We built up an application of the AD5933 impedance to digital converter IC. We
already tested the device using standard resistors soldered directly to the
pins of the AD5933. Unlikely the imaginary part of the impedance is not close
to zero. On the other hand measuring capacitances yields real parts of the
impedance not clos to zero. Thus calculation of the resistance and capacitance
values from the phase and the impedance is far away from accurate.
I did not see any mention of calibration of the phase in your description –
which is required.
This involves placing a known resistor across Vin and Vout and measuring he
phase of the system.
This needs to be subtracted then from future readings for accurate results.
This is described briefly in the datasheet on page 22 of the Evaluation Board
datasheet, 'MEASURING PHASE ACROSS AN IMPEDANCE'.
I believe this will fix the problem.
"Prior to determining the phase of the unknown impedance connected between Vout
and Vin the user must calibrate the
AD5933 system phase. This can be done by placing a resistor across Vout and Vin
and calculating the phase at after each
sweep point. The resistor will not contribute any additional phase lead or lag
and the resulting phase will be due to the
internal poles along the signal path."