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ADC Peculiar readout - ADS8321

Keith Wakeham
Prodigy 20 points
Other Parts Discussed in Thread: ADS8321

I've been using the ADS8321 a lot lately in prototypes I'm working on for use with strain gauges. I'm using an Analog Devices AD623 to amplify the signal and then into the ADS8321. It's been working perfectly with several setups on proto-boards and a couple of PCB's I design. However recently two started reading out in a peculiar way.

Say it's reading out -5096. If I apply load to the strain gauge instrumented lever, it'll jump to -5104 then -5108, 5110,5111 then 5623, 5879. So it's cycling changing only each bit (8, 4, 2, 1, 512, 256 onwards )and inbetween not changing the lower bits until it reaches 0 and then jumps to 512.

However the loads related to these values are generally correct. It's like variable length dead zones.

What's strange is that there are two on the current board and both started acting up at the same time.

Has anyone experienced TI ADC's with such a werid output? should I jump to replacing the IC's? Is something bridged? I haven't done any work on the board in a few weeks?

  • 0
    TI__Guru**** 174226 points

    Hi Keith,

    Is it possible to share a schematic and some screen shots of the SPI interface to your board?  Was there anything you can think of that might have caused an over voltage condition on the ADS8321 analog or digital inputs?  If you do remove and replace the chip, does this jump go away?

  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    I'll see what I can do about posting schematic however I think it's not the fault of the ADS8321. I've spent the last few hours debugging.

    I have small individual boards from an earlier project that is the same amp and ADC. I decided to wire this into my new project. With the old microcontroller it works fine, with my latest one it reads as previously described. If I disable the new microcontroller and wire the old controller to the SPI interface via the ISP header, the old microcontroller reads peculiar on the new board. Thus the only common part is the board. I'm suspecting I have a partial short or something leaking voltage to the data line or grounding it out.

    I've tried several times. The standalone ADS8321 seems to work fine with one controller and not the other. The ADC seems to be fine each time I move it back to the old controller. Doesn't make a lot of sense, but seems it's not the ADC.

    Sorry and thank you

  • 0 in reply to Keith Wakeham
    TI__Guru**** 174226 points

    No worries -

    If you can send a screen shot of the SPI transmissions of the 'good vs. bad', we may still see something going on that might point you to a solution.