ADC Channel Selection Circuit With Dynamic Substrate Biasing
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Solution Overview
Problem
Existing channel selection circuits in analog-to-digital converters (ADCs) using CMOS switches suffer from body-bias effects and threshold voltage variations, leading to non-linearity and poor performance, especially at low power supply voltages, making them unsuitable for high-speed, high-accuracy applications.
Innovation Solution
A channel selection circuit design that includes a switch control circuit, MOS switch channel circuit, and substrate potential control circuit, where the substrate voltage of MOS switches is set to the voltage of the analog input signal during the sampling phase, eliminating the body-bias effect and ensuring constant on-resistance, thereby improving linearity and accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If CMOS switches are used in the channel selection circuit, then the circuit can be implemented with standard semiconductor processes, but the body-bias effect and threshold voltage variations cause non-linearity and poor accuracy
Solution Approach 1:
A substrate potential control circuit is introduced as an intermediary component between the CMOS switches and the substrate. This circuit dynamically adjusts the substrate potential of each MOS switch to match its source potential during the sampling phase, thereby eliminating the body-bias effect and threshold voltage variations that would otherwise degrade measurement precision.
Solution Approach 2:
The invention changes the substrate potential parameter of the MOS switches from a fixed value (typically ground or VDD) to a dynamic value that tracks the source potential. By making the substrate potential a variable parameter that adapts to the input signal conditions, the circuit maintains constant on-resistance and eliminates non-linearity caused by body-bias effects.
2Device complexity
If MOS switches are used for channel selection, then the circuit complexity is reduced, but the on-resistance varies with input voltage due to threshold voltage modulation
Solution Approach 1:
The substrate potential control circuit serves as an intermediary that decouples the relationship between source potential and substrate potential. By actively controlling the substrate potential to match the source potential, it compensates for threshold voltage modulation effects, thereby stabilizing the on-resistance without requiring complex switch designs.
Solution Approach 2:
The invention creates an equipotential condition between the source and substrate of each MOS switch during the sampling phase. By setting the substrate potential equal to the source potential, the body-bias voltage becomes zero, which eliminates threshold voltage modulation and ensures constant on-resistance regardless of input voltage variations.
3Use of energy by moving object
If standard CMOS switches are used, then power consumption is moderate, but harmonic distortion increases and linearity deteriorates at low power supply voltages
Solution Approach 1:
The invention changes the operating parameter of the MOS switches from fixed substrate potential to dynamically adjusted substrate potential that matches the source potential. This parameter change eliminates body-bias effects and threshold voltage variations, thereby improving linearity and reducing harmonic distortion even at low power supply voltages where conventional CMOS switches would fail to maintain adequate performance.
Data Source
AI summary
A channel selection circuit, an analog-to-digital converter (ADC) and a system-on-chip (SoC) are provided. During a sampling phase of an ADC circuit, a substrate voltage of a MOS switch in a selected channel is set to a voltage of analog input signal to which the channel is coupled. This eliminates the body-bias effect of the MOS switch, greatly improving linearity of the channel selection circuit and enabling the ADC to provide the required high speed and high-accuracy and exhibit excellent performance even at a low power supply voltage. Moreover, harmonic distortion around the dominant frequency can be suppressed, enabling the ADC to provide the required high accuracy (e.g., a 16-bit or even higher resolution).