Capacitance Sensing Circuit With Dual Edge Coupling For Noise Cancellation
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Solution Overview
Problem
Conventional capacitance sensing circuits face challenges in enhancing signal-to-noise ratio (SNR) and power efficiency due to fixed reference capacitance, which limits their effectiveness in touch sensing applications.
Innovation Solution
A capacitance sensing circuit employing a difference comparing unit that alternates between sensing and referencing waveforms to calculate positive and negative edge differences, thereby canceling external noise and reducing driving pulses, enhancing SNR and achieving power-saving through dual edge coupling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a one-end sensing circuit with fixed reference capacitance is used, then the circuit structure is simple, but the signal-to-noise ratio cannot be enhanced effectively because external noise cannot be canceled
Solution Approach 1:
The sensing circuit is divided into two separate sensing channels: a first sensing channel for sensing the test waveform and a second sensing channel for sensing the reference waveform. This segmentation allows independent optimization of each channel and enables differential processing to cancel external noise, thereby improving signal-to-noise ratio while maintaining manageable circuit complexity through modular design.
Solution Approach 2:
The reference capacitance is changed from a fixed value to a variable value that can be dynamically adjusted. The second sensing channel senses a reference waveform with variable reference capacitance, allowing the system to adapt to different noise conditions and optimize the signal-to-noise ratio by adjusting the reference capacitance parameter to match the actual sensing conditions.
2Use of energy by moving object
If conventional capacitance sensing is used, then power consumption is较高, but dual edge couple can reduce driving pulses and achieve power-saving
Solution Approach 1:
The circuit employs dual edge coupling with periodic action by utilizing both the rising edge and falling edge of the clock signal for sensing operations. The first sensing channel senses during the rising edge period while the second sensing channel senses during the falling edge period, allowing complete sensing cycles to be completed in one clock period rather than requiring separate periods, thus reducing overall power consumption while maintaining sensing speed.
3Measurement precision
If single edge couple is used to store voltage variation, then the circuit is simple, but SNR enhancement is limited
Solution Approach 1:
The external noise is extracted and canceled by introducing a second sensing channel that specifically senses the reference waveform. By taking out the reference sensing function as a separate channel, the system can differential process the test and reference waveforms to extract and eliminate common-mode external noise, thereby enhancing SNR while adding only moderate circuit complexity through the additional sensing channel.
Data Source
AI summary
A touch sensing system which includes a touch input interface and a capacitance sensing circuit is provided. The touch input interface includes a plurality of sensing capacitors which output at least one waveform under test and at least one reference waveform. The capacitance sensing circuit includes a difference comparing unit. The difference comparing unit receives the waveform under test and the reference waveform and outputs a differential signal according to at least one positive edge difference and at least one negative edge difference between the waveform under test and the reference waveform. Furthermore, a capacitance sensing method is also provided.