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AD7691 Pulsar Differential ADC Common Mode Input Spec

AD7691 spec sheet says Common-Mode Range needs to be Vref/2 +-0.1V.

I have a system who's inputs to the ADC is In-=0.1V In+=1.4V(up to 1.8V), and I don't see any issues due to this.

Can someone help me understand when this specification is important.

My guess is that it has to due with the input voltage range and max input voltage.  Is this spec just to ensure a full range (+-Vref) signal stays within the absolute input voltage range?

Thanks in advance,

Shane

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  • Lloben,

    Thanks for the reply, and the references they helped a lot.

    I am using a ADR361(2.5V) reference, the conversions are bursted at about 50kSPS; AD7691 draws 60uA @250kSPS & 5V ref, I estimated this will cause a 2.2uV error on Vref(<1/2LSB).  Hopefully my estimates here are correct, I have not measured it.

    The AD7691 inputs will be buffered with a ADA4805, the Iqq/low noise and low offset drove this choice.

    The 2 suggested Vrefs draw too much Iqq &/or have to large packages.     I believe the ADR 361 is okay.

    I believe the steps were causes by the sample/hold kickback and lack of recovery in time for the next conversion.  I have tested 1 circuit with a dramatic reduction in step size.  I will be testing a better circuit in the next few days.

    Thanks,

    Shane

Reply
  • Lloben,

    Thanks for the reply, and the references they helped a lot.

    I am using a ADR361(2.5V) reference, the conversions are bursted at about 50kSPS; AD7691 draws 60uA @250kSPS & 5V ref, I estimated this will cause a 2.2uV error on Vref(<1/2LSB).  Hopefully my estimates here are correct, I have not measured it.

    The AD7691 inputs will be buffered with a ADA4805, the Iqq/low noise and low offset drove this choice.

    The 2 suggested Vrefs draw too much Iqq &/or have to large packages.     I believe the ADR 361 is okay.

    I believe the steps were causes by the sample/hold kickback and lack of recovery in time for the next conversion.  I have tested 1 circuit with a dramatic reduction in step size.  I will be testing a better circuit in the next few days.

    Thanks,

    Shane

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