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

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

What is signal-to-noise ratio?

Signal-to-noise ratio is a performance metric for touchscreen controllers that is now accepted as an industry standard. The problem with signal-to-noise ratio is that there is no industry standard way to measure, calculate, or report it, especially when the noise is highly variable in typical systems, such as mobile phones. The measurement and calculation of these two components (signal and noise) depends largely on the device under test (DUT), which is typically a mobile phone. It is important to note that while signal-to-noise ratio is widely accepted as a performance metric, industry experts understand that most marketing claims of high signal-to-noise ratios are not guaranteed in real-world applications. In addition, providing a high signal-to-noise ratio may not fully meet functional specifications in a noisy environment.

In capacitive touch screens, the signal to noise ratio is the change in actual capacitance after adding the measured finger capacitance. Finger capacitance depends on the thickness of the sensor overlay, finger size, parasitic capacitance from the DUT to ground, and the sensor pattern. The noise component depends on internal controller noise and external noise sources, which will be discussed in this article.

Projected capacitive touch screen touch technology has been used in many new smartphones, and touch sensors will encounter noise when used. Noise is coupled from the display (which may be LCD or AMOLED) to the touch sensor, and the closer the distance, the greater the noise. Unlike analog displays, this type of LCD noise is usually spike noise. USB charger noise is also usually spike noise. It is also the most variable because the structure and components of the AC/DC transformer are different in each device.

Third-party low-cost chargers are particularly prone to this noise spike. Therefore, USB chargers are the biggest headache for OEMs when the touch controller does not have noise suppression technology like cypressChargerArmor. When all these external noises are present, we expect the touch controller to not falsely report finger touches or finger positions. They cannot be classified as normal, or Gaussian, or distributed noise. This creates a problem for engineers and marketers to distinguish the signal-to-noise ratio of the ADC when there is no noise.

It is a wonder that SNR is a consistent metric in so many measurement conditions. Furthermore, SNR cannot predict the most important and quantifiable touchscreen noise-related parameters: jitter (also known as noise-free resolution) and false touch reports. Fortunately, there is an SNR measurement technique that can predict jitter in the presence of non-Gaussian noise.

[page] How noise affects touch screen systems

A poor signal-to-noise ratio can affect system robustness, causing false touches and position jumps. When a finger approaches the touch screen, it interferes with the fringe electric fields of two intersecting transparent electrodes. This capacitance is called mutual capacitance. This changes the capacitance of the sensor. The intersection occurs where the transmit and receive electrodes cross at right angles. There are hundreds of these intersections on a mobile phone touch screen. The touch screen controller measures the change in capacitance at all intersections and converts the measurement data into quantized raw data. By measuring each intersection, rather than the entire electrode, the controller is able to create a two-dimensional graph of the touch screen sensor capacitance.

If a large noise spike occurs near the intersection of the finger, an error flag is added to the position calculation algorithm. The algorithm then converts the raw data into coordinates; depending on the size of the noise spike, the coordinates reported for the finger position may be jittery, alternating between two coordinates when the finger is still. This can happen with some unintentional inputs or selections when the smartphone uses a touchscreen interface and is plugged into a USB charger.

We can conclude that in the absence of standardized measurement methods, signal-to-noise ratio can be used as a performance metric, but it is not perfect. There are defined performance indicators, measurement procedures, and calculation methods that touchscreen controller vendors (see Cypress specification 001-49389) and mobile device OEMs can use to quantify touch performance. These specifications are necessary to ensure repeatable test results, verify touchscreen performance, and reduce touchscreen test hardware and firmware changes.

Typical performance testing requires metal finger simulators, fixtures, oscilloscopes, function generators, and robotics in addition to the touchscreen hardware and controller interface. For example, the standard jitter measurement process is a seven-step process that records the temporal noise on the finger position coordinates. The measurement here indicates how much movement, and how much distance, we would expect for a stationary finger. This is a relatively simple parameter to measure, and it has a direct and immediate impact on the user interface. In contrast, the impact of signal-to-noise ratio on touchscreen performance is not as direct. Even in a noisy environment, digital filters and position calculation algorithms are able to remove jitter, but it reduces the signal-to-noise ratio value (as a performance measure). It is not advisable to use signal-to-noise ratio as a performance indicator because it does not ultimately give you a true sense of system performance.

This article is to tell you that signal-to-noise ratio does not tell us whether the system responds well to touch. This is why leading manufacturers of touch controllers, such as Cypress TrueTouch, have a set of tests and measurements to evaluate the performance of new touch screen designs.