Active Electrode Gain Calibration for High-CMRR Biopotential Sensing
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Solution Overview
Problem
Active electrode biopotential signal acquisition systems face limitations due to common mode interference, particularly from mains power supply lines, which can cause significant errors due to voltage gain mismatch and contact-impedance mismatch between electrodes, leading to reduced common mode rejection ratio (CMRR) and interference with biopotential signals.
Innovation Solution
A biopotential signal acquisition system with integrated pre-amplifiers and analogue to digital converters in active electrodes, utilizing a test signal for CMRR calibration, where a digital signal processor adjusts the gain of active electrodes to minimize voltage gain differences and includes a common mode feedback system to reduce interference, ensuring robustness against cable motion and electromagnetic interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If active electrodes with integrated pre-amplifiers are used to minimize signal path length and reduce noise pickup, then noise rejection and signal quality improve, but voltage gain mismatch between electrodes increases due to process variation and component mismatch
Solution Approach 1:
The patent applies preliminary action by performing CMRR calibration before actual biopotential signal acquisition. A test signal is injected through the electrodes to measure and characterize gain mismatch and common mode rejection characteristics in advance, allowing the system to compensate for these parameters before they affect the actual measurement signals
Solution Approach 2:
The patent implements feedback by using a digital signal processor to analyze the responses from test signal injection, calculate the actual gain and CMRR values, and use this information to adjust subsequent signal processing parameters. The system continuously monitors and compensates for electrode mismatches through digital feedback mechanisms
2Object-affected harmful factors
If high common mode rejection ratio (CMRR) is required to eliminate common mode interference signals, then interference rejection improves, but the system becomes more sensitive to voltage gain mismatch between active electrodes
Solution Approach 1:
The system performs preliminary CMRR calibration by injecting a test signal at a known frequency and measuring the differential mode error that results from gain mismatch. This allows the system to characterize and compensate for CMRR limitations before actual biopotential measurement, separating the calibration function from the measurement function
Solution Approach 2:
The patent introduces a test signal as an intermediary element to probe and characterize the system's CMRR performance. This test signal acts as a mediator that allows the digital signal processor to measure gain mismatch and common mode rejection characteristics without affecting the actual biopotential signals being measured
3Reliability
If multiple active electrodes with variable gain are used to compensate for mismatch, then CMRR calibration capability improves, but device complexity increases due to additional components and control circuitry
Solution Approach 1:
The patent merges the CMRR calibration function with the existing active electrode structure by integrating test signal injection capability into the electrode system. The calibration process uses the same pre-amplifiers and ADCs already present in the active electrodes, combining multiple functions into existing components rather than adding separate dedicated hardware
Solution Approach 2:
The active electrodes are designed with multi-functionality, serving both as signal measurement devices and as test signal response sensors for CMRR calibration. The same hardware components (pre-amplifiers, ADCs, cables) are used for both calibration and actual biopotential measurement, eliminating the need for separate calibration hardware
Data Source
AI summary
A biopotential signal acquisition system, comprising: a first active electrode including an integrated pre-amplifier and an analogue to digital converter; a second active electrode including an integrated pre-amplifier and an analogue to digital converter, wherein the second active electrode has variable gain; a test signal generator for generating a test signal at a test frequency and coupling the test signal to the first and/or second active electrodes; and a digital signal processor configured to: process the digital outputs of the first and second active electrodes to derive a gain control signal based on a difference between the first and second active electrode outputs at the test frequency, and apply the gain control signal to the second active electrode. The disclosure also relates to an electronic circuit or device and a biopotential signal acquisition method.