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LMC6484 unexpectedly high input bias current

Jeremy Hicks
Prodigy 20 points

Hi, forgive me if this is a stupid question but I am not good with circuits.  I am using the LMC6484AIMX op-amp as the basis of a transimpedance amplifier.  I have included a picture of my circuit.  It simulates correctly with the parameters shown, and I have demonstrated experimentally that it works (i.e., produces a detectable voltage at the OUT yellow marker) over a range of IPD from 2nA to 2mA.  IPD is a photodiode.  D2 is a Schottky diode with turn-on voltage around 0.5V.

The below circuit is only for negative voltages from the SMUs.  SMU force and SMU sense boxes are models for voltage/current measurement/source units.  Sense has very high impedance and force has very low.

The problem is that when I try to not use the amplifier but instead measure low currents using SMU force and see what the leakage current is when I force a certain voltage.  I get very high currents of 1mA when I expect the leakage current to be around 100nA from the photodiode.  I suspected an extra leakage path from the 6484, so I measured a 6484 IC at various terminals (no power supplied) and found that resistances between any two pins range from 3M-ohm to 30M-ohm.  This is much different from the <1pA input bias current in the spec sheet, from which I would expect G-ohm or higher input impedance instead of M-ohm.  What am I doing wrong here?  

  • 0
    TI__Guru 53846 points

    Hello Jeremy,

    What are the supply voltages for the amplifiers?

    You cannot simply "measure" the resistance of the inputs with a ohmmeter. You are looking at a diode.

    There are ESD protection diodes from each pin to the supplies.

     

    Depending on the open-circuit voltage of your ohmeter, and the applied direction, you will get different "resistance" values.

    Normally, with power applied, the diodes are reversed biased and out-of-the-way.

    As long as the applied voltage to the inverting inputs are within the amplifiers supply voltage, the leakages should be sub-picoamp.

    When using SMU's, you must pay attention to polarity, and how you set the current value and compliance limit. SMU's are "four-quadrent" devices, capable of sinking and sourcing. If you set the polarity wrong, it could be trying to force a voltage the opposite of what you expect.

    Again...what are the supply voltages? You cannot apply a voltage to the inputs that exceed the supply rails.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 20 points
    Paul, thank you for the reply. The schematic of the inside of the IC is illuminating, and this could possibly be what's going on.

    My supply voltages are 10V and -5V. At the time, I was biasing up the system using -2.5V on the sense. The force would have been some value that is lower (more negative) such that the voltage at sense = -2.5V. I do not believe that the force voltage would fall below -5V because the diode between force/sense would begin to conduct significantly if Vense-Vforce > 0.5V. I am confident that the polarity was correct on the SMUs.

    About the multimeter, I don't have access to the open circuit voltage, but safe to say that I would be happy if the resistance were "only" 30M-ohm at this point. The bigger problem is the mA current readings while biased up.