Capacitive Interface Circuit Using CTS for Low-Noise MEMS Sensing
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Solution Overview
Problem
Micromachined systems, such as MEMS sensors, face challenges with low signal accuracy due to high noise levels from kT/C noise, flicker noise, and amplifier offset, which are exacerbated by small capacitance changes and limited power constraints in low-power applications like implantable medical devices.
Innovation Solution
A capacitive interface circuit utilizing a differential amplifier and correlated triple sampling (CTS) to reduce noise components and an output stage with ping-pong demodulator to stabilize signals, allowing for low-power operation with reduced noise and glitching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional switched-capacitor capacitance sensing circuit is used, then sensor signal can be processed, but kT/C noise and amplifier offset undermine sensor accuracy
Solution Approach 1:
The patent implements feedback mechanisms through correlated double sampling where the output of the first sampling capacitor is fed back to subtract from the second sampling capacitor output. This feedback process eliminates kT/C noise and amplifier offset by comparing two samples taken at different times, thereby resolving the technical contradiction between achieving accurate measurements and dealing with noise and offset errors.
Solution Approach 2:
The patent applies preliminary action by performing correlated double sampling where the first sampling capacitor captures the signal plus noise and offset before the second sampling. By taking the first sample in advance and using it to create a feedback correction, the system preliminarily accounts for noise and offset effects, thereby improving measurement precision while managing harmful factors.
2Measurement precision
If feedback capacitance is made approximately the same size as sense capacitance, then sensor signal range is optimized, but output voltage range becomes limited to 10 μV to 1 mV
Solution Approach 1:
The patent uses feedback through correlated double sampling to extend the effective output voltage range beyond what would be achieved by simple voltage amplification alone. By subtracting the feedback signal from the current sample, the system effectively linearizes the transfer function and extends the measurable voltage range while maintaining precision, resolving the contradiction between optimized signal range and limited output range.
3Ease of operation
If reset switch of switched capacitor circuit is used, then circuit operation is enabled, but kT/C noise is sampled onto sensing node
Solution Approach 1:
The patent applies preliminary action by performing correlated double sampling where the first sampling capacitor captures the signal plus noise and offset before the second sampling. By taking the first sample in advance and using it to create a feedback correction, the system preliminarily accounts for noise and offset effects, thereby improving measurement precision while managing harmful factors.
Solution Approach 2:
The patent implements feedback mechanisms through correlated double sampling where the output of the first sampling capacitor is fed back to subtract from the second sampling capacitor output. This feedback process eliminates kT/C noise and amplifier offset by comparing two samples taken at different times, thereby resolving the technical contradiction between achieving accurate measurements and dealing with noise and offset errors.
4Power
If amplifier is used to amplify sensor signal, then signal strength is increased, but amplifier offset and flicker noise are introduced
Solution Approach 1:
The patent implements feedback mechanisms through correlated double sampling where the output of the first sampling capacitor is fed back to subtract from the second sampling capacitor output. This feedback process eliminates kT/C noise and amplifier offset by comparing two samples taken at different times, thereby resolving the technical contradiction between achieving accurate measurements and dealing with noise and offset errors.
Data Source
AI summary
This disclosure describes a capacitive interface circuit for a low power system. The capacitive interface circuit is configured to achieve very low noise sensing of capacitance-based transducers, such as a micro-electro-mechanical system (MEMS)-based sensor, with high resolution and low power. The capacitive interface circuit uses a differential amplifier and correlated triple sampling (CTS) to substantially eliminate, or at least reduce, kT/C noise, as well as amplifier offset and flicker (1/f) noise, from the output of the amplifier. The capacitive interface circuit may further include an output stage that reduces glitching, i.e., clock transients, in the output signal by allowing transients in the amplifier output to settle. In this manner, the circuit can be used in a low power system to produce a stable, low-noise output.