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ADS1226 self calibration mode

DOV M.
Prodigy 200 points
Other Parts Discussed in Thread: ADS1226

Hi,

I am using the ADS1226 and run the first cycle after power up as a self calibration cycle. During the calibration time the ADC gives a digital output of 0x000000 and usually this lasts for up to 300ms as stated in the datasheet. So far so good. Sometimes though, the 0x000000 output continues for ~900ms. I assume that the calibration is taking longer than expected. 

  • Is this a known issue?
  • Is there a work around?
  • What is the penalty of not calibrating?

Thanks, Dov

  • 0
    TI__Guru** 113765 points

    Hi Dov,

    The length of time for calibration is based on the internal clock.  This can vary some, but not to the degree you are seeing.  It should always be very close to 300ms (313ms) or less. 

    You have given a basic procedure, but it is not entirely clear.  There will be a time period required for initial power up of the device and oscillator start.  The START pin must also be a valid high before conversion will start to take place.  If you attempt to calibrate prior to completed device power up you may see some delay.

    The best thing to do is wait until the first conversion is complete before attempting to calibrate.  This will be indicated by a high to low transition on DRDY/DOUT.  How are you determining completion of conversion?  Are you polling the pin, or using an interrupt?

    If you decide not to calibrate, there will be some error in the measurement.  The amount of error will vary from part to part.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 200 points

    Thanks, Bob,

    I indeed do sync on to the completion of the first conversion in order to do calibration. Here is a more detailed description of the state machine handles the interface to the A/D:

    1. Wait 20us from power up with START = 0;
    2. Set START = 1 and poll on high to low transition of DO/DRDY.
    3. In case of 1st cycle do self calibration, otherwise read A/D
    4. go to step 2 (START remains high).
    When I increased the delay of step 1 to 10ms the length of calibration was always according to the spec. Could it be that the supplies need more stabilization time? If so, why does the device assert DO/DRDY?
    Thanks again,
    Dov
  • 0 in reply to DOV M.
    TI__Guru** 113765 points

    Hi Dov,

    I think 20us is a little short.  It is possible that the state machine is still in early stages and the transition of DRDY/DOUT may be in an initialization state and not truly signalling end of conversion.  There is not any specific characterization data to show start up time.  As I mentioned previously there are a number of factors at play including the initial startup of the internal oscillator. There is an internal timer based off the internal oscillator that controls the POR process and I think that this timer is set for about 500us.  I think you need to wait at least that long for DRDY/DOUT to be valid and probably should delay longer to be on the safe side.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 200 points

    OK - thanks so much. your advice was extremely helpful!

    Dov