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Original question:

ADS124S08: Range of REFN0 and REFP0 ...and resulting digital code?

ADS124S08: Can I do single ended and bipolar with AVSS=-2.5 and AVDD=2.5v?

Rodo
Prodigy 70 points
Part Number: ADS124S08

Hi,

Can I do both single ended (VREF internal 0-2.5v )and bipolar conversions (REFN0=-2.5v and REFP0=2.5v) with AVSS=-2.5 and AVDD=2.5v? or does AVSS=0 and AVDD=+2.5v  are required for single ended conversions? The assumption is the single ended analog input signal will be from 0 to +2.5v and the differential analog signal will be -2.5v to +2.5v.

Thanks

  • 0
    TI__Guru** 113765 points

    Hi Rodo,

    Yes, you can do single-ended measurements with bipolar supply configuration.  Using the internal reference, the full-scale range is +/-Vref/Gain.  Your measurement will only use the positive range from 0V to +2.5V when using the internal reference (or 1/2 of the full-scale range). 

    One thing I should note is the internal reference should be referenced to AVSS (REFCOM connected to AVSS).  When the reference is enabled it will appear to be 0V if you measure from AGND.  When measuring the REFOUT voltage you should measure with respect to AVSS.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 70 points

    I'm confused by "REFCOM connected to AVSS". AVSS is -2.5v.  I was expecting to connect REFCOM to the board GND with is the same as DGND. That way I have the internal VREF at +2.5v and REFCOM at 0v. Making VREF=2.5v.

    Thanks

  • 0 in reply to Rodo
    TI__Guru** 113765 points

    Hi Rodo,

    The internal reference with respect to bipolar analog supply can be confusing and that is why I mentioned it in my previous post.  You can see the proper connections in Figure 119 on page 98 of the ADS124S08 datasheet.

    The internal reference is derived from the analog supply, so if REFCOM is connected to AGND, it is very difficult to maintain an output of 2.5V from REFOUT if the supply is also at 2.5V as there is no headroom to establish a good reference output.

    The datasheet on page 6 has a Pin Functions table where it is stated REFCOM should connect to AVSS.  REFOUT will be 2.5V relative to AVSS, so if you measure from AVSS to REFOUT you would measure 2.5V which is the internal reference voltage output.  So the reference is still 2.5V.

    Best regards,

    Bob B