Other Parts Discussed in Thread: FDC1004
I believe that there is an error in the TIDA-00317 reference design which causes the environmental reference electrode (RE) to output invalid data.
The design uses an out-of-phase (OOP) reference for each electrode. CIN1 and CIN2 use OOP reference SHLD2, CIN3 and CIN4 use OOP reference SHLD1. When in-phase shielding is required, SHLD1 and SHLD2 are swapped.
CIN1 (LEVEL) and CIN2 (RL) are correctly paired with SHLD2 as their reference, but CIN3 (RE) shares it reference with RL (SHLD2) which can't be correct. This means that RL is referred to its in-phase shield and hence there should be minimal capacitance between the two electrodes - any changes in capacitance in fact become dominated by the self-capacitance of the lead-in trace, which varies with liquid level as this trace runs the full length of the sensor.
Could someone from TI please either point out what I am misunderstanding or confirm that TIDA-00317 has an error?