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Original question:

ADS1298: Input Differential Dynamic Range

ADS1298: Input Common Mode Range

Colleen Leung
Prodigy 40 points
Part Number: ADS1298
I had a difficult time understanding the way the data sheet was presented and I just wanted to make sure my understanding was correct.
If my Vmax_diff was 0-2.5V with a Vdd=3V, Vss=0 and a PGA gain=1, setting my CM to 1.25V this would cause the output of the PGA to saturate at the correct conditions correct? 
In the case where I do not connect any of the channels to RLD drive, the RLD_out will act like a buffered output of RLDREF which I can set to (AVdd+AVss)/2 which I can use to bias the body correct? 
Thank you.
  • 0
    TI__Guru 66446 points

    Hi Colleen,

    This may have already been communicated to you offline via email, but I will post the reply here as well just in case.

    If you set the common-mode to 1.25 V you would be out of the CM range with a 2.5 V maximum differential. The equation is saying the PGA output requires 200 mV on either side of headroom which means 2.5 V would only barely fit in there.

    Using the internally generated mid-supply RLD voltage helps keep you in the correct range. It can also help with common-mode rejection as explained in this app note.

    Let me know if you have any further questions!

    Best regards,