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ADS124S08: Command Sequence to Perform Offset Calibration When Running in Single-Conversion Mode

Paul White
Prodigy 20 points
Part Number: ADS124S08


We are working on a new board design that is using two ADS124S08 tied to a single SPI bus.  Each converter is used to read 4 thermocouples (or RTDs, depending on how the board is stuffed) plus its internal temp sensor.  I'm using single-conversion mode, with SPI commanding to to set the input mux, start conversions and issue the RREG command to read the conversion results.  Everything works fine, we get good data, but now want to add periodic offset calibrations. 

I see that the SYOCAL command must be issued in 'conversion mode', which seems to conflict with me using the converter in single-conversion mode where the converter is in standby after each conversion.  Is there a way to do the offset calibration by sending a START command followed by an immediate SYOCAL? ... or will I have to switch to continuous conversions, issue the START command followed by the SYOCAL, then switch back to single-conversion mode once the cal completes?

As I think this through as I write this, I'm getting the feeling that I answered the question myself! ...but perhaps you guys can point out a straightforward way of doing this from single-conversion mode.


Thanks!

Paul White

  • 0
    TI__Guru** 113765 points

    Hi Paul,

    Welcome to the E2E forum!  You more or less answered your own question.  When using single-shot mode, you need to issue the START command and before the conversion ends issue the calibration command.  Once the calibration completes the ADS124S08 will go back to standby.

    Best regards,

    Bob B

  • 0
    Prodigy 20 points

    Thanks Bob!  

    One more thing ... I currently drive the Start/~Sync line low when the board boots up and keep it there 'forever'.  Do I need to do anything different with that line in order to do the offset calibration as you have suggested?

    Thanks again!

  • 0 in reply to Paul White
    TI__Guru** 113765 points

    Hi Paul, 

    If you use the SPI commands, then yes you should keep the START/SYNC pin low.  Using the pin to control start of conversions is an alternate method to using the commands.  However, the pin control takes precedence to the SPI commands.  That is why it needs to be low when using the commands.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hi Bob,

    Today I tried out the offset calibration from single-conversion mode as you suggested, but I appear to have had no success.

    I issue the START command as I typically would each cycle, and then follow it immediately with the SFOCAL command.  The ~DRDY line falls to indicate the end of 'something' ... either a conversion or the cal ... but when I read the offset cal registers, they all are equal to 0.

    Thinking the conversion delay time maybe is involved, I added a short delay between the START and SFOCOL commands, but that made no difference (admittedly I don't know the actual delay create as my added code was just a look).  Is there any additional configuration required?

    The data rate selection is 50 sps, so the conversions are not exceptionally fast.

    Any ideas would be appreciated!

    Thanks,

    Paul White

  • 0 in reply to Paul White
    TI__Guru** 113765 points

    Hi Paul,

    The programmable conversion delay is discussed in section 9.4.4.3 of the datasheet.  By default the value is 14*tmod, which is about 55us delay before the actual conversion process begins.  So the time from the 7th falling edge of the START command to the falling edge of the SFOCAL command must be greater than 55us.  However, there may be some additional overhead as it relates to the first conversion.

    The length of time for a single-shot conversion period will depend on which digital filter is used (either Sinc3 or low-latency).  Depending on which filter you are using I would add the additional delay that occurs between the first data and the second data for continuous mode.  Table 13 times are for the low-latency and Table 15 for the Sinc3.  In any case using 500us should be sufficient for either filter, however you could go longer.

    By default, the number of samples for calibration will be 8 samples, so it will take 8*50sps before you see DRDY fall if the SFOCAL actually starts.  Using an oscilloscope or logic analyzer you can verify the communication and timing.

    Best regards,

    Bob B