Differential Circuit for Capacitance Detection Noise Reduction
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Solution Overview
Problem
Existing self-capacitance detection methods have low sensitivity due to large self-capacitance values, requiring large and costly cancel capacitors, and are affected by noise signals like 1/f noise and low-frequency noise, which degrade the signal-to-noise ratio.
Innovation Solution
A differential circuit with a front-end circuit and processing circuit that converts capacitance signals into voltage signals, performs a difference operation, and uses a correlated double sampling method to reduce noise, improving the signal-to-noise ratio and accuracy of capacitance detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If self-capacitance detection method is used, then capacitance measurement capability is provided, but sensitivity is low due to large self-capacitance values
Solution Approach 1:
The detection process is segmented into multiple stages: a first detection stage where the first capacitor is connected to the detection circuit to obtain a first detection signal, and a second detection stage where the second capacitor is connected to obtain a second detection signal. This segmentation allows differential processing to eliminate noise while maintaining simple circuit structure.
Solution Approach 2:
A differential circuit is introduced as an intermediary component to process the detection signals from both capacitors. The differential circuit subtracts the second detection signal from the first detection signal, effectively canceling out common-mode noise and low-frequency interference while preserving the capacitance difference information.
2Measurement precision
If cancel capacitor is used to improve sensitivity, then detection sensitivity increases, but the cancel capacitor becomes large and cannot be integrated with high cost
Solution Approach 1:
Instead of using a large physical cancel capacitor, the patent creates an electrical copy of the capacitance effect through software-controlled switching. The control circuit selectively connects the first and second capacitors to the detection circuit at different times, replicating the noise-cancellation function of a physical cancel capacitor without requiring large component values.
Solution Approach 2:
The patent changes the operational parameters by using dual-capacitor alternating detection instead of a single capacitor with cancel capacitor. The detection method transitions from continuous monitoring with analog cancellation to time-multiplexed digital sampling, enabling noise rejection through differential processing rather than physical component matching.
3Measurement precision
If differential circuit is used to enhance sensitivity, then capacitance detection sensitivity improves, but noise signals such as 1/f noise and low frequency noise still affect signal-to-noise ratio
Solution Approach 1:
The patent implements periodic switching between the first capacitor and second capacitor connections to the detection circuit. This periodic action, controlled by the control circuit, creates alternating detection cycles that enable correlated double sampling. The periodic nature allows synchronization of sampling with the noise characteristics, facilitating noise cancellation through differential processing.
Solution Approach 2:
The patent performs preliminary noise characterization by detecting signals from both capacitors under identical noise conditions before the actual capacitance measurement. The first and second detection stages capture the noise profile, which is then subtracted in the differential circuit to eliminate noise before the final capacitance calculation, achieving noise pre-rejection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The proposed solution enhances the sensitivity of capacitance detection by reducing noise and improving the signal-to-noise ratio, allowing for more accurate detection of capacitance values and touch positions in terminal devices.
Implementation Method 1
control, at a first stage of a detection period, the power supply to charge the two capacitors connected to the front-end circuit and control the two capacitors connected to the front-end circuit to discharge to the PGA circuit
Implementation Method 2
perform a difference operation on the voltage signals of the two capacitors connected to the front-end circuit, to obtain a differential signal of voltages corresponding to the two capacitors connected to the front-end circuit
Implementation Method 3
the processing circuit is configured to determine, according to the differential signal output by the front-end circuit at the first stage and the differential signal output by the front-end circuit at the second stage, a target differential signal
Data Source
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AI summary
The present disclosure provides a differential circuit, a capacitance detection circuit and a touch detection apparatus, which can reduce a noise of the differential circuit and improve a signal-to-noise ratio of an output signal. When the differential circuit is applied in a capacitance detection circuit, accuracy of capacitance detection can be improved. The differential circuit includes a front-end circuit and a processing circuit, where the front-end circuit includes a control circuit and a (PGA) circuit. The control circuit is connected to a power supply, two capacitors and a (PGA) circuit and configured to: control, at a first stage, the power supply to charge the two capacitors and control the two capacitors to discharge to the (PGA) circuit; and control, at a second stage, the (PGA) circuit to charge the two capacitors. The (PGA) circuit is configured to convert capacitance signals of the two capacitors into voltage signals at the first stage and the second stage respectively, and perform a difference operation to obtain differential signals of voltages corresponding to the two capacitors at the first stage and at the second stage. The processing circuit is connected to the front-end circuit and is configured to determine, according to the differential signals output by the front-end circuit at the first stage and the second stage, a target differential signal of voltages corresponding to the two capacitors.