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TLV8541: What does "Input impedance: 2pF mean on datasheet?

David Linker
Prodigy 60 points

Part Number: TLV8541

I am looking for a very high input impedance op-amp, and was looking on this data sheet, and under input impedance it listed 2pF. I can understand that as the input capacitance, and that is what is in that part of the table on other parts. Maybe this is just a typo.

How I can find out the input of an op amp?

Thanks,

  • 0
    TI__Mastermind 27580 points
    David
    Sorry for the confusion, it really isn't a typo but it should be labeled input capacitance. If you want to get a better feel for input impedance from a resistance (DC) perspective, I suggest you look at the input bias versus input common mode voltage plots (figures 16 to 21) where RIN = VCM/Ibias. the input impedance is quite high on this family of nanopower amplifiers.
    Chuck
  • 0
    Guru 140200 points
    Hi David,

    specifying the input impedance with the input capacitance makes sense, because for nearly all AC applications the input capacitance will play the major role when looking at the input impedance. This has to do with the extreme high DC input impedance of this Chip. An input bias current of about 100fA means an DC input impedance in the Terraohm range. But a 4pF common mode input capacitance, on the other hand, makes the input impedance drastically decrease in an AC application. At 1kHz 4pF gives an impedance of 40MOhm, at 100Hz 4pF gives 400MOhm, and so on. Only at frequencies below 0.04Hz the 4pF common mode input capacitance gives an impedance of more than 1TOhm.

    Kai