Integrated impedance measurement system adaptive to biological signal acquisition front end

Abstract

The invention discloses an integrated impedance measurement system adaptive to a biological signal acquisition front end. The integrated impedance measurement system comprises a sine wave current generator, a band-pass filter, an amplifier module BPFA, a multiplexer and an analog-to-digital converter ADC which are communicated in sequence, the sine wave current generator is used for generating low-amplitude alternating current for measuring tissue-electrode impedance, and the alternating current acts on a measured electrode to generate corresponding alternating voltage; the band-pass filter and amplifier module BPFA is used for amplifying and filtering the AC voltage; an analog-to-digital converter (ADC) receives the amplified and filtered alternating voltage through a multiplexer to perform digital signal conversion, and tissue-electrode impedance measurement of the measured electrode acting on the tissue is completed; according to the invention, the amplitude of the AC voltage is controlled to accurately adapt to the + / -5mV input range of the existing acquisition front end, and the amplification and filtering modules are directly multiplexed to complete impedance signal processing without additionally configuring a special analog front end for impedance measurement, so that the integration level and the stability of a chip are improved.

Description

Technical Field

[0001] This invention relates to the field of tissue-electrode impedance measurement technology, and more specifically to an integrated impedance measurement system adapted to a biosignal acquisition front end. Background Technology

[0002] The stimulation acquisition chip is the core integrated circuit of biomedical electronic devices, primarily used in implantable medical devices. It integrates two core functions: electrical stimulation output and bioelectrical signal acquisition, enabling bidirectional electrical interaction between the device and biological tissues. At the stimulation level, the chip can output precise electrical stimulation signals according to preset frequencies, amplitudes, and pulse widths. These signals are applied to target biological tissues such as nerves, muscles, and myocardium through electrodes, thereby achieving physiological function regulation or treatment. For example, electrical stimulation of the myocardium can correct arrhythmias, and stimulation of specific brain regions can alleviate Parkinson's disease symptoms. At the acquisition level, the chip can capture weak bioelectrical signals such as electrocardiograms (ECG), electroencephalograms (EEG), and electromyograms (EMG), typically with amplitudes in the μV~mV range. These signals are then processed through built-in amplification, filtering, and analog-to-digital conversion modules, outputting digital signals suitable for analysis and real-time monitoring of the body's physiological state. Its core value lies in achieving closed-loop control of "stimulation-acquisition-feedback." Compared to traditional separate stimulation and acquisition devices, this chip boasts high integration, small size, and low power consumption, effectively reducing hardware redundancy, lowering the design complexity of implantable and portable devices, and improving device stability and clinical applicability.

[0003] During stimulation, the electrode-tissue interface impedance is a key parameter for stimulation signal transmission, dynamically changing with electrode material, tissue type, and implantation duration. Real-time impedance measurement allows for dynamic adjustment of parameters such as stimulation amplitude and pulse width. During acquisition, bioelectrical signals are weak and susceptible to interference. The matching degree between electrode impedance and the input impedance of the acquisition front-end directly determines signal quality. Impedance mismatch leads to signal voltage attenuation, reflection interference, and, combined with link noise, easily drowning out weak signals, resulting in distorted acquisition data. Real-time impedance measurement optimizes the input impedance of the acquisition front-end based on the impedance value, maximizing signal reception efficiency and improving the signal-to-noise ratio.

[0004] The existing technology is as follows: CN109363674 B discloses a multi-mode bioimpedance measurement system and method, with the core objective of accurate multi-mode bioimpedance measurement in medical and health monitoring scenarios. The system adopts a full-link architecture of "excitation generation - signal conversion - acquisition and processing - data output." The excitation module consists of a DDS (direct digital frequency synthesizer) and a pseudo-random sequence generator. Under the control of an FPGA, it can flexibly output single-frequency sine waves, swept-frequency signals, and multi-frequency pseudo-random signals, supporting dynamic adjustment of key parameters such as frequency and amplitude to adapt to the impedance measurement needs of different biological tissues. The voltage-to-current conversion stage is designed based on the Howland current source principle, using an operational amplifier and five precision resistors to form a closed-loop feedback network, stably converting the AC voltage signal output by the excitation module into a constant current signal, ensuring that the current injected into the biological tissue is not affected by changes in load impedance. The signal acquisition adopts a time-division multiplexing front-end architecture, using a high-performance analog switch to achieve switching control of multiple electrode channels. The acquisition link incorporates a dedicated low-noise amplifier and low-pass filter to suppress and amplify the weak voltage signals generated at both ends of the electrodes, effectively improving signal quality. Control and data processing are performed using an FPGA. With this as its core, the system coordinates the timing logic of each module and performs algorithmic calculations on the acquired voltage signals to generate impedance parameters and frequency characteristic curves of biological tissues, providing data support for clinical diagnosis (such as human body composition analysis and organ function monitoring).

