Interface Circuit for Fully-Differential ADCs
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Solution Overview
Problem
Existing interface circuits struggle to accurately transform single-ended input signals into fully-differential signals for fully-differential ADCs, facing challenges with common-mode rejection and noise levels that are not commensurate with the performance of state-of-the-art ADCs, especially when the common-mode voltage is not tightly specified or controlled.
Innovation Solution
An interface circuit comprising a regulator circuit and two impedance-matched networks that receive input voltages and produce output voltages, with the regulator circuit providing a voltage that decreases with increasing average input voltage, ensuring the output voltage difference is independent of the input voltage average and common-mode voltage, and utilizing matched impedance ratios to achieve high accuracy and low noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a differential amplifier is used to transform single-ended input signals to fully-differential output signals, then the interface circuit can provide voltage transformation, but the common-mode rejection ratio deteriorates when common-mode voltage is not tightly controlled
Solution Approach 1:
The interface circuit is segmented into three functional blocks: a regulator circuit that generates a regulator voltage based on input voltages, and two impedance-matched networks that independently process each input signal. This segmentation allows the common-mode voltage control to be decoupled from the differential signal processing, enabling high common-mode rejection ratio without requiring tight common-mode voltage control.
Solution Approach 2:
A regulator voltage serves as an intermediary element that mediates between the input voltages and the output voltages. The regulator voltage is generated to have a negative correlation with the average input voltage, and both impedance-matched networks use this regulator voltage in their processing. This intermediary mechanism enables the circuit to achieve high common-mode rejection ratio while accepting a wide range of common-mode voltages.
2Measurement precision
If traditional interface circuits are used to drive fully-differential ADCs, then basic signal transformation is achieved, but noise levels exceed the performance requirements of state-of-the-art ADCs
Solution Approach 1:
The circuit changes the parameter of impedance matching between the two networks. By ensuring that the impedance ratios of the two impedance-matched networks are substantially equal, the circuit achieves balanced differential signal generation with minimized noise. This parameter change enables the interface circuit to meet the stringent noise requirements of state-of-the-art ADCs while maintaining reliable operation.
3Measurement precision
If a differential amplifier with high differential gain is used, then accurate voltage transformation is achieved, but the circuit complexity and operational amplifier requirements increase
Solution Approach 1:
The high-gain differential amplifier is segmented into a regulator circuit and two separate impedance-matched networks. Each network independently processes one input signal using the regulator voltage, eliminating the need for a single high-gain differential amplifier. This segmentation reduces operational amplifier requirements and simplifies the overall circuit while maintaining voltage transformation accuracy.
Data Source
AI summary
An interface circuit for driving a fully-differential circuit has a first circuit configured to decrease the voltage at its output in response to an increase in an average value of first and second input voltages. A first network receives the first input voltage and the output voltage of the first circuit to provide a first output voltage for driving the fully-differential circuit. A second network receives the second input voltage and the output voltage of the first circuit to provide a second output voltage for driving the fully-differential circuit. An impedance ratio of the first network is substantially matched to an impedance ratio of the second network.