventional spinal cord and peripheral neuromuscular system, and realizes real-time interaction between the human brain and the external environment. Brain-computer interface involves fields such as information science, cognitive science, materials science and life science, and has an increasingly important impact on artificial intelligence, bioengineering and neurorehabilitation.

[0003] Among them, the front-end amplifier of the brain-computer interface signal acquisition circuit is one of the core structures of the brain-computer interface chip. As the core structure of the signal preprocessing module, its main function is to collect and amplify the weak electrical signals received by the electrodes, filter out various interference signals, and pass the processed signals to the analog-to-digital converter to convert them into digital signals for subsequent digital circuits to perform more detailed analysis and processing. The characteristic of EEG signals is that the amplitude is very weak, with a typical value of only tens of μV, and the signal frequency is generally lower than 1kHz. In addition, in the human brain environment, there are many interference signals, such as electrode misalignment caused by mismatched electrode potentials, whose amplitude is often as high as hundreds of mV; in addition, the huge biological impedance of human tissue will also cause the amplitude of the collected signal to be further attenuated.

[0004] However, the quality of the EEG signal output by the amplifier in the prior art is poor, resulting in poor brain-computer interface signals collected by the acquisition circuit. Summary of the invention

[0005] Based on this, it is necessary to provide a brain-computer interface signal acquisition circuit to address the above-mentioned technical problems. The sensing amplifier in the acquisition circuit can output high-quality EEG signals, thereby making the collected brain-computer interface signals of higher quality.

[0006] The present invention adopts the following technical solutions:

[0007] The present invention provides a signal acquisition circuit for a brain-computer interface, the acquisition circuit includes a sensing amplifier, the sensing amplifier includes: a chopper-modulated capacitor-coupled instrument amplifier and a bandpass filter connected in series; the chopper-modulated capacitor-coupled instrument amplifier includes an input main chopper modulator, an input capacitor, a negative feedback circuit, a first amplification circuit and a second amplification circuit connected in series in sequence;

[0008] A precharge circuit is connected in parallel to the input main chopper modulator, a positive feedback enhancement loop is connected in parallel to the input capacitor, the first amplifier circuit and the second amplifier circuit, a first DC servo loop and a negative feedback circuit are connected in parallel to the first amplifier circuit and the second amplifier circuit respectively, and a ripple suppression loop is connected in parallel to the second amplifier circuit; the second amplifier circuit is connected to the signal output terminal through a bandpass filter.

[0009] Preferably, the ripple suppression loop comprises a sampling capacitor, an RRL chopper modulator, a switched capacitor zero integrator and a transconductance amplifier which are connected in series in sequence.

[0010] Preferably, the pre-charging circuit comprises a pre-charging chopper modulator, a pre-charging buffer, a pre-charging chopper demodulator and an input auxiliary chopper modulator which are sequentially connected in series.

[0011] Preferably, the positive feedback enhancement loop comprises a feedforward capacitor and a feedforward chopper modulator connected in series in sequence.

[0012] Preferably, the first amplifier circuit includes a first transconductance amplifier, the second amplifier circuit includes a second transconductance amplifier, the switched capacitor low-pass filter includes a third transconductance amplifier, the first transconductance amplifier and the third transconductance amplifier both adopt a folded common source and common gate structure, and the second transconductance amplifier adopts a fully differential five-tube operational amplifier structure.

[0013] Preferably, the negative feedback circuit includes a negative feedback capacitor and a negative feedback chopper modulator connected in series in sequence; the loop composed of the negative feedback capacitor, the negative feedback chopper modulator, the first amplification circuit and the second amplification circuit is used to determine the closed-loop gain of the sensing amplifier; the negative feedback capacitor is a capacitor array, which is used to modify the closed-loop gain of the sensing amplifier.

