Error and accuracy of CapSense

Question: What is the accuracy with which capacitance can be measured using CapSense controllers?



In the CSD algorithm, the absolute measurement of capacitance is not very important. The change in capacitance when there is a finger touch is the important thing to be detected. There can be drifts in raw counts (measured absolute capacitance) due to temperature, voltage, board-to-board, and device-to-device variations. This gradual drift in raw counts is tracked by the baseline algorithm (built in to the user module), effectively nullifying the effect of low frequency or DC noise and errors in measured absolute capacitance.

What is important is making sure the nominal raw counts value of a sensor has enough margin to account for the total possible error so that the raw counts do not saturate at the maximum raw counts value.

The total possible error comes from a number of factors. See below:


       Rb resistance tolerance


       Reference voltage tolerance


       The sensor capacitance will always have some error. This is a property of the physical electrode and does not have anything to do with the PSoC device.


       The series resistor errors.


       An overall drift error due to temperature changes.

In the user module datasheet for the CSD User Module, you can see that there is about a ±8% drift in raw counts as temperature varies. You can also factor in the other errors in the system, such as the Rb resistance error. It is recommended to use a wide error (noise) margin in the measurements. Using ±25% error margin is good. This means that it would be best to set the average raw count values (when sensor not active) at 70% of the full scale. This allows the counts to go all the way up to 95% of the full scale due to error. This still leaves a 5% head room of the difference counts.