[0005] CN204600474U discloses a bioimpedance measurement circuit for portable health monitoring devices. It focuses on low-power, high-precision bioimpedance measurement, with a circuit architecture emphasizing hardware simplification and power consumption control. The excitation module uses a general-purpose voltage generator to output a fixed-frequency sine or square wave AC signal. An external resistor voltage divider network adjusts the excitation amplitude, simplifying circuit design while meeting basic measurement requirements. The voltage-to-current conversion stage integrates a dedicated conversion chip, forming a stable conversion link with an external current-limiting resistor. The chip's internal closed-loop control ensures accurate voltage-to-current conversion, while the current-limiting resistor limits the maximum output current to prevent damage to biological tissue. The signal acquisition front-end employs a fully differential amplitude modulation demodulation architecture, integrating a demodulation chip, an instrumentation amplifier, and a low-pass filter. The instrumentation amplifier differentially amplifies the electrode voltage signal to suppress common-mode noise, the demodulation chip processes the signal to extract impedance information, and the low-pass filter removes demodulation noise, resulting in a clean impedance measurement signal.

[0006] The disadvantages of existing technologies are as follows: Existing impedance measurement excitation source designs using DDS (Direct Digital Synthesizer) or dedicated signal generators can output high-precision sine waves, but they rely on complex control devices such as FPGAs, resulting in high circuit integration difficulty and high hardware costs.

[0007] Existing voltage-to-current conversion schemes often rely on complex active devices, resulting in structural redundancy and poor load adaptability. This not only increases circuit size and power consumption but also causes the output current stability to be affected by load impedance.

[0008] Existing technologies are designed only for independent impedance measurement functions, requiring separate design of dedicated analog front-end circuits for demodulation, amplification, and acquisition. They do not consider reuse with the front-end of other systems, increasing circuit complexity and design costs. Summary of the Invention

[0009] The purpose of this invention is to address the aforementioned problems by providing an integrated impedance measurement system adapted to a biosignal acquisition front-end. Based on integrated stimulus acquisition, this system precisely adapts the AC voltage amplitude to the input range of an existing acquisition front-end, directly reusing its amplification and filtering modules to complete impedance signal processing. This eliminates the need for an additional dedicated analog front-end for impedance measurement, thereby improving chip integration and stability.

[0010] The technical solution adopted in this invention is as follows: An integrated impedance measurement system adapted to a biosignal acquisition front end, the system comprising a sinusoidal current generator, a bandpass filter and an amplifier module (BPFA), a multiplexer, and an analog-to-digital converter (ADC) connected in sequence. The sinusoidal current generator is used to generate a low-amplitude alternating current for measuring tissue-electrode impedance. The alternating current acts on the electrode under test to generate a corresponding alternating voltage. The bandpass filter and amplifier module BPFA is used to amplify and filter the AC voltage; The analog-to-digital converter (ADC) receives the amplified and filtered AC voltage through a multiplexer and performs digital signal conversion to complete the tissue-electrode impedance measurement of the tested electrode acting on the tissue.

[0011] Furthermore, the sinusoidal current generator includes a digital-to-analog converter (DAC), a low-pass filter (LPF), and a conversion capacitor C. S ; The input of the 8-bit digital-to-analog converter (DAC) is connected to a bandgap reference (BG) to convert the digital excitation signal into a stepped sinusoidal voltage signal. This voltage signal is then filtered by a low-pass filter (LPF) and passed through a series conversion capacitor (C). S The voltage signal is converted into an alternating current and applied to the electrode being measured.

[0012] Furthermore, the conversion capacitor C S The capacitance value can be selected and adjusted through a register.

[0013] Furthermore, the digital-to-analog converter (DAC) generates an amplitude of V.A and DC offset V off The sinusoidal voltage is specifically expressed as follows: (1) In the formula, f t represents the frequency of the sinusoidal signal and t represents time.

[0014] Furthermore, the alternating current injected into the electrode under test is a cosine wave current with zero offset and amplitude, as shown in the following equation: (2).

