Problem with impedance phase measurement

Hi,

      The default AD5940 configurations given in examples are mainly for load impedance measurements done at 16 bit resolution.

       Since there is very small variation in imaginary part for large change in capacitance, due to resolution limitation of DFTImag register,

       it is required to choose more precise value of RTIA for getting accurate capacitance measurement at output.

      External RTIA value may be required.

Below is an example:

For capacitances in pF range, small current is expected. Hence larger RTIA value may be required as shown below:

Thank you very much for the reply Akila,

in the meantime I had already reduced the Rtia set to the minimum and got much more stable results with an absolute value much closer to the calculated values. However, the sign of the phase is inverted for me, could it still be a saturation problem and therefore I am making a wrong measurement or should a negative sign be added in the phase calculation of the example code?

Hi,

 Yes. Sign must be reversed:

RzPhase = atan2(-pDftRz->Image,pDftRz->Real);

AD5940_ComplexPhase() function doesn't have -ve sign:

float AD5940_ComplexPhase(fImpCar_Type *a)
{
return atan2(a->Image, a->Real);
}

Thank you very much for the help Akila  :) 

Have a good day!