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Original question:

FDC1004: Initial Calibration Difficulties.

FDC1004: How to adjust offset and gain on the FDC1004 in differential mode

Andreas Hansen
Intellectual 410 points
Part Number: FDC1004

Hi

I'm using the FDC1004 for liquid level sensing just as described in the TIDU736A reference design.

The FDC1004 is configured in differential mode measuring a LEVEL sensor, REFERENCE LIQUID sensor and REFERENCE ENVIRONMENT sensor. The three measurement channels are configured as follows:

MEAS1 = CIN1 (CHA) - CIN4 (CHB)
MEAS2 = CIN2 (CHA) - CIN4 (CHB)
MEAS3 = CIN3 (CHA) - CIN4 (CHB)

THe problem is that the measurements seem to have some offset (seen when measuring the air) and saturates before a the tank is filled.

Instead of just designing a new level sensing PCB (with a smaller sensing area), I would like to see what can be done in terms of adjusting gain and offset. But I'm a bit confused about how this can be done when the FDC1004 is operating in differential mode?

From what I can read from the datasheet the CAPDAC setting has no effect for a differential measurement.

What is left, is the OFFSET_CAL_CINx and GAIN_CAL_CINx (x=1,..,4) registers. How should these be interpreted?

Best Regards,
Andreas

  • 0
    TI__Mastermind 23810 points
    Andreas,

    I need to verify if the OFFSET and/or GAIN settings will work with differential mode, but I will not be able to do this until after the New Year's holiday. You could also check this by using the EVM.
  • 0 in reply to Clancy Soehren
    Intellectual 410 points
    Hi Clancy

    I have experimented a bit with the various OFFSET and GAIN settings.

    From what I can see, it is possible to make a "common" offset on all my channels by putting an offset on OFFSET_CAL_CIN4. The individual channels can be shifted by applying an offset OFFSET_CAL_CINx, x=1,2,3.

    Regarding the GAIN settings, by applying a gain GAIN_CAL_C4 does not seem to affect any of the channels. However, each of the three measurements are affected by GAIN_CAL_Cx, x=1,2,3.

    I don't quite understand the saturation limits. If for instance I put an offset of OFFSET_CAL_CIN1=-15 pF, the maximum possible readout of MEAS1 turns out to be 14.04 pF?

    If I apply the gain GAIN_CAL_C1=0.5, the maximum capacitance readout of MEAS1 is 14.52 pF?

    How can I calculate what the measurement ranges will be given some OFFSET/GAIN setting?

    Are there any best practices as to how large OFFSET/GAIN values are recommended to use?

    Best Regards,
    Andreas
  • 0 in reply to Andreas Hansen
    TI__Intellectual 1020 points
    Hi Andreas,

    Thanks for following up. We will get back to you after the holidays.

    Regards,

    Vaibhav
  • 0 in reply to Vaibhav Garg10
    TI__Expert 6625 points
    Hi Andreas,
    What's the max possible readout for MEAS1, when the OFFSET is 0pF and the GAIN = 1? If you're using the FDC1004EVM, can you please share a screen shot of the "Registers" setting tab?

    Best Regards,
    Bala Ravi
  • 0 in reply to Bala Ravi
    TI__Expert 6625 points
    Hi Andreas,
    I've not heard back from you. I will go ahead and close this thread for now, if your issue is not resolved please start a new thread. The full-scale range for FDC1004 is -15pF to +15pF. The gain and offset are factored in after the measurement is taken, meaning the gain and offset features are for post-processing not pre-processing. That's why I wanted to check with you your max readout without any gain or offset. This way we will know if the device already saturated or not. In case, the device did not saturate, please check your 2's complement.

    Best Regards,
    Bala Ravi

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