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Question: What is the measurement accuracy of CapSense® controllers?
CapSense controllers convert Capacitance into a digital value. Let us consider an example where we are representing a capacitance value between 0-45 pF with a 14-bit digital value. Now with 1LSB we can represent 45 pF/16384 = 2.7fF approximately 3fF. Thus, we could say that the accuracy is 3fF. But because of the inherent noise in the system we will always observe around 5-15 counts of noise (this noise is system dependent). So, in reality we can measure a minimum of only 5*3fF = 15pF.
Note that absolute capacitance measurement (raw count) is not very important in realizing a capacitive touch interface. Instead, measuring change in capacitance (signal) is critical for detecting when a finger touches a sensor. So, for the above system, it is not possible to successfully differentiate finger touch from noise if the finger produces only 15 fF signal (which means finger signal is equal to noise). Hence, it is recommended to have at-least 5:1 SNR which means finger signal should be a minimum of 75fF to enable a safe detection.