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ADS1282: calibrate gain ads1282

wivach rungshawang
Prodigy 30 points
Part Number: ADS1282
Other Parts Discussed in Thread: REF6250

Hello,all


Please explain. How to  calibrate gain ads1282 in bipolar mode. I'm not sure how to manage INP and INN pin when gain calibrate and full scale voltage input in bipolar mode.

Best Regards,

Wivach

  • 0
    TI__Mastermind 44640 points

    Hi Wivach,

    Welcome to the TI E2E Forums!

    The gain calibration requires you to provide an accurate full-scale input signal. For bipolar supplies (+/- 2.5V), your input signal will be biased around a 0-V common mode voltage. Therefore you will want to make INP =  +VREF / (4 x PGA) and INN =  -VREF / (4 x PGA). (CORRECTION)

    Notice that the exact values for INP and INN will depend on your reference voltage and PGA setting. Also, the common-mode voltage doesn't need to be exactly 0V, but the differential voltage (INP - INN) should be very close to VREF / PGA; otherwise you'll end up with a poor calibration result and increased gain error in your measurements.

    Does that answer your question?

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 30 points
    Hi Chris,
    Thank you for answer.
    for your answer INP = +VREF / (2 x PGA) my Vref is 2.5 and gain 1. so INP = +2.5 / (2 x 1) = 1.25V and INN -1.25V. that correct?
    Now I move avdd to 5v and avss to 0v vref is ref6250 and run gain cal with 5.02040V within 103% of full scale but output code is max at 7ffffdbe is eqal 3.18V (gdm8261a 6 1/2 digital multi meter) and now my +VREF = 5.0 and -VREF = 0 so my INP = +2.5V and INN = 0V. that correct?

    Best Regards,
    Wivach
  • 0 in reply to wivach rungshawang
    TI__Mastermind 44640 points

    Hi Wivach,

    wivach rungshawang said:
    INP = +VREF / (2 x PGA) my Vref is 2.5 and gain 1. so INP = +2.5 / (2 x 1) = 1.25V and INN -1.25V. that correct?

    My apologies, I had made a mistake above...

    The differential input voltage (INP - INN) should not exceed +/- VREF / (2 x PGA). Or in your orginal case, +/- 1.25V.

    Therefore, the difference (INP - INN) should be 1.25V, and the individual voltages on INP and INN should be +/- VREF / (4 x PGA) or +/- 0.625V, respectively.

     

    wivach rungshawang said:
    Now I move avdd to 5v and avss to 0v vref is ref6250 and run gain cal with 5.02040V within 103% of full scale but output code is max at 7ffffdbe is eqal 3.18V (gdm8261a 6 1/2 digital multi meter) and now my +VREF = 5.0 and -VREF = 0 so my INP = +2.5V and INN = 0V. that correct?

    I'm not quite sure what you mean by "running gain cal with 5.0204V". Is that the REF6250's output voltage? In this case, your input signal should be 2.5-V.

    The 103% full-scale tolerance only applies to the input signal and just allows for you to performe gain calibration when your reference voltage is less than 5V.

    Now that you've moved to a 5V unipolar supply, you'll also need to increase the input signal's common-mode voltage. You won't be able to connect INN to 0V. I would suggest INP = 3.475V and INN = 0.975V, since this places the common-mode voltage in the center of the "absolute input range" specification.

     

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 30 points

    Hi Chris,

    many thank for your help.

    1. In my Bipolar mode when i run gain cal must supply +0.625V to INP and - 0.625V to INN  that correct?

    2. Sorry for unclear question. In my Unipolar mode  i running gain cal with supply 5.0204V  to INP and 0V to INN. From your suggestion "You won't be able to connect INN to 0V. I would suggest INP = 3.475V and INN = 0.975V" That my mistake i stop do that.

    Best regards,
    Wivach

  • 0 in reply to wivach rungshawang
    TI__Mastermind 44640 points
    Hi Wivach,

    1. Correct. In that case you had a 2.5V reference, so the differential input signal should be 1.25V (for PGA = 1 V/V).

    2. No problem. Likewise with a 5V reference (and PGA = 1 V/V), the differential input signal should not exceed 2.5V.

    For differential inputs, you not only need to pay attention to the differential amplitude, but also the common-mode voltage. The common-mode voltage for the ADS1282 is actually specified as absolute input voltages. No input (either VINP or VINN) should be less than AVSS + 0.7, and no input should exceed AVDD - 1.25V.

    Best regards,
    Chris