Current Conveyor Instrumentation Amplifier With High CMRR
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Solution Overview
Problem
Conventional instrumentation amplifiers face challenges in achieving high common mode rejection ratio (CMRR) due to resistor mismatch and voltage coefficient effects, leading to distortion and increased costs from resistor trimming and semiconductor area requirements.
Innovation Solution
The use of single or dual current conveyors with resistive elements and optional buffers to determine differential gain independently of resistor matching, achieving high CMRR and reduced distortion with polysilicon resistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If resistor pairs are used to set gain in instrumentation amplifiers, then the amplifier can achieve differential gain, but resistor mismatch and voltage coefficients cause poor common mode rejection ratio (CMRR) and distortion
Solution Approach 1:
The patent replaces the conventional voltage-mode operational amplifier circuit with a current-mode circuit using current conveyors. This substitution fundamentally changes the operating principle from voltage-based to current-based, eliminating the harmful voltage coefficients of resistors and the sensitivity to resistor mismatch, thereby achieving high CMRR without requiring precise resistor matching.
Solution Approach 2:
The invention changes the fundamental operating parameter from voltage to current. By using current conveyors that operate in the current domain, the circuit achieves gain through current relationships rather than voltage relationships, which eliminates the problems associated with resistor voltage coefficients and mismatch sensitivity.
2Measurement precision
If resistor trimming is performed to alleviate discrepancies between resistors, then CMRR improves, but manufacturing cost and complexity increase
Solution Approach 1:
By substituting the voltage-mode operational amplifier architecture with a current-mode current conveyor architecture, the patent eliminates the need for resistor trimming entirely. The current-mode operation inherently rejects common-mode signals without requiring precise resistor matching, thereby simplifying manufacturing and reducing costs.
3Ease of manufacture
If polysilicon resistors are used in instrumentation amplifiers, then the amplifier can be fabricated in standard processes, but voltage coefficients create significant distortion and non-linearity
Solution Approach 1:
The patent replaces the voltage-mode operational amplifier circuit that uses polysilicon resistors with a current-mode circuit using current conveyors. This substitution eliminates the voltage coefficients of polysilicon resistors because the circuit operates in the current domain where these coefficients do not exist, thereby eliminating the distortion and non-linearity while maintaining fabrication compatibility.
Data Source
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AI summary
Current conveyor based instrumentation amplifiers are disclosed. Such instrumentation amplifiers may have the higher common mode rejection ratios (CMRR), lower area requirements in integrated circuits, fewer resistors, fewer resistor matching requirements, less noise, and less distortion than prior art instrumentation amplifiers. One embodiment, with two input voltage lines and one output voltage line, comprises a single current conveyor and two resistors. Another embodiment, with two input voltage lines and two output voltage lines, comprises two current conveyors and four resistors, possibly in two matched pairs. Buffers may be used for impedance, frequency, and phase delay adjustment on any or all of the voltage lines.