[0014] Preferably, the bandpass filter comprises a switched capacitor low-pass filter and a second DC servo loop, the second amplifier circuit is connected to the signal output terminal via the switched capacitor low-pass filter, and the second DC servo loop is connected in parallel to the switched capacitor low-pass filter;

[0015] The switched capacitor low-pass filter and the second DC servo loop both include adjustable capacitor arrays. The low-pass angular frequency of the sensing amplifier is adjusted by adjusting the capacitance of the switched capacitor low-pass filter; and the high-pass angular frequency of the sensing amplifier is adjusted by adjusting the capacitance of the second DC servo loop.

[0016] Preferably, the first DC servo loop and the second DC servo loop have the same structure, and both include a loop capacitor, a DSL chopper modulator and an ultra-large time constant integrator connected in series in sequence.

[0017] At least one of the above technical solutions adopted by the present invention can achieve the following beneficial effects:

[0018] In the present invention, the input impedance of the sensing amplifier can be improved through the pre-charging circuit and the positive feedback enhancement loop, ensuring that the output signal of the sensing amplifier has a higher amplitude, and the ripple current generated by the offset voltage of the operational amplifier being modulated to a high frequency by the input main chopper modulator can be suppressed through the ripple suppression loop, so that the shape of the output signal of the sensing amplifier is smoother, and the DC offset carried in the input signal can be filtered out through the first DC servo loop. In this way, the above-mentioned pre-charging circuit, positive feedback enhancement loop, ripple suppression loop and first DC servo loop jointly ensure that the output signal of the sensing amplifier has a higher quality, so that the brain-computer interface signal collected by the acquisition circuit has a higher quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0020] Figure 1 A schematic diagram of the structure of a sensing amplifier provided by the present invention;

[0021] Figure 2 A schematic diagram of the structure of another sensing amplifier provided by the present invention;

[0022] Figure 3 It is a principle diagram and clock schematic diagram of the pre-charging path input impedance enhancement technology of the present invention;

[0023] Figure 4 is a circuit diagram of a ripple suppression loop in the present invention;

[0024] Figure 5 A circuit diagram of a first DC servo loop and a second DC servo loop in the present invention;

[0025] Figure 6 is a circuit diagram of the first, third, fourth, fifth and sixth transconductance amplifiers in the present invention;

[0026] Figure 7 The frequency relationship between the chopper-modulated capacitive-coupled instrumentation amplifier and the bandwidth-programmable bandpass filter in the present invention;

[0027] Description of reference numerals:

[0028] 100, sensing amplifier; 101, chopper modulated capacitor coupled instrument amplifier; 102, bandpass filter; 103, input main chopper modulator; 104, input capacitor; 105, negative feedback circuit; 106, first amplifier circuit; 107, second amplifier circuit; 108, pre-charge circuit; 109, positive feedback enhancement loop; 110, first DC servo loop; 111, ripple suppression loop; 112, signal output terminal;

[0029] 301, pre-charge chopper modulator; 302, pre-charge buffer; 303, pre-charge chopper demodulator; 304, input auxiliary chopper modulator;

[0030] 401, sampling capacitor; 402, RRL chopper modulator; 403, switched capacitor self-zero integrator; 404, transconductance amplifier;

[0031] 501, loop capacitor; 502, DSL chopper modulator; and ultra-large time constant integrator 503. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention and the corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0033] It is understood that the serial numbers assigned to the components in this application, such as "first", "second", etc., are only used to distinguish the objects described and do not have any order or technical meaning. The "connection" and "coupling" mentioned in this application, unless otherwise specified, include direct and indirect connections (couplings). In the description of this application, it should be understood that the orientation or position relationship indicated by directional words such as "upper" and "lower" is based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing this 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 this application. In this application, unless otherwise clearly specified and limited, the first feature "upper" or "lower" of the second feature can be the first and second features directly in contact, or the first and second features indirectly in contact through an intermediate medium. In this application, the difference in name is not used as a way to distinguish elements, but the difference in function of the elements is used as the principle of distinction.

