How to measure the actual signal-to-noise ratio of a capacitive touch screen

Touch screen controller manufacturers often take a variety of specifications and standards to differentiate their products. The most frequently mentioned is the signal-to-noise ratio (SNR). However, when noise is present, even if the numbers look good, it does not mean that SNR is a good system performance indicator. This article will discuss what is the signal-to-noise ratio, how it is calculated, what it means for system performance, and whether it is a good measure of touch performance.

What is the signal to noise ratio?

The signal-to-noise ratio is a performance indicator for touch-screen controllers and is now accepted as an industry standard. The problem with signal-to-noise ratio is that there are no industry-standard methods of measurement, calculation, and reporting, especially in some typical systems where noise is highly variable, such as mobile phones. The measurement and calculation of these two parts (signal and noise) is largely dependent on the device under test (DUT), typically a mobile phone. It is worth noting that although signal-to-noise ratio has been widely accepted as a performance measure, industry experts understand that the high signal-to-noise ratio that most markets promote is not guaranteed in practical applications. In addition, in a noisy environment, providing a high signal-to-noise ratio does not fully comply with its functional specifications.

In a capacitive touch screen, the signal in the signal-to-noise ratio is the amount of change in the actual capacitance after the measured finger capacitance is added. Finger capacitance depends on sensor cover thickness, finger size, DUT to ground parasitic capacitance, and sensor mode. The noise component is dependent on internal controller noise and external noise sources, which are discussed in this article.

Projected capacitive touch screen touch technology has been used in many new smartphones, and touch sensors are exposed to noise. Noise is coupled from the display (possibly LCD or AMOLED) to the touch sensor, the closer the distance the noise is. Unlike analog displays, this type of LCD noise is usually spiked. USB charger noise is also usually spiked. It is also the easiest to change because the structure and components of the AC/DC transformer are different in each device.

Third-party low-cost chargers are particularly prone to such noise spikes. Therefore, when the touch controller does not have noise suppression technology like cypressChargerArmor, the USB charger is the most troublesome thing for OEMs. When all of this external noise is present, we expect the touch controller not to report a finger touch or finger position by mistake. They are not classified as normal, or Gaussian, or distributed noise. This poses a problem for engineers and marketers to distinguish the signal-to-noise ratio of the ADC when there is no noise.

Under a wide range of measurement conditions, the signal-to-noise ratio has always been a metric and cannot be said to be a miracle. In addition, the signal-to-noise ratio cannot predict the most important and quantified touch-screen noise-related parameters: jitter (also known as noise-free resolution) and false touch reporting. Fortunately, there is a SNR measurement technique that predicts jitter in the presence of non-Gaussian noise.

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