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ADS1292 OFFSETCAL instructions

James Whong
Prodigy 180 points
Other Parts Discussed in Thread: ADS1292

Hi Forum,

I have an ADS1292 and I am trying to run the OFFSETCAL command as described in the ADS1292 datasheet page 36.  After changing the PGA gain I run the OFFSETCAL command, but the reported offsets don't change at all.  The datasheet is very short on details for how to run this command... are there timing details that must be respected?  Has anyone successfully used this command?

Presently I am working around it by having the host MCU put the ADS1292 in "inputs shorted" mode, sampling, averaging, and then subtracting the result from subsequent readings.

Thank you

~James

  • 0
    Intellectual 340 points

    Hi James,

    Did you ever figure this out? I am trying to calibrate the ADS1292 as well. I think you have to set the calibration bit in the RESP2 register. Also, I am thinking you may have to short the channels in the channel setting registers. However, I can't find any information regarding timing of the calibration process. 

    Any information regarding calibration would be much appreciated.

    Corey

  • 0 in reply to ccent
    Prodigy 180 points

    Hi Corey,

    No joy.  I played with the calibration bit in the RESP2 register but still couldn't get the instruction to work.  I've started just working around the lack of OFFSETCAL instruction and compensating in the host microcontroller.

    ~James

  • 0 in reply to James Whong
    TI__Guru 57336 points

    Hi James and Corey,

    From what you mentioned, it does seem that you are calling the OFFSETCAL function correctly, but I can give you a better explanation about what goes on inside so that you know what to expect.

    With RESP2 [7] = 1, you can send the OFFSETCAL command at any time following the SPI timing protocols. When this command is issued, the Data Rate is changed to the lowest available. Then the digital filter is reset, which requires a settling time of 4*tDR. 16 samples will be recorded, averaged and stored on-chip, and this value is automatically subtracted from the PGA output. The value that is stored is only 16-bits, so there is still a possibility that you will see 28 = 256 codes worth of offset (post-gain) following the OFFSETCAL.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 180 points

    Thank you Ryan!  This is extremely helpful.  I wasn't giving enough time after the OFFSETCAL command.

    I've got compensation on the microcontroller side working well now and it avoids the 256 lsb of offset issue, so I'll probably stick with my workaround.

    Any chance this information will make it in to the next ADS1292 datasheet?

    Thank again

    ~James

  • 0 in reply to Ryan Andrews
    Intellectual 340 points

    Thanks Ryan - very helpful. One related question, does the OFFSETCAL command take care of internally shorting the channels or must the channels be shorted in the relevant registers prior to running the calibration command?

    Thank you,

    Corey

  • 0 in reply to ccent
    TI__Guru 57336 points

    Hi Corey,

    Yes, the OFFSETCAL command does internally short the channels. The user does not have to do this explicitly.

    I will make a note to better explain OFFSETCAL in the next datasheet revision.

    Best Regards,