The FDC1004 measures capacitance between CINn and ground. That means any capacitance to ground on signal path between the FDC1004 CINn pins and sensor is included in the FDC1004 conversion result.

In some applications, the parasitic capacitance of the sensor connections can be larger than the capacitance of the sensor. If that parasitic capacitance is stable, the capacitance can be treated as a constant capacitive offset. However, the parasitic capacitance of the sensor connections can have significant variation due to environmental changes (such as mechanical movement, temperature shifts, humidity changes). These changes are seen as drift in the conversion result and can significantly compromise the system accuracy.

To eliminate the CINn parasitic capacitance to ground, the FDC1004 SHLDx signals can be used for shielding the connection between the sensor and CINn. The SHLDx output is the same signal waveform as the excitation of the CINn pin; the SHLDx is driven to the same voltage potential as the CINn pin. Therefore, there is no current between CINn and SHLDx pins, and any capacitance between these pins does not affect the CINn charge transfer. Ideally, the CINn to SHLD capacitance does not have any contribution to the FDC1004 result.

In differential measurements, SHLD1 is assigned to CHn and SHLD2 is assigned to CHm, where n < m. For instance in the measurement CIN1 – CIN2, where CHA = CIN1 and CHB = CIN2 (see Table 6-4), SHDL1 is assigned to CIN1 and SHDL2 is assigned to CIN2.

In a single-ended configuration, such as CINn versus GND, SHLD1 is internally shorted to SHLD2. In a single ended configuration, such as CINn versus GND with CAPDAC enabled, SHLD1 is assigned to the selected channel, SHLD2 is floating.

For best results, locate the FDC1004 as close as possible to the capacitive sensor. Minimize the connection length between the sensor and FDC1004 CINn pins and between the sensor ground and the FDC1004 GND pin. Shield the PCB traces to the CINn pins and connect the shielding to the FDC1004 SHLDx pins. In addition, if a shielded cable is used to connect the FDC1004 to the sensor, connect the shield to the appropriate SHLDx pin. In applications where only one SHLDx pin is used, the unused SHLDx pin can be left unconnected.

For more information on shielding, refer to Capacitive Sensing: Ins and Outs of Active Sensing application note.