Auto-Zeroed Amplifier Circuit With Dual Nulling and Switch Overlap
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Solution Overview
Problem
Existing auto-zeroed operational amplifiers suffer from switching noise, particularly around the sampling clock frequency, which can result in low-frequency distortion and glitches due to changes in open-loop gain during compensation and nulling cycles, and common-mode voltage mismatches between null amplifiers.
Innovation Solution
The introduction of a second null amplifier to maintain constant open-loop gain, along with a make-before-break overlap in switch operations and a common-mode zeroing loop, reduces distortion and glitches. Additionally, precharging the null amplifiers to match the main amplifier's auxiliary port voltage and using fully-differential topology with two pairs of zeroing switches helps average out glitch energy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single nulling amplifier is used to compensate the main amplifier during zeroing cycles, then offset correction is achieved, but the open-loop gain changes between compensation and nulling cycles causing low-frequency distortion and switching noise
Solution Approach 1:
The single nulling amplifier is segmented into two separate nulling amplifiers. Each nulling amplifier handles different phases of the zeroing cycle independently, allowing the system to maintain stable open-loop gain by ensuring one amplifier is always active during both compensation and nulling cycles.
2Device complexity
If switch operations are performed without overlap, then circuit simplicity is maintained, but impedance variations cause glitch energy at the output
Solution Approach 1:
The switch operations are designed with make-before-break overlap, where the new switch closes before the old switch opens. This preliminary action ensures continuous low impedance at the auxiliary port during transitions, preventing glitch energy at the output without significantly increasing circuit complexity.
3Measurement precision
If null amplifiers have different common-mode voltages, then differential mode offset correction is achieved, but common-mode voltage mismatches cause additional glitches
Solution Approach 1:
A common-mode feedback mechanism is implemented to monitor and equalize the common-mode voltages of the two nulling amplifiers. This feedback loop adjusts the common-mode levels to match, eliminating common-mode voltage mismatches and the resulting glitches while preserving differential mode offset correction capability.
Data Source
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AI summary
Two nulling amplifiers are used with an auto-zeroed differential amplifier. While one nulling amplifier is compensating the main amplifier, the other nulling amplifier is being zeroed for both differential mode (DM) and common mode (CM) offsets. By using two nulling amplifiers, one always connected to the main amplifier, a relatively constant open- loop gain is maintained for the main amplifier. A further improvement is make-before-break timing overlap of the switch operations between the two nulling amplifiers and the main amplifier. This ensures that the main amplifier is continuously driven by one or both null amplifiers, thereby maintaining a low impedance at the main amplifier auxiliary port. Both DM and CM offset sampling and precharging of each of the two nulling amplifiers is performed so as to substantially reduce switching glitches in the output of the main amplifier.