Hi,
I have designed a data acquisition board using the ADS1298. I am using differential inputs with RDL electrode also connected to body as return path. I have the AC lead off feature activated and have managed to take accurate measurement for resistive inputs (used a know resistor to calibrate all the channels). This circuit (and the impedance calculation to find Z_IN) also works accurately when electrodes are applied on skin. However when electrodes lose contact with skin (basically an open circuit) then the impedance value suddenly go out of range and show odd behaviour (for some of the lead, other leads just show a very high value compared to when there is skin contact, which I assume is the expected behaviour).
My sampling rate is 2000, which makes the AC lead signal at 500Hz. looking at the raw data I can see this 500Hz signal clearly for the leads that behave as expected but for the leads that have have impedance measurement out of range I get no 500Hz in the raw data (or a significantly lower amplitude than I would have expected).
I have a differential capacitor setup as for the input stage as shown below:
I would appreciate any help you would be able to give me
