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ADS1220: single ended measurement = differential meas. with one input on GND?

seb
Genius 4170 points
Part Number: ADS1220

Hello there,

maybe a very simple question for some of You :)

We are using the 24bit ADS1220 Sigma-delta ADC, everything is basically working the way it should, now come the difficult tricky parts as we are doing some noise evaluations and stuff in our actual environment.

Principally we want to measure a single ended signal voltage coming from a transimpedance amplifier, so maybe from 0V = GND to 2Vpeak, but it can also be very low, as low as for example 1mV.

So we thought the PGA would come in handy to measure the really small and the really big signals.

So we tried the ADS1220 in different config settings and are now a bit confused about wheather to measure single ended ( = no PGA allowed, due to Common-mode restrictions, right?), or ( in our previous tests proven less noise ) differential mode measurments.

But now the tricky question: Is it still pseudo differential if we put GND on eg channel 1, and our real signal on channel 0, then we do a differential measurement from CH0 - CH1, is this allowed or is it the same thing as a simple single ended measurment for CH0 to GND???

Right now i am assuming it is the same as a single ended one, since internally the PGA will produce error results, since its range is not specified any more, right?

Thanks for reading and replies.

seb

  • 0
    TI__Guru** 113765 points

    Hi Seb,

    For the ADS1220 internally the measurement is always differential (AINP with respect to AINN).  In the case of a single-ended measurement, only the positive portion of the scale is used as AINP cannot go less than AINN.  It makes no difference if the mux connection is internal where ANN is connected to AVSS (where AVSS is AGND when using a unipolar analog supply), or if you connect to one of the analog pins to AGND.  The common-mode restrictions will apply in either case, and you need to disable and bypass the PGA. 

    If you want to use the PGA there are a couple of possible options.  You could force the TIA to use some reference point above ground (1/2 of AVDD for example) where the output of the TIA is now setting the common-mode at the reference point (which will be connected to an analog input pin assigned to AINN) and the TIA output connected to AINP.  This approach will allow you to use the PGA.

    Another approach would be to use bipolar analog supplies (+/-2.5V) where AGND is within the input common-mode allowed for the PGA.

    Best regards,

    Bob B