[0034] The technical solutions provided by various embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

[0035] In an exemplary embodiment, the acquisition circuit includes a sense amplifier, such as Figure 1 As shown, Figure 1 The schematic diagram of the structure of a sense amplifier in the present invention is a sense amplifier 100, which includes: a chopper-modulated capacitor-coupled instrument amplifier 101 and a bandpass filter 102 connected in series; the chopper-modulated capacitor-coupled instrument amplifier 101 includes an input main chopper modulator 103, an input capacitor 104, a negative feedback circuit 105, a first amplifier circuit 106, and a second amplifier circuit 107 connected in series in sequence; a precharge circuit 108 is connected in parallel to the input main chopper modulator 104, a positive feedback enhancement loop 109 is connected in parallel to the input capacitor 104, the first amplifier circuit 105, and the second amplifier circuit 106, a first DC servo loop 110 and a negative feedback circuit 105 are connected in parallel to the first amplifier circuit 106 and the second amplifier circuit 107, and a ripple suppression loop 111 is connected in parallel to the second amplifier circuit 107; the second amplifier circuit 107 is connected to the signal output 112 terminal through the bandpass filter 102.

[0036] The input impedance of the sensing amplifier is increased by the pre-charging circuit 108 and the positive feedback enhancement loop 109, so that the output signal of the sensing amplifier has a larger amplitude; the ripple current generated by the offset voltage of the operational amplifier being modulated to a high frequency by the input main chopper modulator is suppressed by the ripple suppression loop 111, so that the shape of the output signal of the sensing amplifier is smoother; the DC offset of the input signal is filtered out by the first DC servo loop 110, so that the output signal of the sensing amplifier has a DC component of the same size.

[0037] Preferably, the negative feedback circuit is connected in parallel with the first amplifying circuit and the second amplifying circuit to determine the closed-loop gain of the amplifier. The negative feedback circuit includes a negative feedback capacitor and a negative feedback chopper modulator connected in series in sequence; the loop composed of the negative feedback capacitor, the negative feedback chopper modulator, the first amplifying circuit and the second amplifying circuit is used to determine the closed-loop gain of the sensing amplifier; the negative feedback capacitor is a capacitor array, which is used to modify the closed-loop gain of the sensing amplifier.

[0038] Preferably, the positive feedback enhancement loop 109 includes a feedforward capacitor and a feedforward chopper modulator connected in series.

[0039] Preferably, the first amplifier circuit includes a first transconductance amplifier, the second amplifier circuit includes a second transconductance amplifier, the switched capacitor low-pass filter includes a third transconductance amplifier, the first transconductance amplifier and the third transconductance amplifier both adopt a folded common source and common gate structure, and the second transconductance amplifier adopts a fully differential five-tube operational amplifier structure.

[0040] It should be noted that the signal input terminal of the sensing amplifier includes a positive and negative signal input terminal, and the input capacitor includes a first input capacitor and a second input capacitor; Figure 2 As shown, Figure 2CH in,main That is the input main chopper modulator, C in1 and C in2 are the first input capacitor and the second input capacitor, G m1 is the first transconductance amplifier, G m2 is a second transconductance amplifier; in addition, the second amplifier circuit also includes an output chopper demodulator (CH out ) and the capacitor C connected in parallel to the second transconductance amplifier m1 and C m2 PCL is a precharge circuit, and the feedforward capacitor of the positive feedback enhancement loop includes the first feedforward capacitor C pf1 and the second feedforward capacitor C pf2 , the first feedforward capacitor (C pf1 ), the second feedforward capacitor (C pf2 ) and feed-forward chopper modulator CH pf A positive feedback enhancement loop is formed; RRL is a ripple suppression loop, DSL1 is a first DC servo loop, and the sensing amplifier also includes a negative feedback circuit, which is connected in parallel with the first amplifier circuit and the second amplifier circuit. The negative feedback circuit consists of a first feedback capacitor (C fb1 ), the second feedback capacitor (C fb2 ) and chopper modulator CH fb constitute.

