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ADS1220: ADS1220 AC excitation

jogi loew
Intellectual 290 points
Part Number: ADS1220

Hi,

I am using the ADS1220 with a load cell. I am using AC excitation (polarity of the bridge can be reversed) and ratiometric supply (the ADC is supplied with the same voltage as the bridge).

Is there a way to have the ADC working in continuous mode at 20Hz and reverse the polarity of the bridge at the same frequency? I would like to have AC excitation working continuously without having to trigger each conversion manually, like they do in this app note:

http://www.ti.com/lit/ug/tidubl0a/tidubl0a.pdf

I guess it would be a matter of perfectly timing the polarity switch, but I am not sure if that would work with a delta sigma ADC. Another way I could imagine would be to operate the ADC at 40Hz and throw away every second sample (where the polarity switch happens), but then I lose simultaneous 50/60Hz rejection.

Am I totally wrong with my assumptions or could this be made working somehow?

Many thanks,

Jos

  • 0
    TI__Guru** 113765 points

    Hi Jos,

    Notice in Figure 23 of the TI Design you reference, there is a delay for analog settling (approximately 20ms) after the change of excitation polarity.  Running in continuous conversion mode will add settling error to the result.

    Also, there is no strong benefit to running in continuous conversion mode aside from not sending the START/SYNC command.  This is really very little overhead.  Actually the START/SYNC command can be sent during the last byte read of the data.  You still get 50/60 Hz rejection at very close to 20sps if you read the data as soon as it becomes available.

    As an example, let's say you want to run in continuous mode and can accept any settling error (which I would then ask why you need to do AC bridge excitation in the first place), one way of accomplishing the task would be:

    1. When DRDY transitions from high to low, switch excitation polarity.
    2. Read results
    3. Repeat

    You can save some of the startup overhead of single-shot mode by issuing the START/SYNC command when reading the data which I would highly recommend for consistency.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 290 points

    Hi Bob,

    Thanks a lot for the explanation.

    I will start without AC for now and then do some further testing.

    Best regards,

    Josef