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AFE4300: Calibrating Phase Offset

Mark Newman
Prodigy 20 points
Other Parts Discussed in Thread: AFE4300EVM-PDK, AFE4300

I have been calibrating my AFE4300 using the AFE4300EVM-PDK according to the guidlines in the PDF: Impedance Measurement with AFE4300. However when calibrating the phase offset (using the method described in section 2.2 on page 4 of the PDF) I hit a problem.

I am only interested in making measurements using the I/Q mode at a DAC DDS frequency of 64000 Hz. However, when I tried to measure the phase offset using the AFE4300EVM-PDK on board resistors, I measured a different offset for each resistance.

For the 100R resistor I measured a phase offset of 22 degrees (I = 0.00981V, Q = 0.00394V)

For the 200R resistor I measured a phase offset of 17 degrees (I = 0.01831V, Q = 0.0056V)

For the 750R resistor I measured a phase offset of 9 degrees (I = 0.06189V, Q = 0.01493V)

For the 1000R resistor I measured a phase offset of 9 degrees (I = 0.08161V, Q = 0.02097V)

Shouldn't the phase offset be constant irrespective of resistance? It looks like there is some stray capacitance somewhere. Am I doing something wrong? Which offset to I subtract from my phase measurement when I test an unknown impedance as described in section 2.2 on page 4 of the PDF?

Thanks,

Mark.

  • 0
    TI__Mastermind 49916 points
    Hi Mark,

    The DAC frequency generator DDS is initialized on the register update of the DAC frequency register. The IQ demod clock divider is updated on the divider register value. Since the registers are written through the SPI interface (that is asynchronous to the device clock), every time either of these registers are written, the phase relation between the DAC output and the I-Q demod clock can get altered. This can cause the phase of the I-Q measurement to be non-deterministic.

    One way of circumventing this issue is to do a fresh calibration (measuring all the calibration impedances) each time the registers are reprogrammed for a new excitation frequency.

    Do you issue a reset before switching to the different resistor networks?
    The on-board resistors of the resistor bank are selected using the on-board switches. Not sure if these are contributing to the error.
    Instead of using the on-board resistors, can you use external resistors for the measurement?
  • 0 in reply to Praveen Aroul
    Prodigy 20 points
    I was wondering whether the inaccuracy might be because of the 1uF capacitors that are between the RP0 RN0 and RP1 and RN1 inputs on the demo board. They would obviously add a phase difference with the increase in resistance. I can try shorting them and seeing if that helps.