LDC2114: waveform of INx does not drop to zero

Hello,

The second waveform you're showing discharges to a lower voltage because of the additional oscilloscope probes loading on the other input pins. If you add a second scope probe to the other inputs you should see it discharge to a lower voltage. Also, it looks like your ground connection is not very secure. I would try to ground the scope probe to the same ground potential near the LDC2114. Also, to prevent loading the sensor with the probe capacitance (~10-12pF), we typically recommend to isolate the probe with a 1kΩ resistor. When measuring the waveforms, the critical behavior is when the button is sampling. The amplitude should be around 900mVpp and the DC offset should be 900mV and the frequency should be in range (1< fsensor < 30MHz). This can be measured by zooming into the waveform:

Additionally, you should set the sample window to be around 1ms and ensure your sample rate is correct for your given operating mode (Normal power vs low power).

For help on setting the correct values (LCDIV, SENSCYC, CNTSC), you can refer to the LDC2114_Config tab of the LDC tools spreadsheet which simplifies the design process: http://www.ti.com/lit/zip/slyc137

For example, if my sensor frequency measured to be 19MHz and I want a 1ms sample interval, the computed values are LCDIV = 3, Sensor Cycle Count = 18, and CNTSC = 0

For more information, take a look at the application notes section for LDC devices: http://www.ti.com/analog/docs/analogtechdoc_hh.tsp?viewType=mostuseful&rootFamilyId=353&familyId=3244&docCategoryId=1 

Regards,

Luke