Reservoir Capacitor DAC for SAR ADC Common-Mode Rejection

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Solution Overview

Problem

Successive approximation routine (SAR) analog-to-digital converters (ADCs) face challenges in accommodating varying input common mode voltages, leading to nonlinear errors and requiring additional circuitry to maintain a fixed common mode, which increases space, power consumption, and introduces noise.

Innovation Solution

A differential digital-to-analog (DAC) circuit with a reservoir capacitor and switches that couple the bottom plates of input capacitors to reference voltages, allowing the reservoir capacitor to provide differential charge and reference voltages to provide common mode charge, thereby accommodating varying common mode voltages without additional circuitry.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If additional circuitry is used to translate input common mode to the required common mode, then the common mode can be maintained at a fixed value, but the circuit space increases, power consumption increases, and noise sources are introduced

Engineering Contradiction:

Improvecommon mode stabilityVSAvoidcircuitry complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The reservoir capacitor performs multiple functions: it provides differential charge to the input capacitors for signal conversion and simultaneously provides common mode charge to maintain the required common mode voltage. This multi-functionality eliminates the need for separate common mode translation circuitry, reducing device complexity while maintaining common mode stability.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The system uses its own existing components (reservoir capacitor and reference voltages) to provide the common mode charge needed for stable operation, rather than requiring external dedicated circuitry. The reservoir capacitor serves itself by dual-purpose operation, reducing overall system complexity.

Inventive Principle:

Principle #25Self-service

2Reliability

If additional circuitry is used to translate input common mode to the required common mode, then the common mode can be maintained at a fixed value, but power consumption increases

Engineering Contradiction:

Improvecommon mode stabilityVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The reservoir capacitor provides both differential and common mode charge functions, eliminating the need for separate power-consuming common mode translation circuitry. This reduces overall power consumption while maintaining common mode stability.

Inventive Principle:

Principle #6Universality (Multi-functionality)

3Reliability

If additional circuitry is used to translate input common mode to the required common mode, then the common mode can be maintained at a fixed value, but noise sources are introduced

Engineering Contradiction:

Improvecommon mode stabilityVSAvoidnoise

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

By using the reservoir capacitor for dual purposes (differential and common mode charge), the invention eliminates additional active circuitry that would generate noise. The approach maintains common mode stability through passive charge redistribution rather than active translation, reducing noise in the signal chain.

Inventive Principle:

Principle #6Universality (Multi-functionality)

4Reliability

If additional circuitry is used to translate input common mode to the required common mode, then the common mode can be maintained at a fixed value, but the circuit space increases

Engineering Contradiction:

Improvecommon mode stabilityVSAvoidcircuit space

Core Design Contradiction:

ReliabilityVSArea of stationary object

Solution Approach 1:

The reservoir capacitor serves dual functions by providing both differential and common mode charge to the input capacitors. This eliminates the need for separate common mode translation circuitry, reducing the overall circuit space required while maintaining common mode stability.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This solution reduces nonlinear errors and eliminates the need for additional circuitry, improving the SAR ADC's ability to handle varying common mode voltages while minimizing space and power consumption, and reducing noise in the signal chain.

Implementation Method 1

control operation of a first set of switches to transfer a differential residue charge from the reservoir capacitor to the pair of input capacitors

Methodology Applied

Scientific EffectCharge transfer: Electrostatics

Implementation Method 2

control operation of a second set of switches to transfer a common-mode residue charge from a reference voltage to set the pair of input capacitors

Methodology Applied

Scientific EffectCharge transfer: Electrostatics

Data Source

PatentUS20200021305A1Common mode rejection in reservoir capacitor analog-to-digital converter

Publication Date: 2020.01.16 ANALOG DEVICES GLOBAL UNLTD

AI summary

A differential digital-to-analog (DAC) circuit that can include a reservoir capacitor and various switches to couple the bottom plates of the input capacitors, e.g., bit-trial capacitors, to reference voltages, e.g., REF+ or REF−. In this manner, the reservoir capacitor can be used to provide any differential charge to the input capacitors, e.g., bit-trial capacitors, and the reference voltages, e.g., REF+ and REF−, can be used to provide any common mode charge to the input capacitors.