Hello, we are working on a new design around the FDC2212 and are experiencing some anomalies. We're seeing occasional "steps" in the data, even in a purely static test set up. Occasionally, one or both channels with go completely "off the rails" with excursions way outside the normal operating range. I've collected 3 runs at different drive level settings to illustrate the phenomenon - it's not consistently reproducible, but seems to behave better at higher IDRIVE settings (closer to the 1.8V max).
Design details - the device is battery powered, the PCB is custom, the capacitive element is a 2-electrode printed electronic sensor with a hatched top ground shield. Both electrodes are single-ended. Of the 2 sensing electrodes, there is a single environmental reference electrode that operates at 1.25MHz nominal and a level sensing electrode that operates at 1.10MHz nominal. The FDC2212 is operating from its internal oscillator at 43.40MHz. Both channels of the FDC2212 use a 100uH inductor and a 68pF capacitor, along with the 18pF filter caps as per the EVM design (see schematics).
Settings are as follows:
#define DEFAULT_FDC2212_DEGLITCH_BW FDC2212_DEGLITCH_BW_3_3MHZ
#define DEFAULT_FDC2212_REFCNT_CH0 (12288)
#define DEFAULT_FDC2212_SETTLECNT_CH0 (128)
#define DEFAULT_FDC2212_CLKDIV_CH0 (7) // 43.4MHz nominal / div, lvl Fref = 6.2MHz
#define DEFAULT_FDC2212_FINSELDIV_CH0 FDC2212_FIN_SEL_DIV2
#define DEFAULT_FDC2212_REFCNT_CH1 (8192)
#define DEFAULT_FDC2212_SETTLECNT_CH1 (128)
#define DEFAULT_FDC2212_CLKDIV_CH1 (7) // 43.4MHz nominal / div, ref Fref = 6.2MHz
#define DEFAULT_FDC2212_FINSELDIV_CH1 FDC2212_FIN_SEL_DIV2
I've attached here schematics, as well as 3 different captures and waveform pictures at different drive strengths. All test runs are in a completely static office environment. Any help or advice you could provide would be greatly appreciated. We haven't tried ferrite beads or CMC's yet, but wanted to reach out for some advice prior to going that route. There are clearly harmonics in the waveforms - would that cause this behavior?
Thanks in advance!
Schematics...
1.18v IDRIVE, output and waveforms...
1.32v IDRIVE, output and waveforms...
1.58v IDRIVE, output and waveforms...