Input Common Mode Circuit for Fully Differential Amplifier Drift Control
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Solution Overview
Problem
Fully differential amplifiers experience common mode output drifts and current flow due to changes in input common mode, especially when the input impedance is high, leading to suboptimal performance and unwanted differential output voltage changes.
Innovation Solution
Incorporating an impedance component coupled to the inputs and outputs of the differential amplifier to set and match the input common mode voltage with the output common mode voltage, minimizing common mode current and drift through a controlled impedance network.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high input impedance is used in the fully differential amplifier, then sensitivity and signal amplification are improved, but common mode current flow and input common mode drift increase
Solution Approach 1:
A common mode control circuit is introduced as an intermediary between the high-impedance amplifier and the common mode disturbances. This control circuit actively compensates for common mode voltages by injecting compensating currents, thereby preventing common mode current flow into the high-impedance input nodes while preserving the sensitivity benefits of high input impedance.
2Stability of the object's composition
If additional circuits are added to control input common mode voltage, then common mode stability is improved, but device complexity and power consumption increase
Solution Approach 1:
The common mode control functionality is merged with the existing amplifier structure by integrating the control circuit within the feedback network. The common mode control operational amplifier shares the feedback path with the main signal path, and common mode control transistors are integrated into the differential pair structure, reducing overall device complexity while maintaining stability.
Solution Approach 2:
The feedback network is designed to serve dual purposes: maintaining signal amplification and providing common mode control. The same feedback resistors and operational amplifiers that maintain signal integrity also participate in common mode voltage regulation, eliminating the need for separate dedicated common mode control components.
3Power
If feedback resistors are increased in value to improve amplification, then signal amplification is improved, but input common mode impedance increases causing greater common mode shifts
Solution Approach 1:
A dedicated common mode feedback loop is implemented that continuously monitors the input common mode voltage and adjusts the common mode control signal to counteract drifts. This feedback mechanism allows the use of high-value feedback resistors for signal amplification while actively compensating for the resulting common mode shifts through real-time adjustment of common mode control currents.
Data Source
AI summary
This application describes a system for minimizing the common mode voltage drift at the input of a fully differential amplifier. An impedance component is coupled to the inputs and outputs of the differential amplifier. The impedance component optimizes the common mode resistance or impedance to ground without significantly affecting the differential impedance, matches the input common mode voltage to the output common mode voltage and reduces the input common mode voltage drift in presence of leakage currents.