C-2C Capacitor Array for Low THD in Switched-Capacitor PGA
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Solution Overview
Problem
Switched capacitor programmable gain amplifiers (PGAs) face challenges with high harmonic distortion and gain non-linearity due to non-ideal effects like charge injection and clock feed-through, especially as target resolutions and operating speeds increase, making it difficult to achieve practical capacitor sizes and efficient signal processing.
Innovation Solution
The implementation of a C-2C capacitor array with constant VGS switches in the PGA circuit reduces harmonic distortion by optimizing the sampling and feedback capacitance ratio, minimizing charge injection and clock feed-through errors, and allowing for digital programmability of gain settings without requiring impractically small capacitors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If switched capacitor PGA circuits are used to achieve adjustable gain and wide signal range amplification, then gain programmability and signal range are improved, but harmonic distortion increases and gain linearity deteriorates due to charge injection and clock feed-through effects
Solution Approach 1:
The capacitor array is divided into multiple segmented capacitors (C1, C2, C3, C4) that can be independently switched. This segmentation allows selective connection of capacitor pairs to achieve different gain settings while maintaining symmetry, thereby reducing harmonic distortion caused by charge injection and clock feed-through effects.
Solution Approach 2:
The patent employs symmetric capacitor pairing (C1-C2, C3-C4) with equal capacitance values within each pair. This symmetric arrangement compensates for charge injection and clock feed-through errors by ensuring that distortion-generating effects are equal and opposite, thereby canceling each other out and improving gain linearity.
2Speed
If capacitor sizes are reduced to achieve higher operating speeds and resolutions, then operating speed and resolution are improved, but manufacturing precision becomes difficult to achieve and circuit performance deteriorates
Solution Approach 1:
The patent combines multiple capacitor pairs in parallel to achieve the required total capacitance values. By merging capacitors C1 and C2, as well as C3 and C4, the circuit achieves sufficient capacitance for stable operation at high speeds without requiring impractically small individual capacitor sizes, thereby maintaining manufacturing precision.
3Measurement precision
If fine control over gain is implemented to achieve high resolution, then gain control precision is improved, but circuit complexity increases and non-ideal effects become more pronounced
Solution Approach 1:
The switched capacitor network serves multiple functions simultaneously: it provides gain programmability through selective capacitor connection, maintains gain linearity through symmetric pairing, and reduces harmonic distortion through balanced charge injection cancellation. This multi-functionality achieves fine gain control without proportionally increasing circuit complexity.
Data Source
AI summary
A switched-capacitor programmable gain amplifier (PGA) has improved circuit performance that avoids impracticably small capacitors, while providing low total harmonic distortion (THD) and reasonable gain linearity. The sampling capacitor (CS) in the PGA is designed with a C-2C capacitor array. Gain settings from the C-2C capacitor array arranged for selection with constant VGS switches. The gain of the resulting PGA corresponds to the ratio of the effective capacitance of the C-2C array and the effective capacitance value of the feedback capacitor (CF). The resulting performance of the PGA has reduced charge injection and clock feed-through errors, and thus reduced THD.