Differential Sampling Circuit Harmonic Cancellation
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Solution Overview
Problem
Differential sampling circuits in analog-to-digital converters suffer from phase imbalance-induced second harmonic generation due to non-linearity in MOS sampling switches, which degrades the Spurious-Free Dynamic Range (SFDR) by causing harmonic content to be present in the output.
Innovation Solution
A compensation circuit using field effect transistors operating in saturation mode is added in parallel with the differential switch, redirecting harmonic content away from sampling capacitors and through the compensation circuit, thereby canceling out the second harmonic current and reducing parasitic capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If MOS sampling switches are used in differential sampling circuits, then the circuit can perform sampling function, but phase imbalance-induced second harmonic generation occurs which degrades SFDR
Solution Approach 1:
The patent converts the harmful second harmonic generation from MOS sampling switches into a beneficial effect by introducing compensation transistors that generate equal and opposite second harmonic currents. These compensation transistors use the same non-linear characteristics but in reverse to cancel the unwanted harmonics, thereby improving SFDR while utilizing the inherent device properties.
Solution Approach 2:
The patent introduces compensation transistors as intermediary elements between the differential sampling switches and the output. These compensation transistors act as mediators that generate counteracting second harmonic currents to cancel the harmful harmonics produced by the sampling switches, without interfering with the normal sampling function.
2Reliability
If compensation circuit is added to cancel second harmonic, then SFDR is improved, but device complexity increases
Solution Approach 1:
The patent merges the compensation transistors with the existing differential sampling circuit structure. The compensation transistors are integrated into the same circuit topology and share common nodes and signal paths with the sampling switches, thereby reducing the overall device complexity compared to adding separate compensation circuits.
Solution Approach 2:
The compensation transistors serve multiple functions: they generate second harmonic cancellation currents while also maintaining the differential signal path and operating within the existing circuit voltage and current ranges. This multi-functionality reduces the need for additional dedicated compensation circuits and simplifies the overall design.
Data Source
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AI summary
A differential sampling circuit (100) includes a compensation circuit (140) for cancelling harmonic content resulting from a phase imbalance. The compensation circuit (140) includes a pair of field effect transistors (142, 143) operating in saturation mode, each field effect transistor (142, 143) coupled in parallel with the differential switch (122) of the sampling circuit (100), which operates in linear mode. The saturation region transistors (142, 143) across the differential switch (122) allow the harmonic content to flow through the compensation circuit (140) instead of the sampling capacitors (12, 13) of the sampling circuit (100).