[0041] The positive and negative input terminals are connected to the positive input signal (V IP ) and negative input signals (V IN ); The chopper modulated capacitor coupled instrumentation amplifier filters and amplifies the two input signals for the first time, removes the interference caused by extremely low frequency components such as DC offset and baseline drift through the first DC servo loop, and the ripple suppression loop suppresses the ripple current generated by the offset voltage of the op amp being modulated to high frequency by the input main chopper modulator. The amplification factor of the signal is determined by the capacitor ratio, which is easy to achieve accurate amplification factor in modern CMOS technology. The closed-loop gain of the chopper modulated capacitor coupled instrumentation amplifier can be flexibly adjusted by changing the size of the feedback capacitor, and the output signal is transmitted to the bandwidth programmable bandpass filter.

[0042] The bandpass filter in the present invention can flexibly adjust the bandwidth of the output signal without affecting the quality of the output signal. Optionally, the bandpass filter includes a switched capacitor low-pass filter and a second DC servo loop, the second amplifier circuit is connected to the signal output end through the switched capacitor low-pass filter, and the second DC servo loop is connected in parallel to the switched capacitor low-pass filter; the switched capacitor low-pass filter and the second DC servo loop both include an adjustable capacitor array, and the low-pass angular frequency of the sensing amplifier is adjusted by adjusting the capacitance of the switched capacitor low-pass filter; the high-pass angular frequency of the sensing amplifier is adjusted by adjusting the capacitance of the second DC servo loop.

[0043] Please continue to see Figure 2 The bandpass filter consists of two parts. The first part is a switched capacitor low-pass filter, including the third input capacitor (C in3 )、The fourth input capacitor (C in4 ), the third feedback capacitor (C fb3 ), the fourth feedback capacitor (C fb4 ), switched capacitor low-pass filter sampling capacitor array (C array ), a first sampling switch (S1), a second sampling switch (S2), a third sampling switch (S3), a fourth sampling switch (S4), a third transconductance amplifier (G m3 ), the low-frequency gain and low-pass corner frequency of the bandwidth programmable bandpass filter are adjusted by adjusting the size of the input capacitor, feedback capacitor, and sampling capacitor array; the second part is the second DC servo loop (DSL2), which is only used to generate a high-pass corner, and the high-pass corner frequency of the bandpass filter is adjusted by changing the size of the capacitor.

[0044] The pre-charging circuit includes a pre-charging chopper modulator, a pre-charging buffer, a pre-charging chopper demodulator and an input auxiliary chopper modulator connected in series in sequence; the pre-charging circuit is used to pre-charge the input capacitor when the main chopper is not turned on. Figure 3 As shown, Figure 3 The structure and clock diagram of the pre-charging circuit. The pre-charging circuit uses the pre-charging path impedance improvement technology to pre-charge the input capacitor when the main chopper is not turned on, thereby improving the input impedance; see Figure 3 As shown in Figure (a), the pre-charge circuit 108 includes a pre-charge chopper modulator 301, a pre-charge buffer 302, a pre-charge chopper demodulator 303, and an input auxiliary chopper modulator 304. The pre-charge circuit 108 is connected across the two ends of the input main chopper.

[0045] like Figure 4As shown, the ripple suppression loop 111 includes a sampling capacitor 401, an RRL chopper modulator 402, a switched capacitor zero integrator 403 and a transconductance amplifier 404 connected in series in sequence; the ripple suppression loop is used to realize the conversion from the output ripple voltage to the compensation ripple current. Specifically, the switched capacitor zero integrator 403 includes a capacitor C int2 , C int3 , C az1 and C az2 , switches S3 and S4, and a fourth transconductance amplifier; transconductance amplifier 504 is a fifth transconductance amplifier.

