Buffered SAR ADC Charging Path for Low Input Current Non-Linearity
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Solution Overview
Problem
Capacitive SAR ADCs are susceptible to non-linearity due to switching activities of the switches, leading to signal-dependent current variations that affect performance and cost-effectiveness.
Innovation Solution
Incorporating an amplifier as a buffer between the first switch and the input node, and controlling switches with a predetermined timing to isolate the input node from switching operations, using larger impedance switches to reduce non-linear components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If switches are operated to perform analog-to-digital conversion, then conversion function is achieved, but non-linearity and signal-dependent current variations occur
Solution Approach 1:
An amplifier is introduced as an intermediary component between the switch and the input node. The amplifier buffers the switching operations, isolating the input node from direct switch operations. This mediator transfers the conversion function while preventing the harmful switching transients and non-linearities from reaching the input signal path.
Solution Approach 2:
The circuit is segmented into distinct functional blocks: the switching network, the amplifier buffer stage, and the input node. By separating the switching operations from the input signal path through the amplifier, each segment can be optimized independently - switches for conversion function and amplifier for linearity preservation.
2Manufacturing precision
If amplifier is added as buffer, then input current non-linearity is reduced, but device complexity increases
Solution Approach 1:
A single amplifier is strategically placed as the minimum necessary intermediary to achieve linearity improvement. This focused use of one active component provides the needed buffering function without unnecessarily complicating the overall circuit architecture.
Data Source
Figure 1
Figure 2A~2B
Figure 3
AI summary
A system includes an amplifier to receive a signal, an analog-to-digital converter (ADC), a first switch coupled to a capacitor to receive an output from the amplifier, the capacitor to provide the output to the ADC, a second switch coupled between the capacitor and the ADC to turn on/off the ADC, a third switch coupled between the amplifier and the first switch to connect/disconnect the output to/from the first switch, a fourth switch coupled between the amplifier and the first switch to bypass the amplifier, and circuitry. The circuitry turns on the first switch and the second switch to initiate charging the capacitor, turns on the fourth switch and turns off the third switch to complete the charging, and turns off the second switch and the first switch to control the ADC to convert the output to a digital signal.