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LDC1101: DIG_CONF Measurement Response Time

Arthur Thompson27
Expert 1745 points
Part Number: LDC1101

Tool/software:

Hello,

We are seeing our Rp/L measurements take longer than predicted by Formula 10 in Section 9.1.8 of the datasheet.

Our sensor frequency is 4 MHz.

Here are the values we measured experimentally, by using an oscilloscope to measure the time between the sensor waveform turning on and the LDC generating the INTB signal to indicate the measurement is ready.

RESP_TIME     Measured Conversion Time

768                    270 usec

1536                  538 usec

3072                  1064 usec

6144                  2128 usec

The expected measured conversion time, e.g. for a RESP_TIME of 6144, should be 6144/(3*4e6) = 512 usec.

Can TI explain why my measured conversion time is taking longer than predicted by the datasheet formula?

Thanks,

Arthur

  • 0
    TI__Mastermind 22775 points

    Hey Arthur,

    Thanks for considering to use Texas Instruments.  Could you provide the scope capture you used for determining the conversion time discrepancy?  We would like to see if the sensor waveform profile aligns with what we would expect.

  • 0 in reply to Patrick Simmons
    Expert 1745 points

    Hello Patrick,

    Pink is the interrupt pin, and Blue is the single-ended sensor waveform (just half of the total differential signal) with a setting of 6144. 

  • 0 in reply to Arthur Thompson27
    TI__Mastermind 22775 points

    Hey Arthur,

    Thanks for providing that.  Will try to have an update later today regarding debug steps or what we think might be wrong.

  • +1 in reply to Patrick Simmons
    TI__Mastermind 22775 points

    Hey Arthur,

    We did some testing and it looks like we duplicated your issue. From our tests, we startup the device from sleep mode and then start taking samples.  Looks like the initial conversion period runs ~4x longer then all subsequent conversions.  As of now, we do not have a work around for this behavior.  Are you planning to use only one conversion cycle when waking from sleep mode?

  • 0 in reply to Patrick Simmons
    Expert 1745 points

    Hi Patrick,

    Yes, for maximum power saving we are planning to just use one conversion and then go to sleep.  We have already shown that setting RESP_TIME to 1536 allows the measurement to complete fast enough for our application.  I had been wanting to increase RESP_TIME to 6144 in order to maximize resolution while staying within our time budget, but due to the first conversion being longer than predicted in the datasheet, we can just leave RESP_TIME at 1536.

    Thanks for your support,

    Arthur