[0015] Furthermore, the digital-to-analog converter (DAC) is an 8-bit analog-to-digital converter.

[0016] Furthermore, the multiplexer is a 16:1 MUX multiplexer.

[0017] Furthermore, the analog-to-digital converter (ADC) is a 16-bit analog-to-digital converter.

[0018] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. The system of the present invention generates a stepped sine wave basic signal through an 8-bit resistor string DAC, smooths the waveform initially through a low-pass filter, and optimizes the signal quality by combining subsequent filtering links, thus avoiding high-cost devices such as DDS and FPGA.

[0019] 2. The system of the present invention uses a conversion capacitor to replace the active circuit of the traditional operational amplifier and multi-resistor combination, so as to achieve stable AC current output without complex hardware. This can reduce the circuit size and power consumption, reduce the impact of load impedance changes on the output current, ensure the stability of current injection in different tissue scenarios, and meet the integration requirements of stimulation acquisition chip.

[0020] 3. The purpose of this invention is to achieve efficient reuse of analog front-ends for impedance measurement and biosignal acquisition. Based on the integrated stimulus acquisition background, by controlling the AC voltage amplitude to accurately adapt to the ±5mV input range of the existing acquisition front-end, the amplification and filtering modules of the existing front-end can be directly reused to complete impedance signal processing. There is no need to configure an additional dedicated analog front-end for impedance measurement, thereby improving chip integration and stability. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of an integrated impedance measurement system adapted to a biosignal acquisition front end according to the present invention; Figure 2 This is a circuit diagram of a switching capacitor in an integrated impedance measurement system adapted to a biosignal acquisition front end, according to the present invention. Figure 3The diagram shows the stepped voltage output from the DAC, the output voltage after preliminary filtering by the low-pass filter, and the AC current waveform generated by the conversion capacitor in the system of this invention. Figure 4 The diagram shows the AC voltage generated on the electrode under test by the AC current in the system of this invention, and the voltage waveform after passing through the bandpass filter and amplifier module BPFA. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings.

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0024] Example like Figure 1 As shown, this embodiment provides an integrated impedance measurement system adapted to the front end of biological signal acquisition. Specifically, the stimulus acquisition chip has a built-in impedance measurement module, which includes a sinusoidal current generator, a bandpass filter and an amplifier module (BPFA), a multiplexer, and an analog-to-digital converter (ADC) connected in sequence. In this embodiment, the impedance measurement module provides selectable access to 16 input pins to measure the tissue-electrode impedance of the electrode applied to the tissue. A sinusoidal current generator is used to generate a specified low-amplitude AC current waveform for measuring the tissue-electrode impedance. By closing any of the switches S0 to S15, the on-chip current generator can be connected to a selected electrode, and the AC current waveform applied to the electrode will generate a corresponding AC voltage waveform. The high-gain AC-coupled amplifier is mainly used for the acquisition, amplification, and filtering of bioelectrical signals. In impedance measurement mode, multiplexing this amplifier can amplify and filter the AC voltage, which is then quantized by a 16-bit ADC. Finally, the tissue-electrode impedance can be calculated as the ratio of peak voltage to peak current.

[0025] The waveform generator includes a bandgap reference (BG), an 8-bit digital-to-analog converter (DAC), a low-pass filter (LPF), and a conversion capacitor (C). S ; Specifically, the sinusoidal current generator includes an 8-bit digital-to-analog converter (DAC), such as... Figure 3 As shown, the DAC is connected to a fixed-frequency second-order low-pass filter to smooth the stepped edges of the DAC waveform; the voltage generated by the DAC varies from a minimum of 0V to a maximum of (255 / 256)Vref, where Vref is the reference voltage provided by the bandgap reference BG to the DAC; the resulting test waveform is passed through a series capacitor C. SConnect to the selected electrode under test to convert AC voltage into AC current; the value of this series capacitor can be selected via a register, and its value can be 0.1C, C, or 10C. The specific circuit structure is as follows: Figure 2 As shown.

[0026] Specifically, let the voltage waveform generated by the DAC / filter be... The current injected into the electrode under test can be given by the following formula. (3) If the DAC output remains unchanged, then Therefore, the SPI master device must periodically update the 8-bit digital code to create an AC voltage waveform so that an AC current waveform can be generated through a series capacitor; thus, the DAC can generate an approximate AC current waveform with an amplitude of V. A and DC offset V off A sine wave is given by the following formula: (1) In the formula, f t represents the frequency of the sinusoidal signal, and t represents time. The current injected into the electrode under test will be a cosine wave with zero offset and amplitude, given by the following equation. (2).

