Current Conveyor Instrumentation Amplifier Without Resistor Trimming
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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-induced distortion, particularly when using polysilicon resistors, which complicates gain control and increases costs.
Innovation Solution
The use of single or dual current conveyors with strategically placed resistive elements and optional buffers to determine differential gain independently of resistor matching, thereby enhancing CMRR and reducing distortion, while employing polysilicon resistors without voltage coefficient modulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If resistor pairs are used to control gain in instrumentation amplifiers, then gain control is achieved, but manufacturing precision deteriorates due to resistor mismatch requiring expensive trimming
Solution Approach 1:
The patent changes the fundamental parameter used for gain control from resistor values to capacitor ratios. By using capacitors in the feedback path of operational amplifiers, the gain is determined by capacitor ratios rather than resistor values. This parameter change eliminates the need for precise resistor matching while maintaining accurate gain control, as capacitor fabrication can achieve better matching characteristics.
Solution Approach 2:
The patent replaces the resistor-based gain control mechanism with a capacitor-based mechanism. This substitution fundamentally changes how gain is controlled in the instrumentation amplifier, moving from resistive elements to capacitive elements in the feedback path, thereby avoiding resistor mismatch issues entirely.
2Area of stationary object
If polysilicon resistors are used in instrumentation amplifiers, then compact design is achieved, but distortion increases due to voltage coefficients modulating differential gain
Solution Approach 1:
The patent replaces polysilicon resistors with capacitors in the gain control path. By using capacitors instead of resistors in the feedback mechanism, the voltage coefficient-induced distortion is eliminated while maintaining the compact semiconductor area advantage. The capacitive feedback path does not suffer from the same voltage coefficient modulation issues as resistive paths.
3Manufacturing precision
If resistor trimming is performed to alleviate discrepancies, then manufacturing precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent eliminates the need for resistor trimming by substituting the resistor-based gain control with a capacitor-based system. This substitution removes the entire trimming process and associated complexity, as capacitors can be fabricated with inherent matching characteristics that do not require post-fabrication adjustment.
4Measurement precision
If conventional operational amplifier-based instrumentation amplifiers are used, then common mode rejection is achieved, but CMRR is limited to 30-60 dB due to resistor mismatch
Solution Approach 1:
The patent changes the critical parameter from resistor ratios to capacitor ratios for determining amplifier gain. This parameter change enables higher CMRR because capacitor fabrication processes can achieve better matching ratios than resistors, and the capacitive feedback mechanism is less sensitive to process variations. The improved ratio matching directly translates to enhanced common mode rejection performance.
Data Source
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.