Expected raw values from DFT on AD5940?

What is the formula for the expected results from the DFT conversion for impedance measurement with the AD5940?

The formulas that I see in the documentation show how to use a calibration resistor to adjust the result, but I don't see a direct formula.

I ask primarily because raw measurements of RCAL consistently have a phase angle of roughly 70-80 degrees.  I understand that the intended use is to use RCAL to calibrate magnitude and phase, but I was surprised that the uncalibrated raw results were so severely rotated.

  • 0
    •  Analog Employees 
    on Sep 6, 2021 11:30 AM

    Hi,

    If

    rZ = real part of DFT result with Z load impedance,

    iZ = imaginary  part of DFT result with Z load impedance,

    rCAL =  real part of DFT result with RCAL load impedance,

    iCAL =  imaginary  part of DFT result with RCAL load impedance,

    then,

  • Apologies, my question must not have been clear.

    The formulae you give are how to use a pair of raw DFT results to calculate the calibrated impedance.  I want to predict the raw DFT results.

    For example, a 10 bit ADC with a 2.5V reference and a 2x PGA gain should give a raw result of:    (input)/(2.5V) * 2^10 * 2

    What is the equivalent formula for this measurement?

    If I measure a resistor, should I expect the raw DFT imaginary result to be close to zero?

  • 0
    •  Analog Employees 
    on Sep 7, 2021 7:47 AM in reply to EricV

    Hi,

    The DFT block does Discrete Fourier Transform of DFTNum of ADC samples and gives the result in cartesian form.

    When the load is a resistor, imag(DFT) need not be zero.

    Ex: Mag:69661.554688,Phase:0.911263,Real:42685,Imag:207092