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ADS1191 OFFSETCAL: Channel Offset Calibration

Babak Madjidian
Prodigy 20 points
Other Parts Discussed in Thread: ADS1191

Hi,

According to the ADS1191/92 datasheet (P.33), OFFSETCAL command must be executed every time there is a change in the PGA gain settings.

In our application we use an ADS1191 and need to switch between two different inputs with different gains. Question is that what is happening after execution of the OFFSETCAL command? When is the device ready to accept new command and must the CALIB_ON bit in the MISC2 register be reset?

I can not find any comments about it!

Thanks in advance,

/Babak

  • 0
    TI__Guru 57296 points

    Hello Babak,

    Thank you for your post and welcome to our forum!

    The OFFSETCAL command works as described in this post. When you are sending this command, make sure that /CS is low the whole time as the command is decoded on the seventh falling edge of SCLK.

    I see that we do not specify the amount of time required before using the device again, but I believe it would be 19*tDR = 152ms (DR = 125SPS). Please let me confirm this timing with the designers.

    ** EDITED to say 19*tDR = 152ms **

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 20 points
    Hello Ryan and thanks a lot for your answer. Some supplementary questions:
    1. Does /DRDY signal indicate the end of Offset calibration?
    2. As I understand, the CALIB_ON bit needs to be set only once! then the Offsetcal command can be executed every time we are switching inputs? Is it correct?
    Look forward to your confirmation about the timing.
    Best Regards,
    /Babak
  • 0 in reply to Babak Madjidian
    TI__Guru 57296 points
    Hi Babak,

    You should be able to use /DRDY to indicate when the OFFSETCAL routine is complete. During the OFFSETCAL routine, I'm honestly not sure what the /DRDY pin will do - it may be held high or it may continue to toggle. I cannot verify it on the bench since our evaluation platform does not have this SPI command included. After OFFSETCAL is complete, you should see /DRDY toggle again at the previous data rate, assuming START is high and the device is in Continuous Mode.

    I have to correct what I said in my previous post - the total time for OFFSETCAL should take 19*tDR. The falling edge of /DRDY after tSETTLE (4*tDR) indicates the first of 16 samples used in the OFFSETCAL routine. So the total time is equal to 4*tDR + 15tDR = 152ms.

    The CALIB_ON bit should be set to '1' in the beginning and it should remain '1'. You do not need to issue the OFFSETCAL command after switching inputs - you will only need to wait for tSETTLE so that the digital filter outputs settled data. OFFSETCAL should only be issued with the PGA gain is changed.

    Best Regards,