CFIA Tail Current Swapping for Gain Error Correction
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Solution Overview
Problem
Current-feedback instrumentation amplifiers face challenges in minimizing gain error due to transistor and tail current mismatches, which affect noise levels and frequency compensation complexity.
Innovation Solution
A circuit architecture that includes a modulator circuit to dynamically correct gain error by periodically swapping tail current sources between input and feedback transconductors, using differential pairs and degeneration resistors, and controlled by alternating phase signals to average out tail current differences.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If preamplifiers are used in each transconductor to create a transfer function proportional to the ratio between degeneration resistors, then gain error is reduced and depends almost exclusively on resistor matching, but noise adds to the total noise of the instrumentation amplifier making it less suitable for low noise designs
Solution Approach 1:
The patent extracts the preamplifier stage from the transconductor and replaces it with a differential pair configuration. This removes the noise-generating preamplifier while maintaining the degeneration resistor ratio approach for gain error correction, thus eliminating the harmful noise effect while preserving the benefit of resistor-matching-dependent gain error
Solution Approach 2:
The patent changes the operating parameters by using differential pair transistors instead of preamplifiers, and introduces dynamic tail current modulation to achieve gain error correction. This parameter change allows the system to achieve low gain error through current modulation rather than through preamplifier-based resistor ratios, avoiding the noise penalty
2Measurement precision
If preamplifiers with feedback loops are used in each transconductor, then gain error is reduced, but frequency compensation becomes rather complex and difficult
Solution Approach 1:
The patent removes the complex feedback loops around preamplifiers and replaces them with a simplified differential pair architecture. The gain error correction is achieved through dynamic tail current modulation rather than through complex feedback compensation networks, thus eliminating frequency compensation complexity while maintaining gain error performance
Solution Approach 2:
The patent introduces periodic modulation of the tail currents to dynamically correct gain error. By modulating the tail currents at a specific frequency and using differential pairing, the system achieves gain error correction without requiring complex continuous feedback loops, thereby simplifying frequency compensation
3Object-generated harmful factors
If differential pairs with degeneration are used in transconductors, then noise is significantly lower and there are no extra feedback loops, but gain error is affected by mismatch between degenerated differential paired transistors and tail currents
Solution Approach 1:
The patent applies periodic modulation to the tail currents of the differential pairs. By dynamically modulating the tail currents and using differential pairing, the system converts static mismatch errors into dynamic signals that can be differentially rejected, thus maintaining low noise while correcting gain error caused by transistor and current mismatches
Solution Approach 2:
The patent introduces a feedback mechanism through differential pairing and tail current modulation. The differential configuration inherently provides feedback that rejects common-mode mismatch effects, and the modulated tail currents create a feedback path that corrects gain error caused by transistor and current source mismatches
Data Source
AI summary
A current feed-back instrumentation amplifier (CFIA) comprises a differential pair with degeneration for amplifying small differential voltages in the presence of large common-mode voltages. The CFIA includes input and feedback transconductors and a chopping modulator circuit that continuously swaps tail current sources between the transconductors. This tail current swapping reduces the contribution to the CFIA's gain error caused by random mismatch between the tail currents of the input and feedback transconductors. The modulator circuit operates on a clock cycle to periodically swap the tail current sources. As a result, even if the tail currents are mismatched, on average the tail currents (transconductor gains) will approximately equal out, and the contribution of the tail current difference to the gain error is canceled out.