Track-and-Hold Circuit RC Zero Shift for Low Distortion ADCs
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Solution Overview
Problem
High-speed analog-to-digital converters (ADCs) face performance limitations due to harmonic distortion in their track-and-hold circuits, which affect the accuracy of signal conversion.
Innovation Solution
A track-and-hold circuit design incorporating a bi-directional current source, an RC network, and a unity gain amplifier, with additional capacitors and resistors to reduce signal-dependence of the sampling instant and minimize harmonic distortion, is introduced.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a conventional track-and-hold circuit is used, then the ADC can operate at high speed, but harmonic distortion occurs that degrades performance
Solution Approach 1:
An RC network is introduced as an intermediary element between the track-and-hold circuit and the quantizer. This RC network acts as a mediator that conditions the sampled signal by adjusting its frequency response characteristics, thereby reducing harmonic distortion while preserving the high-speed sampling capability of the original circuit
Solution Approach 2:
The circuit modifies the electrical parameters of the signal path by introducing resistive and capacitive elements with specific time constants. These parameter changes alter the signal transmission characteristics to minimize distortion effects during the hold phase, enabling accurate signal conversion at high speeds
2Device complexity
If the sampling instant is signal-dependent, then circuit operation is simplified, but distortion increases adversely affecting ADC performance
Solution Approach 1:
The RC network provides a form of frequency-dependent feedback that compensates for the signal-dependence of the sampling instant. By carefully selecting the RC time constant, the network introduces phase and amplitude adjustments that counteract the distortion effects, thereby improving signal conversion accuracy while maintaining the simplicity of the sampling mechanism
Data Source
AI summary
A track-and-hold circuit is provided. This track-and-hold circuit is adapted to track an analog input signal and hold a sampled voltage of the analog input signal at a sampling instant for processing by other circuitry, in response to a track signal that alternates with a hold signal. Preferably, the track-and-hold circuit includes a bi-directional current source that sources and sinks current through a first output node and a second output node, a unity gain amplifier that is coupled to first and second output nodes of the bi-directional current source and that receives the analog input signal, a resistor coupled to an output of the unity gain amplifier, and a capacitor coupled between the resistor and ground. Of interest, however, is the bi-directional current source, which includes a differential input circuit that is adapted to receive the track signal and the hold signal and that is coupled to the first and second output nodes and an RC network that is coupled to the differential input circuit. The RC network receives the analog input signal and is scaled to change the location of a zero to reduce the signal-dependence of the sampling instant.