Dynamic Comparator Topology for Thermal and Kickback Noise
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Solution Overview
Problem
Comparators in successive approximation register digital-to-analog converters (SAR ADCs) face challenges with high thermal noise and kickback noise, which degrade the signal-to-noise ratio and introduce non-linearity, necessitating trade-offs between conversion speed and noise levels.
Innovation Solution
The comparator circuit design incorporates specific configurations of transistors and control signals to reduce thermal noise by accelerating the regeneration phase without affecting the integration time, and includes cascode transistors to mitigate kickback noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the comparator operates faster to improve conversion speed, then productivity increases, but thermal noise increases degrading signal-to-noise ratio
Solution Approach 1:
The comparator operation is divided into two distinct phases: integration phase where inputs are sampled and held, and regeneration phase where the differential signal is amplified. This segmentation allows optimization of each phase independently - the integration phase can be extended to reduce thermal noise, while the regeneration phase can be accelerated to improve conversion speed.
Solution Approach 2:
The integration phase performs preliminary action by sampling and holding the input voltages before the regeneration phase. This preliminary action establishes a stable differential voltage that can be rapidly regenerated without introducing additional thermal noise, thereby separating the noise-sensitive operation from the speed-critical operation.
2Productivity
If dynamic comparator is used to improve conversion speed, then productivity increases, but kickback noise increases introducing non-linearity
Solution Approach 1:
Cross-coupled transistors are introduced as intermediary elements between the differential input pair and the output latch. These transistors act as a buffer that isolates the input stage from the output stage, preventing kickback noise from propagating back to the input while still enabling rapid signal regeneration.
Solution Approach 2:
The cross-coupled transistors provide preliminary anti-action by counteracting the kickback effect before it can degrade linearity. The negative feedback mechanism established by the cross-coupling preemptively compensates for the non-linear effects, maintaining accuracy even at high conversion speeds.
Data Source
AI summary
A comparator circuit includes a first transistor configured to receive a first input and a second transistor configured to receive a second input. The comparator circuit further includes a third transistor coupled to a terminal of each of the first and second transistors. The third transistor is configured to be controlled by a first control signal. A gate of a fifth transistor is coupled to a terminal of a fourth transistor at a first node and a gate of the fourth transistor is coupled to a terminal of the fifth transistor at a second node. A sixth transistor is coupled between the first and fourth transistors. A seventh transistor is coupled between the second and fifth transistors. A gate of the sixth transistor and a gate of the seventh transistor are coupled together at a fixed voltage level.