General Guidelines for Calculating SNR for Touch Applications

In touch sensing technologies (such as capacitive touchscreens or touch sensors), Signal-to-Noise Ratio (SNR) is a critical parameter that determines the sensor’s ability to accurately detect and distinguish a valid touch from background noise or interference.

What is Touch Delta?

Touch Delta or simply Delta refers to the change in measured capacitance (or signal value) when a finger or conductive object touches or approaches the sensor electrode, compared to its baseline (untouched) state. In capacitive touch sensing, each sensor channel continuously monitors its capacitance. When a touch occurs, the capacitance increases due to the human body’s conductive properties.

The delta is calculated as:
Touch Delta = Measured Value (Touched) - Baseline Value (Untouched)

What Does SNR Mean in Touch Sensing?

• Signal: The change in the sensor’s output when a finger touches the surface (e.g., a change in capacitance)
• Noise: Unwanted variations in the sensor’s output caused by electrical interference, environmental factors, or inherent electronic noise

SNR in touch sensing is the ratio of the touch signal amplitude to the noise amplitude. A higher SNR means the touch event is much more distinguishable from the noise, leading to more reliable and accurate touch detection.

Why Is SNR Important in Touch Sensing?

• Sensitivity: Higher SNR allows the sensor to detect lighter or smaller touches, such as a gloved finger or a fine stylus
• Accuracy: Reduces false touches and missed touches, improving user experience
• Robustness: Helps the sensor perform well in noisy environments (e.g., near power supplies, displays, or in the presence of electromagnetic interference)

SNR Measurement

The following graph shows the touch delta for a set of 'N' samples:

• Delta Max: Highest value of delta recorded in the set of 'N' number of samples
• Delta Minimum: Lowest value of delta recorded in the set of 'N' number of samples
• Average Touch Delta: Average delta value of all 'N' number of samples
• Peak-Peak Touched Noise: Difference between Delta Max and Delta Minimum

Procedure to measure and calculate SNR:

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Example SNR Calculation

Assume the following data is captured from a set of 200 samples.

• Average Touch delta (200 samples) = 30
• Delta Max= 34
• Delta Minimum= 29
• Peak to Peak Touched Noise = (34-29) = 5
• SNR= (30/5) = 6

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Typical SNR Values

• Good SNR: For reliable touch detection, an SNR of at least 10:1 is often recommended.
• Higher SNR: Values above 20:1 are preferred for high-performance or multi-touch applications.

 The table below shows the criteria for SNR.

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