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Parallel RC Issues with AD5941 EIS

Category: Hardware
Product Number: AD5941


I noticed something strange every time I would test EIS on a parallel RC circuit. For example, running a 33k Resistor in parallel with a 10nF capacitor results in data that is far from the theoretical values. However, placing a resistor in series with this parallel circuit yields correct values regardless of how small the resistor is. I've tried this with multiple combinations of HSRTIAs to see if it's a saturation issue, but found no difference. In fact, it seemed to saturate much sooner with just the parallel circuit case. 

I wanted to ask if anyone has run into this issue before? The EIS works fine otherwise in any other circuit configuration, but this one without a series resistance yields a lot of noisy and unreliable results. Is this a limitation of the AD5941 chip? Is it limited to specific loads? From my understanding, the AD5941 is capable of measuring loads without much limitation (i.e. < 50 Ohms and > 300 kOhms), so I wanted to see if it's an issue with the software and how it processes EIS. 

Thank you!