Differential Amplifier Calibration for Common-Mode Error Rejection
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Solution Overview
Problem
Conventional difference amplifiers suffer from significant common-mode rejection errors due to resistor mismatch, which are difficult to accurately trim and maintain, especially when circuit conditions change, leading to inaccuracies in current sensing applications.
Innovation Solution
A common mode rejection calibration scheme that generates a signal varying with the common-mode voltage, scales it, and couples it into the amplifier's signal path to reduce errors, using a voltage-to-current converter and digital-to-analog converter to produce and adjust a compensation current that counters the common-mode error.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If laser trimming is performed to reduce common-mode error, then manufacturing precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent implements a feedback mechanism where the common-mode rejection performance is continuously monitored and adjusted. A calibration circuit generates a common-mode signal that feeds back to the difference amplifier, allowing dynamic adjustment of the common-mode rejection ratio (CMRR) through digital control of resistor values, replacing static laser trimming with adaptive feedback-based calibration.
Solution Approach 2:
The patent transforms the static resistor values into dynamically adjustable parameters. By using digital-to-analog converters and calibration circuits, the resistor values can be modified in real-time based on operating conditions, enabling the system to adapt to temperature variations and other environmental changes that affect CMRR.
2Measurement precision
If fixed-condition trimming is applied, then common-mode error is reduced at trimming conditions, but measurement precision deteriorates when circuit conditions change
Solution Approach 1:
The patent changes the operating parameters of the difference amplifier by dynamically adjusting resistor values through digital control. The calibration circuit modifies the effective resistance values based on the common-mode voltage level and temperature, maintaining optimal CMRR across varying operating conditions rather than being fixed at trim conditions.
Solution Approach 2:
The patent performs preliminary calibration by generating a known common-mode signal and using it to adjust the amplifier's parameters before actual measurement. The calibration circuit pre-adjusts the resistor values to compensate for expected operating conditions, ensuring optimal performance is established before the amplifier processes real signals.
3Reliability
If resistor mismatch is reduced through trimming, then common-mode gain is reduced, but device complexity and cost increase
Solution Approach 1:
The patent replaces the mechanical laser trimming process with an electronic/digital calibration system. Instead of physically altering resistor values through laser trimming, the system uses digital-to-analog converters and calibration circuits to electronically adjust the effective resistance values, eliminating the need for costly and complex laser trimming equipment while achieving superior and adjustable CMRR.
Data Source
AI summary
A common mode rejection calibration scheme for use with a difference amplifier having an associated signal path. A signal is generated which varies with the common mode voltage of the differential input voltage applied to the amplifier. This signal is scaled and coupled into the signal path such that the scaled signal reduces the common-mode error that would otherwise be present in the difference amplifier's output.