Continuous-Time Anti-Aliasing Filter for Capacitive Touch Sensing
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Solution Overview
Problem
Capacitive touch sensing devices face interference from external and internal sources, leading to signal degradation due to high-frequency noise above the Nyquist limit, which cannot be effectively removed by traditional discrete-time signal processing.
Innovation Solution
A continuous-time analog anti-aliasing filter, specifically an infinite impulse response (IIR) filter, is integrated into the analog front end (AFE) of capacitive touch sensing devices to filter out high-frequency interference before sampling, improving the signal-to-interference ratio (SIR) and reducing aliasing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional discrete-time signal processing is used, then the device complexity is reduced, but high-frequency interference above the Nyquist limit cannot be effectively removed, causing aliasing and signal degradation
Solution Approach 1:
The patent applies preliminary anti-aliasing filtering in the continuous-time domain before the signal is sampled by the ADC. This continuous-time IIR filter removes high-frequency interference above the Nyquist limit beforehand, preventing aliasing from occurring in the first place. The filter is implemented using analog components (resistors, capacitors, operational amplifiers) in the signal path before the sampling stage.
2Measurement precision
If continuous-time analog anti-aliasing filter is applied, then high-frequency interference is removed and signal quality is improved, but the device complexity increases due to additional analog components
Solution Approach 1:
The patent combines the anti-aliasing filter with the existing analog front-end circuitry of the capacitive touch sensor. The continuous-time IIR filter is integrated into the signal path using analog components that work together with the touch sensor's existing amplification and conditioning circuits. This merging approach implements effective anti-aliasing filtering while minimizing the addition of separate discrete components.
Data Source
AI summary
Embodiments described herein include an input device with a plurality of capacitive sensor electrodes configured to receive a signal. The input device also includes a processing system coupled to the plurality of capacitive sensor electrodes. The processing system includes an analog front end (AFE). The AFE includes an anti-aliasing filter comprising a continuous time analog infinite impulse response (IIR) filter configured to filter out interference from the received signal at frequencies higher than a signal frequency of the processing system to produce an anti-aliased signal. The AFE also includes a charge integrator configured to integrate the anti-aliased signal.