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ADS1120: Input Range With PGA Bypassed

Jacob Olstad
Intellectual 1320 points
Part Number: ADS1120

Hi,

measuring an input signal centered around mid supply and varying by a few hundred millivolts either direction.  Looking through the datasheet making sure we stay in the common mode range of the PGA may be difficult.  Since we don't need a ton of gain bypassing the PGA seems like a possible way to go.

With the PGA bypassed is the input still differential?  i.e. would a signal centered around 1.5V varying by +/-400mV with a gain of 4 work?

Regards,

Jake

  • 0
    TI__Guru** 113765 points

    Hi Jacob,

    The answer to you question is yes.  The ADS1120 will always measure differentially in the respect of AINP relative to AINN.  If AINP is greater in voltage the output code is positive.  If AINP is less than AINN the output code is negative.  In the single-ended case, you will only see positive output codes as AINP cannot go lower than AINN (AVSS=AGND).

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Intellectual 1320 points

    Thanks Bob,

    My main concern was making sure the switched capacitor stage the datasheet takes about still gained signals differentially,

    I'll take your response to mean that it does.

    Regards,

    Jake

  • 0 in reply to Jacob Olstad
    TI__Guru** 113765 points

    Hi Jake,

    With the PGA_BYPASSED the ADS1120 will work the same was as with PGA enabled with the advantage of a relaxed input common-mode.  You can have a maximum gain of 4 with PGA_BYPASSED.  The downside of the PGA_BYPASSED is lower input impedance.  Otherwise the measurement will work the same way.  So to specifically answer the original question, yes you can measure +/-400mV at 1.5V common-mode and gain of 4 using the internal reference.

    Best regards,

    Bob B