This application note describes the use of special algorithms and techniques with Microchip’s Charge Time Measurement Unit (CTMU) for capacitive touch applications in noisy environments. The CTMU is an excellent peripheral for use in touch sensing applications with many benefits such as high scanning speed, charge current trimming and low component count. Before planning to use capacitive touch interfaces in industrial environments (or any other kind of environment with noisy power supply lines), the user must understand the hazards of conducted noise.

Even if regulators keep a constant voltage drop at the circuit input, human interaction couples the noise into the touch pads. By using a combination of processing techniques (signal envelope and jittered sampling) and good PCB layout, it is possible to detect the capacitive touch keys reliably in the presence of conducted noise. An envelope function (basically a peak detector) of the button readings is calculated for each channel and all decisions are made based on the difference between the envelope and the average derived from the raw data. Under normal conditions, the envelope functions follow each button closely, allowing the same functionality. Under noise conditions, raw button readings look like white noise and the envelope function transforms that into a much steadier signal.

Modulation in the noise amplitude can be sometimes seen as a low-frequency component in the readings because of under sampling. Since buttons are read in sequence the low frequency component is phase-shifted between them (different deviation from average), leading to problems like false triggering or no button detection. Jittering the sampling rate of the buttons solves this problem by distributing the modulation frequency component energy. The resulting signal looks like white noise and is very suitable for use with the envelope function.

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