In capacitive sensing systems, the design of the sensor plays an important role in determining system performance and capabilities. In most cases the sensor is simply a metal plate that can be designed on the PCB.

The sensor used in this example is implemented with a two-layer PCB. On the top layer, which faces the tank, there are the 3 electrodes (Reference Environmental, Reference Liquid, and Level) with a ground plane surrounding the electrodes. The bottom layer is covered with a shield plane to isolate the electrodes from any external interference sources.

Depending on the shape of the container, the FDC1004-Q1 can be located on the sensor PCB to minimize the length of the traces between the input channels and the sensors and increase the immunity from EMI sources. In case the shape of the container or other mechanical constraints do not allow having the sensors and the FDC1004-Q1 on the same PCB, the traces which connect the channels to the sensor need to be shielded with the appropriate shield. In this design example all of the channels are shielded with SHLD1. For this configuration, the FDC1004-Q1 measures the capacitance of the 3 channels versus ground; and so the SHLD1 and SHLD2 pins are internally shorted in the FDC1004-Q1 (see The Shield).