Signal-to-Noise Ratio (SNR)


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Signal-to-Noise Ratio (SNR)

2015-07-24 Technology

Signal-to-Noise Ratio (SNR) SNR is a industry-standard performance metric for p-cap touchscreen systems. However, no standard methodologies exist for measuring, calculating, and reporting SNR. In capacitive touchscreens, the signal in SNR is the measured amount of change in mutual capacitance as a direct result of finger capacitance. Finger capacitance depends on the sensor cover thickness, finger size, DUT stray capacitance to ground, finger size, and sensor pattern (see Figure 1 for an example touchscreen stack-up). The noise component is dependent on internal controller noise and external noise sources.

The two components (signal & noise) depend heavily on the device under test. Noise from displays (LCDs & OLEDs) and from USB chargers is spiky – it doesn’t have a normal (Gaussian) distribution – and spikes create jitter. Yet marketers typically specify SNR in the absence of noise, using the RMS noise (standard deviation) of analog-to-digital convertors (ADCs). With Gaussian noise, you can multiply the RMS noise by 6 to calculate the peak-to-peak noise with 99.7% confidence.

Calculation of SNR: 

· CFinger = Mean (Finger) - Mean (NoFinger)

· CFinger = 1850 - 813 = 1037

· CNS (Standard Deviation) = 20.6 counts

· CNS (Peak-to-Peak) = Max (NoFinger) - Min (NoFinger) +1

· CNS = 900 - 746 +1 = 155 counts

· SNR (Peak-to-Peak) = 1037/155 = 6.7

· SNR (Standard Deviation) = 1037/20.6 = 49.9

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