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Original question:

LMV841 untity gain driving capacitive load with out-of-loop compensation

LMV844: LMV844 impendence test

Hangjie Wang
Intellectual 1230 points
Part Number: LMV844

Hi expert,

When we test Lmv844 the impendence of LMV844's IN+ pin, found that the impendence decreased with the increase of temperature when there was no power supply, Could you pls help to explain that? Thx. 

  • 0
    TI__Guru 67971 points

    The input resistance of any op amp, including LMV844, cannot be measured unpowered because by doing so all you measure is the impedance of the input protection diodes - see below. Since the forward bias voltage of the diodes decreases roughly -2mV per degree C, it will look in your test like the input resistance decreases with increase temperature.

    The actual input common-mode input resistance of LMV844 can be calculated from the graph of IB vs Vcm and it turns out to be 80TΩ at 25C and 533Gohm at 85C - see below.

  • 0 in reply to Marek Lis
    TI__Intellectual 1230 points

    Hi Marek,

    Thanks for your warm support, it's very detail, could you pls help me mark which protection diode it is? Is there a curve of diode forward voltage versus temperature? And could you please combine the test method of the multimeter and talk about why the measured impedance will decrease as the forward voltage of the diode decreases? thanks alot.

  • +1 in reply to Hangjie Wang
    TI__Guru 67971 points

    All diodes shown in Fig 34 are input protection diodes: D1 and D2 for overvoltage protection while remaining four are ESD protection (see label). 

    Below please see the typical diode forward bias voltage vs temperature. 

    What your customer does using multimeter has nothing to do with the input impedance of the op amp - all they do is to test forward (or reverse bias) resistance of the input protection diode - this is meaningless.

  • 0 in reply to Marek Lis
    TI__Intellectual 1230 points

    Hi Marek,

    Thanks for your answer, There are still some confusion is if IN- to V-, what should be the reverse voltage drop for ESD? why is8V/15pA? should be 8V/10pA? Δvalue?

  • 0 in reply to Hangjie Wang
    TI__Guru 67971 points

    Hangjie,

    I hope you understand that all our discussion of the input resistance of forward or reverse bias input protection diodes under unpowered conditions has nothing to do with the actual input resistance specified in the data-sheet since the latter is based on a reverse bias leakage of ESD diodes under powered condition. As far as Rin_cm at 85C goes, it should in fact be 800GΩ. 

  • 0 in reply to Marek Lis
    TI__Intellectual 1230 points

    Hi Marek,

    Your answer is very helpful to me, it is better if you can share me some technical notes which you accumulate, thanks a lot.Slight smile