Isolation Amplifier Feedback Circuit for Higher Common-Mode Rejection
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Solution Overview
Problem
Isolation amplifiers face limitations in rejecting common mode voltage due to non-ideal characteristics and unmatched impedances of input networks, which degrades their ability to suppress noise, particularly in medical applications where leakage current must be minimized.
Innovation Solution
The isolation amplification circuit incorporates a galvanic isolation barrier with input stage circuitry featuring two filter networks and feedback paths for lowpass filtering, using nominally identical passive components and digital or analog lowpass filters to enhance Common Mode Rejection Ratio (CMRR) and minimize noise interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If protective resistors and input filter networks are added to isolation amplifiers, then input protection and noise filtering are improved, but Common Mode Rejection Ratio (CMRR) degrades
Solution Approach 1:
The patent applies feedback by connecting the output of each filter network back to its common node through a feedback path. This feedback mechanism compensates for impedance mismatches in the filter networks, maintaining balanced common mode rejection despite the presence of protective resistors and filter components. The feedback continuously adjusts the common node potentials to reject common mode signals effectively.
Solution Approach 2:
The patent segments the isolation amplifier into two symmetrically configured filter networks (first and second filter networks), each with its own feedback path. This segmentation allows independent optimization and feedback control of each input channel, ensuring that impedance mismatches in one network do not adversely affect the overall CMRR. The symmetric segmentation also facilitates balanced common mode rejection.
2Object-affected harmful factors
If input filter networks are added to remove unwanted frequency components, then noise filtering is improved, but CMRR performance degrades
Solution Approach 1:
The feedback paths compensate for the impedance effects of the filter networks. By feeding back the output signal to the common node, the system counteracts the imbalance introduced by filter components, maintaining CMRR performance while benefiting from the noise filtering capability of the filter networks.
Solution Approach 2:
The patent changes the electrical parameters (impedance, voltage) at the common nodes through feedback control. This dynamic parameter adjustment compensates for the fixed impedance values of the filter networks, allowing the system to maintain optimal CMRR across varying operating conditions while the filter networks continue to provide noise filtering.
3Adaptability or versatility
If unmatched impedances are present in input networks, then design flexibility is improved, but common mode voltage rejection capability deteriorates
Solution Approach 1:
The feedback mechanism automatically compensates for unmatched impedances in the input networks. Each filter network's common node receives feedback from its output, creating a self-correcting system that maintains balanced potentials despite impedance mismatches. This allows designers to use different component values for flexibility while the feedback maintains CMRR performance.
Solution Approach 2:
The feedback paths enable the filter networks to self-correct their own impedance mismatches. Each network's feedback path monitors and adjusts its common node potential based on its own output signal, allowing the system to automatically compensate for component variations and maintain optimal common mode rejection without external intervention.
Data Source
AI summary
An isolation amplification circuit having an input stage circuitry and a control circuitry stage interconnected through a galvanic isolation barrier. The input stage circuitry includes a first filter network and a second filter network for supplying first and second output signals in response to the application of first and second electrical input signals. The input stage circuitry includes a first feedback path configured for applying a first feedback signal to a common node of the first filter network to close a first feedback loop around the first filter network and a second feedback path configured for applying a second feedback signal to a common node of the second filter network to close a second feedback loop around the second filter network.