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ADS8904B: Single-ended input?

James Dupre
Prodigy 70 points
Part Number: ADS8904B
Other Parts Discussed in Thread: ADS8881,

Hi,

I just want to confirm that it's *not* possible to drive the AINP input with a single-ended signal while the AINM input is connected to a fixed DC voltage. I believe this approach can be utilized for pseudo-differential inputs but my understanding is that the ADS8940B has fully differential inputs. Therefore, the common mode voltage requirement that (VAINP + VAINM)/2 = VREF/2 must be met at all times and cannot be met if AINM or AINP is held at a fixed DC voltage. Is my understanding correct?

Regards,
James 

  • +1
    TI__Mastermind 38720 points

    Hello James,

    Yes, your understanding is correct.  If you drive the ADS8904B as either a single-ended input (VAINM=0V) or pseudo-diff input (VAINM=Vref/2), the ADC will still function, but performance will be less than a general purpose 12b ADC.  You can either use a fully differential amplifier to convert a single ended input to fully differential input (Figure 111 in the datasheet), or use a different ADC, such as ADS8881, which can be driven as a fully diff, pseudo-diff, or single-ended input and still maintain 18b level performance.

    Just to be clear, the common mode voltage requirement allows some variation, and is specified in the range of Vref/2-0.1 up to Vref/2+0.1V, or in the case of Vref=5V, the common mode voltage can range from 2.4V up to 2.6V.

    If interested, please refer to the TI Precision Labs - ADCs , ADC Drive Topologies, for more information on this subject.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 70 points

    Thank you very much Keith.