Capacitive Comparator Kickback Compensation in ADC CDAC Inputs
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Solution Overview
Problem
Latched comparators in analog-to-digital converters (ADCs) face issues with kickback noise, which introduces errors due to large signal swings, especially when driven by capacitive sources like charge redistribution CDACs, leading to power consumption inefficiencies and accuracy issues.
Innovation Solution
The implementation of kickback compensation circuits, including edge rate control and bypass circuitry, in ADCs to manage the edge rate of comparator outputs based on bit significance and route kickback current away from comparator inputs, along with differential CDACs that equalize impedance mismatch between signal inputs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If latched comparators are used in ADCs to reduce power consumption, then power efficiency is improved, but kickback noise is generated due to large signal swings
Solution Approach 1:
The patent extracts the harmful kickback current from the comparator inputs by providing dedicated kickback compensation capacitors (Ckp) connected to the comparator inputs. These capacitors capture and isolate the kickback current generated during output state changes, preventing it from coupling into the input signals and degrading measurement accuracy.
Solution Approach 2:
The kickback compensation capacitors act as intermediary elements between the comparator output and inputs. They serve as buffer components that absorb the transient kickback energy and provide a controlled impedance path, preventing direct coupling of harmful signals while allowing the latched comparator to maintain its low-power operation.
2Adaptability or versatility
If capacitive sources like charge redistribution CDACs drive the comparator, then ADC functionality is achieved, but kickback noise is amplified due to impedance mismatch
Solution Approach 1:
The patent applies local quality by providing separate, dedicated kickback compensation capacitors for each comparator input (Ckp+ and Ckp-). This localized compensation approach addresses the impedance mismatch issue at each input node individually, allowing the capacitive CDAC to drive the comparator effectively while minimizing kickback noise at each specific input location.
Solution Approach 2:
The patent changes the electrical parameters at the comparator inputs by introducing compensation capacitors that modify the impedance characteristics. These capacitors alter the frequency response and impedance matching at the input nodes, reducing the amplification of kickback noise while maintaining the capacitive driving capability of the CDAC.
3Use of energy by stationary object
If positive feedback is used to maintain output state, then power efficiency is improved, but kickback current is generated during state transitions
Solution Approach 1:
The patent implements beforehand cushioning by pre-positioning kickback compensation capacitors at the comparator inputs before state transitions occur. These capacitors are ready to absorb and cushion the kickback current when the latched comparator switches states, preventing the harmful current from affecting the input signals while allowing the positive feedback mechanism to maintain power efficiency.
Data Source
AI summary
An analog-to-digital converter (ADC) includes a comparator, a voltage reference circuit, a first capacitive digital-to-analog converter (CDAC), and a second CDAC. The first CDAC includes a plurality of capacitors. Each of the capacitors of the first CDAC includes a top plate coupled to a first input of the comparator, and a bottom plate switchably coupled to an output of the voltage reference circuit. The second CDAC includes a plurality of capacitors. Each of the capacitors of the second CDAC includes a top plate coupled to a second input of the comparator, and a bottom plate switchably coupled to a ground reference.