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ADS7828: Differential input mode confusion?

Jon Fick52
Prodigy 20 points

Part Number: ADS7828

I searched the E2E Forum and found a similar question that I'm presently asking.  However, there is no answer or perhaps the "locked" designation means that the answer is private.  Since there is no discernible post number associated with that post all I can give you is the title of the post: "ADS7828 Absolute Input Range - Negative Input".  Not having benefit from that post, here's our question... 

We are using the ADS7828E ADC in a circuit where we have eight low impedance voltages that we are attempting to measure.

  • If we capture the eight voltages by commanding single-ended mode the eight voltages (counts out of 4096) are correct.
  • If we put channels 0 and 1 into differential mode, the received count is not the difference between the single-ended counts at those two inputs.

We are obtaining these output counts every two seconds. The following image shows seven reads, one every two seconds. There is 1mS between the reads of the diff, and each singled-ended value. (Diff mode ch0-ch1, single ch 0, single ch 1) 

What are we not understanding about differential input mode?

  • 0
    TI__Guru**** 182601 points
    Hi Jon!

    Welcome to our e2e Forum!

    Sorry about that other post, I'm not sure what happened there, but by the ADS7828 datasheet my first instinct is to say that this part is not a 'true differential' device since the IN- is listed as being +/-200 mV. I am curious to know though what you had the COM pin tied to during these tests.
  • 0 in reply to Tom Hendrick
    Prodigy 10 points
    Tom, thanks for the quick reply. COM pin 11 and GND pin 9 are tied together at one point so as to attempt to keep COM as quiet as possible. None of the eight single-ended input pins seem to be limited to +/-200mV. Could they somehow be limited to near zero when in differential mode with that constraint?
  • 0 in reply to Jon Fick
    TI__Guru**** 182601 points
    Hi Jon,

    Unfortunately I don't have access to the design files showing the construction of the inputs, but since these channels can all be used in SE mode with COM as IN(-), I'm not surprised that they don't 'clamp' at +/-200 mV. I did find some old notes that say the ADS7828 is a 'pseudo differential' input when using CH0-CH1, etc. That would fit with the ABS MAX statement saying the negative input can range from +/- 200 mV.
  • 0 in reply to Tom Hendrick
    Prodigy 20 points
    Tom,

    OK, we understand what we need to do. Like a posters had stated in another thread, we can simply do the math with the eight single ended values we get from the chip. None of our connected inputs goes below zero volts...they are all something between 0 and 2.5Vref so I'm not quite sure of the usefulness of this pseudo-differential mode but when re-spinning this board we'll select something else that has true differential so our software can stay on other tasks. Thanks for your help.

    Jon