[0046] like Figure 5 As shown, Figure 5 : is a circuit structure diagram of a first DC servo loop and a second DC servo loop. The first DC servo loop and the second DC servo loop have the same structure, and both include a loop capacitor 501, a DSL chopper modulator 502 and an ultra-large time constant integrator 503 connected in series in sequence; the loop capacitor 501 includes a first DSL loop capacitor (C dsl1 ), the second DSL loop capacitor (C dsl2 ). The ultra-large time constant integrator 503 includes a capacitor C a1 , C a2 , C A1 , C A2 , C a‘1 and C a’2 , switches S1, S2, S3, S4, S5, S6 and S7, and a sixth transconductance amplifier.

[0047] like Figure 6 As shown, Figure 6 The circuit diagram of the first, third, fourth, fifth and sixth transconductance amplifiers in the present invention is shown in FIG. m1 ) and the third transconductance amplifier (G m3 ) both adopt the folded common source and common gate structure, and the second transconductance amplifier (G m2 ) adopts a simple fully differential five-tube operational amplifier structure; its transconductance amplifiers all adopt a structure similar to the first transconductance amplifier; the first transconductance amplifier (G m1 ) and the third transconductance amplifier (G m3 ) structure, mainly including NMOS and PMOS input transistors M1, M2, M3, M4, common source and common gate transistors M7-M 14 , common-mode feedback network transistor M 16 -M 21 , and tail current source transistors M5, M6, M 15 .

[0048] In an exemplary embodiment, please continue to see Figure 2The core structure of the present invention is a chopper-modulated capacitor-coupled second-order Miller compensation instrument amplifier. Since the operational amplifier is a two-stage structure, its gain can be approximated to be infinite. The gain of the chopper-modulated capacitor-coupled instrument amplifier obtained can be expressed as:

[0049]

[0050] Among them C in1,2 is the size of the input capacitor, C fb1,2 is the size of the feedback capacitor in the negative feedback circuit, C fb1,2 Replaced with a capacitor array, programmable circuit closed-loop gain can be achieved by programming the capacitor size. The present invention provides a total of 4 magnifications of 4, 10, 40, and 100 times, which can be freely configured by the user. Various technologies are added to improve the working performance of the sensing amplifier on the basis of the chopper-modulated capacitor-coupled second-order Miller compensation instrument amplifier, including pre-charge path input impedance enhancement technology, positive feedback capacitor input impedance enhancement technology, the first DC servo loop, the ripple suppression loop, and the bandwidth programmable bandpass filter, so that it can have the following capabilities: having an input impedance of hundreds of MΩ, being able to suppress 200mV of electrode offset, being able to suppress 10mV of offset voltage at the input end of the first-stage op amp, and the circuit high and low pass angles are adjustable in 4 levels.

[0051] See also Figure 3 , Figure 3 Figure (b) shows the timing diagram of the pre-charge loop, where the pre-charge chopper modulator CH pc,in , Pre-charged chopper demodulator CH pc,out The complementary clock with a duty cycle of 50% is input to the main chopper modulator CH in,main With input auxiliary chopper modulator CH in,pc The clock relationship is shown in the figure, which is a two-phase non-overlapping clock. When the pre-charge path is turned on, the input capacitor C in1,2 The charge is absorbed from the power supply instead of the signal source, thereby reducing the amount of charge absorbed from the signal source in one cycle, which is equivalent to achieving a higher input impedance. The benefit of using two input impedance enhancement techniques is that it can not only provide a higher input impedance, but also ensure that the sensing amplifier has a higher input impedance if one of the mechanisms fails. As shown in Table 1, Table 1 is a summary of the impedance enhancement results.

