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LTC2494 Datasheet Error

I am looking at the datasheet of LTC2494 (Rev E).

On figure 12 (page 28), it says that the formula for the common mode voltages is half of the difference between the input voltages.

Shouldn't it be half of the sum? (in+ + in-) / 2

Also, the formula for the average input current looks wrong too, shouldn't it be Vin(cm) / Req, since the equivalent resistor is 1/(fswitching*Csample) and fswitching is fsw/4 since there are 4 inputs.

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  • For the purposes of determining the input current, I believe all formulas are correct. For the traditional definition of common mode voltage it would be the addition of the two voltages divided by two.

  • Let's suppose we have the following setup:
    In+=5.1V
    In-=5V
    Ref+=0.1v
    Ref-=0V

    Your formulas as is would indicate that the input current is 0 in this case, which is very strange considering the large voltage difference between the reference and the input.

    Let's suppose we have the following setup:
    In+=0V
    In-=2.5V
    Ref+=5V
    Ref-=0V

    Your formulas as is would indicate that the input current is -2.8uA in this case.

    Let's suppose we have the following setup:
    In+=2.5V
    In-=0V
    Ref+=5V
    Ref-=0V

    Your formulas as is would indicate that the input current is -923nA in this case, even though only the polarity of the input is what changed.

    Can you give an equivalent circuit for the input that would make this all believable, please? Because right now it is hard for me to see how these formulas could be correct.

    Do you have graphs that show actual measurements of this input characteristic?

    It's hard to show that a precision measurement circuitry one designs will have the required characteristics when its hearth is so "mysterious" and things in the datasheet mean the opposite of what they are generally understood to mean.

  • I tried measuring the input current and you are correct. The common mode equations should show addition not subtraction.

  • When do you think, we could see this being corrected in the datasheet?

    It's been a few months since the error has been discovered.

  • Tech Docs has limited resources during this work from home period and most of that time is spent doing data sheets for new products. Patience is required.

  • Hello,

    It has been one and a half years and the datasheet has still not been corrected.

    How much more patience is required?

    Regards,

    Adam

  • Tech Docs is still understaffed and only working on new products. I have been told this situation may change in the near future but even after it has changed there will be a large backlog of data sheets and user guides to get through.

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