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AD8302 - problem with detecting Vphs

Question asked by Majjo on Jul 10, 2015
Latest reply on Sep 29, 2015 by jdobler

Hi everybody,

I have a problem with my AD8302. It cannot detect the phase very well. I want to use the AD8302 for measuring of the bio- impedance in my device. So the impedance can be in range from 3 kohm to 10 ohm.  As a signal generator I use AD9850. My problem is that when my electrical model of the biological tissue has got impedance from 1200 ohm to 470 ohm. The AD8302 is able to detect Vmag and Vphs very well. When I change the model of the biological tissue the impedance is from 440 ohm to 220 ohm….. AD8302 can detect only Vmag and Vphs is totally wrong. Signal from electrical model of the biological tissue is amplify 4x. I measure in frequency range from 10 kHz to 1 MHz. Also I am using attenuator to fit in range to detector (-63 to 0 dBm).  The schematic of my AD8302 looks like this (see picture). I don´t know if I choose the right capacitors.

Thank you very much for your help.