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ADS8354: Valid Common Mode Input Range

Jeff Lee1
Prodigy 80 points
Part Number: ADS8354

The part states for the +/-Vref input range that the common mode voltage should be Vref/2 while for +/-2xVREF the CMV is Vref.  For a 5V analog rail the Vref VCM makes sense with +/-2xVref input range because Vref/2 VCM at 1.25V would clip things for 10Vpp inputs, but I mistakenly found that for the +/-Vref input range (5Vpp) using 2.5V for Vref that having the VCM at 2.5V also looks to work perfectly fine.  It would seem that no matter what the input range is having VCM at 2.5V for a 5V analog input rail would optimize headroom.  Is there anything reliabilty or performance wise that is really preventing me from just using 2.5V VCM for the +/-Vref input range?

  • 0
    TI__Guru 57336 points

    Hi Jeff,

    Thanks for your post. Please excuse the delay, we had a few folks out of the office last week, including myself.

    First, can you confirm that you are truly changing the input range mode by either reading back the CFR register or by comparing digital output values for the same input voltage? I would expect a differential input voltage of VREF to reach near positive full-scale (0x7FFF) for the "+/- VREF Range" and only reach about positive mid-scale (~0x3FFF) when using the "+/- 2xVREF Range." Is that what you're seeing and all you're saying is that the performance still looks ok when VCM is fixed to mid-supply?

    Best regards,

  • 0 in reply to Ryan Andrews
    Prodigy 80 points

    Ryan,

    I confirmed changing the input range scales correctly with always using 2.5V for the CMV. 
    Regards,
    Jeff Lee
  • 0 in reply to Jeff Lee1
    TI__Guru 57336 points

    Thanks, Jeff. I'm waiting to hear back from our systems/design team to understand why this may seem to work and what possible implications it may have.

    Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 80 points

    Ryan,

    Thanks for working this.  Since the ADC is differential and the inputs are within well within the 5V supply and ground rails, (if fact better centered with the 2.5V for the +-VREF range) it doesn't surprise me.  I'm just wondering why the datasheet thinks 1.25V is better since the analog inputs would get closer to rails.

    Regards,

    Jeff

  • 0 in reply to Jeff Lee1
    TI__Guru 57336 points

    Hi Jeff,

    Thanks for your patience.

    The VCM conditions in the data sheet are restricted to VREF/2 (+/-100mV) for the VREF range and VREF (+/-100mv) for the 2xVREF range to cover all combinations of AVDD and reference voltages. In some cases, when VREF is close to AVDD, this VCM restriction becomes very important in order for certain internal nodes to operate properly. I cannot go into more detail about exactly how close VREF and AVDD must be for the VCM limits to apply as it varies from device to device. 

    If it seems that a mid-supply VCM is working well for you, then I think you're ok. Please keep in mind that it does violate the data sheet operating conditions and we will not guarantee that all performance specs will still apply.

    Best regards,