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ADS7947: Max input current? (contemplating input protection)

Aaron Holmes
Prodigy 50 points
Part Number: ADS7947

Unlike some of TI’s ADC datasheets, the datasheet for the ADS794x series provides no maximum input current specification.  This is probably a big hint that I’m approaching my problem the wrong way, but perhaps somebody will give me a nudge in the right direction.

I’m designing an add-on front end for an existing circuit based on the ADS7947.  This circuit uses 3V for AVDD, but input will be provided by an op-amp with a +/-5V supply.  Under normal operating conditions this amp will be outputting a voltage that’s safely between 0V and AVDD, but the order of power rail ramp up/down and other events may result in the amp’s output voltage being momentarily outside of this range.  My first thought was to simply place a BAT54-type Schottky array in front of the ADC to clamp input to between 0V and AVDD +/- diode Vf, with current-limiting resistors on either side, but I can’t feel good about that if I don’t know what current I’m trying to keep it under.

Any advice would be much appreciated!

~Aaron

  • 0
    TI__Mastermind 22950 points

    Hi Aaron,

    I am not sure why this information was not added... There is a maximum input current specification for a similar device family (ADS795x) that has the maximum input current at +/- 10mA. I would say that is a safe bet for this family as well. However, you can limit the input current to +/- 1mA just to be on the safe side. 

    Regards,

    Aaron Estrada

  • 0 in reply to Aaron Estrada
    Prodigy 50 points

    Hi, Aaron - Thank you.  It's a funny omission.  I'd seen the same 10mA spec elsewhere, and TI's own lecture series on protecting A/D inputs references this spec.  I hate making assumptions, but designing for +/- 1mA to start with is obviously trivial and probably among the safer assumptions.  As well, given the settling time requirements of this particular application, I don't think this should have any detrimental effect on system performance.

    Very best,
    ~Aaron

  • +1 in reply to Aaron Holmes
    TI__Mastermind 22950 points

    Hi Aaron,

    I agree with your comments. I have made note of this missing spec and will bring attention to it in case of any future data sheet revisions. 

    Regards,
    Aaron Estrada