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Input current is Very high on LMC660AIM

DNAB
Prodigy 140 points

Other Parts Discussed in Thread: LMC660

We have a LMC660AIM that are connected as an ordinary amplifer. Its powered from an 12VDC supply. On the positive input pin we use a 100k serial resistor. The amplifer have a gain of 4 bu feeback resistors from output. On the 100k input resistor we put 2.5V, and have about 10V on the output. Nothing strange about that so far.

The strange thing is that I measure a 20mV drop over the 100k input serial resistor. This means that it will flow about 250nA to the maplifer positive input pin. And this is strange as datasheet say something about a couple of pA.

Why does if flow 250nA into the positive input pin of the LMC660 ??

(we have tried other internal amplifers and samples without any change)

  • 0
    TI__Guru 69306 points

    Hello DNAB,

    I assume you are measuring this with a DMM - and it probably has a 10Mohm input impedance.

    You are creating a voltage divider with the 100K input resistor and 10Mohm meter resistance.

    A 100K and 10Mohm resistor divider string, fed by 2.5V, would have 2.475V at the tap point (the amplifier input).

    2.5V - 2.475V = 25mV!

    Remember - you cannot observe something without affecting it.....

    The way to confirm this is to observe the output (with another meter) and see if it drops when you measure the input. I'm guessing you will see it move from 10V to 9.9V when you "measure" the input.

    The LMC660 typically has just a few femtoamps (1e-15A) of bias current. It would have to be severely damaged  to draw 250nA.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 140 points

    Thanks tor the answer. I feel pretty stupid right now. Your assumption was right, the DMM forms a voltage devide togather with my 100k.

    Thanks