Considerations for Capacitive Sensing Technology in Mobile Phone Touchscreens 2

How to measure SNR

The SNR in touch sensor applications is determined based on the count value at the sensor output. For example, when there are no fingers on the sensor, the peak to peak noise is 8. When the finger is placed on the sensor, the obtained signal is 118, so the SNR is 118:8, which is about 15:1.  diode

The measurement of SNR should consider both the best and worst case scenarios. The best scenario is to place the larger finger in the center of the sensor pad. The worst-case scenario is that the smaller fingers are placed off center. In the early development of mobile phone systems, using real fingers for testing was an acceptable method. If developers want testing to be more operator independent and reproducible, they can use metal discs or sticks instead of real fingers.  diode

Due to the weakening of signal strength caused by the thickness of the overlay, a more reliable approach is to use a slightly thicker overlay during system development than expected. To avoid the masking effect of higher-level firmware, the original uncompressed sensor count values are used to measure SNR. When there is no finger touch, turn off any self compensation or automatic calibration functions that reset the sensor output to zero.

Figure 1: Finger pressure determines the composition of signal and noise.

Establish noise budget

One of the methods to manage the performance of capacitive sensors is to establish a noise budget, which includes listing noise sources that may reduce the system SNR. For mobile phones, these noise sources include internal IC noise, RF noise, and AC line noise. Estimate the impact of each noise source on the sensor count value. Adding the sum of these calculated values and the additional applied count values as the design margin will result in an SNR greater than 5:1.  diode

The mobile phone itself generates an environment with high RF energy, which may have a greater impact on the system than adding a small amount of noise count to the system. The problem with capacitive sensors working close to RF transmitters is that the sensor's trace acts as an effective antenna. The coupling of a large amount of RF energy with the controller IC may lead to adverse consequences in the sensor system, resulting in the failure of touch sensing. A simple solution to this potential problem is to use series resistors to suppress resonance. A resistor of several hundred ohms is connected in series with the input of the sensor and placed as close as possible to the pins of the control IC to prevent such problems from occurring.  diode