Sensor Trace Length and Width of PSoC 4 CapSense CSD – KBA227570

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The recommended maximum trace length is 12 inches (300 mm) for a standard PCB and 2 inches (50 mm) for flex circuits. The maximum recommended trace width is 7 mil (0.18 mm). You should surround the CapSense® traces with a hatched ground or hatched shield with trace tohatch clearance of 10 mil to 20 mil (0.25 mm to 0.51 mm).

Some practical applications require using longer sensor traces than the recommended maximum value. In such cases, you should pay attention to the equivalent resistance of the CapSense sensor trace, especially the sensor trace on flex circuits. The longer the sensor trace, the larger the equivalent resistance is. For a certain sense clock frequency, too large an equivalent resistance will cause insufficient charge and discharge on sensors, which will lead to low sensitivity and high power consumption.

If longer sensor trace length is used in your design, double-check the sense clock frequency settings. To calculate the exact maximum sense clock frequency, see the “Sense Clock Related Parameters” section in AN85951. In practice, you can change the sense clock, watch if sensor signal would be larger with using lower sense clock frequency, and choose the best sensor clock frequency that can give the best SNR.

Reference Document:

AN85951 - PSoC® 4 and PSoC® 6 MCU CapSense® Design Guide