[0052] Table 1

[0053] Technology Type Input Impedance PCL 196MΩ PFL 514MΩ PCL+PFL 775.39MΩ

[0054] Figure 5The circuit diagram of the DC servo loop is shown. The DC servo loop continuously integrates the DC signal at the output of the chopper-modulated capacitor-coupled instrumentation amplifier, modulates it to a high frequency through the chopper, and then transmits it to the C dsl1,2 Feedback to the input of the chopper modulated capacitor coupled instrumentation amplifier. The DSL integrator will continue to integrate until the DC component in the output signal completely disappears. All switching frequencies in the DC servo loop are the same, and their magnitude is twice the ratio frequency. and For complementary clock, the low-pass angular frequency generated by the DC servo loop can be expressed as:

[0055]

[0056] in That is, the clock frequency of the control switch in the DC servo loop. In the present invention, C a1,2 =C a’1,2 Simplifying the calculation, we can get the above formula. The high-pass corner frequency of DSL in the chopper-modulated capacitive-coupled instrumentation amplifier is configured to be 0.25Hz. Based on the above derivation, DSL can theoretically eliminate a DC offset of up to 450mV.

[0057] Figure 4 The circuit structure of the ripple suppression loop is shown, in which the transconductance amplifier and the fourth transconductance amplifier (G m4 ) and the fifth transconductance amplifier (G m5 ) and the first transconductance amplifier (G m1 ) is the same. The ripple at the output of the chopper modulated capacitive coupled instrumentation amplifier will be removed by the first RRL sampling capacitor C s1 , the second RRL sampling capacitor C s2 After being collected and modulated to a low frequency by the RRL chopper modulator, it is continuously integrated into a voltage by the switch capacitor self-zero integrator, and converted into a compensation current by the fifth transconductance amplifier, and injected into the output of the first-stage transconductance amplifier of the chopper modulated capacitor-coupled instrument amplifier to compensate for the offset current generated by the offset voltage. The RRL circuit will continue to work until the ripple in the circuit is too small to be detected. The ripple suppression effect of RRL can be equivalent to introducing a notch at the ripple frequency. Therefore, in order to ensure that the effective signal is not eliminated by mistake, the notch width must be smaller than the difference between the ripple frequency and the signal bandwidth. In this design, the ripple frequency is the chopping frequency, and the corresponding notch width can be expressed as:

[0058]

[0059] Among them C mThe size of the Miller compensation capacitor in the chopper-modulated capacitor-coupled second-order Miller compensation instrument amplifier can be reasonably designed to ensure that the RRL effectively eliminates the ripple voltage without affecting the accuracy of the effective signal. The ripple suppression loop's ability to suppress ripple can be expressed as a suppression factor K, and its size can be calculated as:

[0060]

[0061] in, is the gain of the integrator op amp, f chop is the chopping frequency of the main chopper.

[0062] The bandwidth programmable bandpass filter is mainly composed of two parts, a switched capacitor low-pass filter that provides a low-pass angle and a second DC servo loop that provides a high-pass angle. Among them, the passband gain of the bandwidth programmable bandpass filter is configured to 1 times, which can ensure the stability of the circuit operation. The passband gain of the bandwidth programmable bandpass filter can be expressed as:

[0063]

[0064] Configure it to unity gain. The low-pass corner of the bandwidth programmable bandpass filter is provided by the switched capacitor low-pass filter, and its low-pass corner frequency can be expressed as:

[0065]

[0066] Where R eq is the switched capacitor low-pass filter sampling capacitor array C array The equivalent impedance of the switched capacitor network composed of and switch, where f SW For switch S 1-4 The frequency is at least twice the frequency of the input signal. The present invention provides 4 high-pass angles to choose from: 0.25Hz, 0.5Hz, 1Hz, 2Hz

[0067] The high-pass corner frequency of the bandwidth programmable bandpass filter is provided by the second DC servo loop. The second DC servo loop has the same structure as the first DC servo loop. The only difference is that C a1,2 and C a’1,2 is replaced by a capacitor array to facilitate modification of the high-pass corner frequency of the bandwidth programmable bandpass filter. The high-pass corner frequency provided by the second DC servo loop can be expressed as:

[0068]

