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THS4541: Common-mode loop supply headroom to negative supply

Dan Welch
Prodigy 130 points
Part Number: THS4541
Other Parts Discussed in Thread: THS4520

I have a design where my ADC's typical OCM voltage is 0.95V, but it could possibly be as low as 0.90V. I see that at 25 degrees C I am within the THS4541 negative headroom limit, but from 0 to 70 degrees C, I am not. I am not as concerned about the higher temperatures, but will be running the part from -10 to 50 degrees C. Do the higher or lower temperatures reduce the headroom voltage and what is the greatest effect of being at the headroom limit. Is it just a OCM Vos  greater than 12mv?

Thanks,

Dan

  • 0
    TI__Genius 15975 points
    Hi Dan,

    Yes, the higher or lower temperatures do reduce the headroom voltage, and have the greatest effect of increased offset voltage OCM Vos along-with signal harmonic distortion. If you are concerned about the OCM voltage being too close to the negative supply, would it be possible for you to shift the negative supply voltage down by -0.5V and subsequently shift the positive voltage down?

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 130 points

    Hello Rohit,

    Thank you for the response. Actually, I was switching from the THS4520 to the THS4541 in order to eliminate the need for the negative supply. I was hoping that my worst case Vocm from the ADC of 0.90v would be within the THS4541 headroom. The minimum specification for the THS4541 from 0 to 70 degrees C is 0.91v, so my worst case is only 0.01V below the specification. I would think that the 0.01V difference would have little affect on the OCM Vos. Maybe it would be 13mv instead of 12mv?

    Dan

  • 0 in reply to Dan Welch
    TI__Genius 15975 points
    Hi Dan,

    I think you should be able to use the THS4541 near 0.9V Vocm from 0 to 70 degrees C without a significant degradation in Vos and HD.

    Best Regards,
    Rohit