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AD7626 input range

Question asked by StephenD on Apr 20, 2012
Latest reply on Apr 26, 2012 by awalshe

Hi all,

I've an issue with driving my AD7526 ADC - I'm driving the ADC inputs differentially and so at the -FSD end of my input range IN- = 4V and +IN+ = just above 0V i.e so (IN-) - (IN+) = -4V. My ADC drivers are powered from a single +5V rail.


My problem is that my sensor waveform which is also differential, can move momentarilary to beyond the -FSD point described above and forces the amp driving IN+ into non-linear negative saturation. In this case I've aberations on the waveform as the op-amp returns to a linear mode before the CNV is applied.


I can add a DC offset to counteract the issue described above and keep the last op-amp from being over-driven. I then have a second issue with this approach - since the over-drive issue is only momentary, but the offset applied is DC, by the time the CNV pulse arrives I've quite a large baseline offset in terms of ADUs.


The AD7626 datasheet shows many of the driver circuits to be powered by dual (+ve and -ve) rails but the datasheet also quotes the input voltage range WRT AGND as -0.1V. I'm wondering what precautions these circuits have from driving a -ve wavrform into the ADC?

     My reason for bringing this point up is a thought that if my last op-amp was powered by a dual supply it would never saturate, as my waveform would only force the ADC input just below 0V for a short period.


Any advice?


Thanking you in advance.