[0069] Take C a1,2 =C a’1,2 Simplifying the calculation, we can get the above formula. By adjusting C a1,2to adjust the size of the high-pass corner frequency introduced by the second DC servo loop. The present invention provides a total of 4 low-pass angles to choose from, namely: 250Hz, 500Hz, 1kHz, and 2kHz. It should be noted that since the signal output by the chopper-modulated capacitor-coupled instrument amplifier already has a bandpass characteristic, if the normal operation of the bandwidth programmable bandpass filter is to be guaranteed, it is necessary to ensure that the 4 high-pass corner frequencies provided by the bandwidth programmable bandpass filter are all higher than the high-pass corner frequency of the output signal of the chopper-modulated capacitor-coupled instrument amplifier; and ensure that the 4 low-pass corner frequencies provided by the bandwidth programmable bandpass filter are all lower than the low-pass corner frequency of the output signal of the chopper-modulated capacitor-coupled instrument amplifier, such as Figure 7 As shown, Figure 7 Frequency relationship for a chopper modulated capacitor coupled instrumentation amplifier and a bandwidth programmable bandpass filter.

[0070] The purpose of the present invention is to overcome the unfavorable factors faced by analog front-end circuits when working in a human brain environment, which mainly include: since the amplitude of the EEG signal is very weak, and since the huge impedance of the human tissue will further reduce the amplitude of the collected EEG signal, the noise of the entire perception amplifier system should be controlled at a very small level, and the capacitor-coupled instrument amplifier using chopper modulation technology can greatly suppress the flicker noise in the circuit, but when the chopper modulator switches at different clock phases, it is inevitable to charge and discharge the input capacitor, resulting in a reduction in input impedance and the introduction of charge and discharge ripple. The ripple can be alleviated to a certain extent by pre-charging technology, and the input impedance can be improved to a certain extent. The input impedance can be further greatly improved by using positive feedback input impedance enhancement technology, but due to the influence of parasitic capacitance, the feedback capacitor needs to be accurately adjusted to prevent circuit oscillation.

[0071] Therefore, the perception amplifier in the present invention is a low-noise, low-power perception amplifier for brain-computer interface signal acquisition circuit, and its main structure includes a chopper-modulated capacitor-coupled instrument amplifier and a bandwidth-programmable bandpass filter connected in sequence. Two input impedance enhancement technologies are innovatively used in the chopper-modulated capacitor-coupled instrument amplifier. The positive feedback input impedance enhancement technology and the pre-charge path input impedance enhancement technology can greatly improve the input impedance. Among them, the pre-charge path input impedance enhancement technology not only further improves the input impedance of the circuit, but also can ensure that the circuit still has a large input impedance under certain extreme conditions (such as the positive feedback capacitor value due to process fluctuations is too large, resulting in the failure of the positive feedback impedance enhancement technology). In addition, the frequency components of bioelectric signals are concentrated in the low frequency band (hundreds of mHz to several kHz). In order to avoid interference caused by extremely low frequency components such as DC offset and baseline drift, while retaining as many low-frequency details of the original signal as possible, the perception analog front-end amplifier needs a high-pass cutoff frequency lower than 0.5Hz. The present invention uses a DC servo loop (DSL) using an ultra-large time constant integrator structure to generate a high-pass angle, which effectively reduces the size of the required capacitor and resistor. The sensing amplifier invented and introduced a ripple suppression loop in the chopper-modulated capacitor-coupled instrumentation amplifier to suppress the ripple current generated by the offset voltage of the operational amplifier, thereby greatly improving the quality of the output signal of the sensing amplifier, thereby improving the quality of the brain-computer interface signal collected by the acquisition circuit.