[0027] The AC current waveform generated above, when applied to the electrode under test, produces a corresponding AC voltage waveform. The high-gain AC coupling amplifier, specifically a bandpass filter and amplifier module BPFA, is mainly used for the acquisition, amplification, and filtering of bioelectrical signals. In impedance measurement mode, this amplifier can be multiplexed using a 16:1 MUX multiplexer to amplify and perform secondary filtering on the AC voltage. The output voltage waveform is as follows: Figure 4 As shown, it is then quantized by a 16-bit ADC; finally, the tissue-electrode impedance can be calculated as the ratio of peak voltage to peak current.

[0028] In summary, the system of this invention generates a stepped sine wave base signal using a low-cost 8-bit resistor string DAC, smooths the waveform through a low-pass filter, and further optimizes the signal quality through a band-pass filter and amplifier, achieving a programmable, high-precision AC excitation source and reducing circuit complexity. It simplifies the voltage-to-current conversion link using a switching capacitor, achieving stable AC current output without the need for complex active components, while improving the circuit's adaptability to different load impedances, reducing overall power consumption and circuit size. Based on the application background of integrated stimulus acquisition, it adapts the AC voltage amplitude to the input range of the existing biosignal acquisition analog front-end in the integrated system, and further optimizes the signal using the band-pass filtering function of this analog front-end, achieving efficient multiplexing of the analog front-end for impedance measurement and biosignal acquisition. Without adding additional hardware circuitry, it reduces signal interference and improves the integration and stability of the integrated stimulus acquisition circuit.

[0029] This article uses specific embodiments to illustrate the principles and implementation methods of the present invention. The descriptions of the embodiments above are only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. An integrated impedance measurement system adapted to a biosignal acquisition front-end, characterized in that, The system includes a sinusoidal current generator, a bandpass filter and amplifier module (BPFA), a multiplexer, and an analog-to-digital converter (ADC) connected in sequence. The sinusoidal current generator is used to generate a low-amplitude alternating current for measuring tissue-electrode impedance. The alternating current acts on the electrode under test to generate a corresponding alternating voltage. The bandpass filter and amplifier module BPFA is used to amplify and filter the AC voltage; The analog-to-digital converter (ADC) receives the amplified and filtered AC voltage through a multiplexer and performs digital signal conversion to complete the tissue-electrode impedance measurement of the tested electrode acting on the tissue.

2. The integrated impedance measurement system adapted to a biosignal acquisition front end according to claim 1, characterized in that, The sinusoidal current generator includes a digital-to-analog converter (DAC), a low-pass filter (LPF), and a conversion capacitor (C). S ; The input of the 8-bit digital-to-analog converter (DAC) is connected to a bandgap reference (BG) to convert the digital excitation signal into a stepped sinusoidal voltage signal. This voltage signal is then filtered by a low-pass filter (LPF) and passed through a series conversion capacitor (C). S The voltage signal is converted into an alternating current and applied to the electrode being measured.

3. The integrated impedance measurement system adapted to a biosignal acquisition front end according to claim 2, characterized in that, The conversion capacitor C S The capacitance value can be selected and adjusted through a register.

4. The integrated impedance measurement system adapted to a biosignal acquisition front end according to claim 2, characterized in that, The digital-to-analog converter (DAC) generates an amplitude of V. A and DC offset V off The sinusoidal voltage is specifically expressed as follows: （1） In the formula, f t represents the frequency of the sinusoidal signal and t represents time.

5. The integrated impedance measurement system adapted to a biosignal acquisition front end according to claim 4, characterized in that, The alternating current injected into the electrode under test is a cosine wave current with zero offset and amplitude, as shown in the following formula: （2）。 6. The integrated impedance measurement system adapted to a biosignal acquisition front end according to claim 1, characterized in that, The digital-to-analog converter (DAC) is an 8-bit analog-to-digital converter.

7. The integrated impedance measurement system adapted to a biosignal acquisition front end according to claim 1, characterized in that, The multiplexer is a 16:1 MUX multiplexer.

8. The integrated impedance measurement system adapted to a biosignal acquisition front end according to claim 1, characterized in that, The analog-to-digital converter (ADC) is a 16-bit analog-to-digital converter.

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