[0072] In summary, the beneficial effects of the present invention also include: the chopper-modulated capacitor-coupled instrument amplifier adopts a two-stage operational amplifier structure, which can provide higher gain, and at the same time directly changes the closed-loop gain of the operational amplifier through the feedback capacitor array in the negative feedback circuit, which can provide higher gain accuracy. Two impedance enhancement technologies are used to prevent drastic changes in input impedance due to fluctuations in the capacitance value caused by the process. The low-pass and high-pass angles of the circuit are changed by a bandwidth-programmable bandpass filter, and the bandwidth adjustment is separated from the gain adjustment to prevent the normal operation of other loops from being affected when adjusting the bandwidth of the sensing amplifier, providing better stability. A ripple suppression loop is added between the two-stage operational amplifiers of the chopper-modulated capacitor-coupled instrument amplifier to suppress the ripple generated by the imbalance of the first-stage operational amplifier, so that the output signal of the sensing amplifier is cleaner and clearer.

[0073] The technical features of the above embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of the present invention.

Claims

1. A signal acquisition circuit for a brain-computer interface, characterized in that: The acquisition circuit includes a sensing amplifier, which includes: a chopper-modulated capacitor-coupled instrument amplifier and a bandpass filter connected in series; the chopper-modulated capacitor-coupled instrument amplifier includes an input main chopper modulator, an input capacitor, a negative feedback circuit, a first amplifier circuit and a second amplifier circuit connected in series in sequence; A precharge circuit is connected in parallel to the input main chopper modulator, a positive feedback enhancement loop is connected in parallel to the input capacitor, the first amplifier circuit and the second amplifier circuit, a first DC servo loop and a negative feedback circuit are connected in parallel to the first amplifier circuit and the second amplifier circuit respectively, and a ripple suppression loop is connected in parallel to the second amplifier circuit; the second amplifier circuit is connected to the signal output terminal through a bandpass filter.

2. The acquisition circuit according to claim 1, characterized in that: The ripple suppression loop includes a sampling capacitor, an RRL chopper modulator, a switched capacitor self-zeroing integrator, and a transconductance amplifier connected in series.

3. The acquisition circuit according to claim 1, characterized in that: The pre-charging circuit comprises a pre-charging chopper modulator, a pre-charging buffer, a pre-charging chopper demodulator and an input auxiliary chopper modulator which are connected in series in sequence.

4. The acquisition circuit according to claim 1, characterized in that: The positive feedback enhancement loop includes a feedforward capacitor and a feedforward chopper modulator connected in series.

5. The acquisition circuit according to claim 1, characterized in that: The first amplifier circuit includes a first transconductance amplifier, the second amplifier circuit includes a second transconductance amplifier, the switched capacitor low-pass filter includes a third transconductance amplifier, the first transconductance amplifier and the third transconductance amplifier both adopt a folded common source and common gate structure, and the second transconductance amplifier adopts a fully differential five-tube operational amplifier structure.

6. The acquisition circuit according to claim 1, characterized in that: The negative feedback circuit includes a negative feedback capacitor and a negative feedback chopper modulator connected in series in sequence; a loop composed of the negative feedback capacitor, the negative feedback chopper modulator, the first amplifier circuit and the second amplifier circuit is used to determine the closed-loop gain of the sensing amplifier; the negative feedback capacitor is a capacitor array, which is used to modify the closed-loop gain of the sensing amplifier.

7. The acquisition circuit according to claim 1, characterized in that: The bandpass filter includes a switched capacitor low-pass filter and a second DC servo loop, the second amplifier circuit is connected to the signal output terminal through the switched capacitor low-pass filter, and the second DC servo loop is connected in parallel to the switched capacitor low-pass filter; The switched capacitor low-pass filter and the second DC servo loop both include adjustable capacitor arrays. The low-pass angular frequency of the sensing amplifier is adjusted by adjusting the capacitance of the switched capacitor low-pass filter; and the high-pass angular frequency of the sensing amplifier is adjusted by adjusting the capacitance of the second DC servo loop.

8. The acquisition circuit according to claim 7, characterized in that: The first DC servo loop and the second DC servo loop have the same structure, and both include a loop capacitor, a DSL chopper modulator and an ultra-large time constant integrator connected in series in sequence.

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