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OPA140 input protection by antiparallel diodes?

Karlheinz Storck
Prodigy 30 points
Other Parts Discussed in Thread: OPA140, OPA197, OPA191, OPA192

I'm using a single gain AD8605 (Supply 5V) and an OPA140 as a non inverting 50x amplifier (Supply: +%/-5).  Both positive inputs are tight together. Small signals are amplified by OPA 140 while higher levels are routed through the single gain amplfier into a different ADC input. In front I have a 10k resistor for current limiting purpose when something goes wrong. With higher voltages (approx. 2.5V at the pos. input) I see a significant current in the pos. input of opa140 (voltage drop at 10k resistor).

Does the opa40 has something like antiparallel diodes between the inputs without notice in the docs?

  • 0
    TI__Expert 3555 points

    Kartheinz,

    Thanks for using the OPA140 for your application. This device has turned out to be a good CMOS amplifier workhorse.

    Concerning your application, I am finding a puzzle with the fact that you are seeing a 'large' drop across your 10 Kohm input resistor. The input bias current of the OPA140 is typically +/-0.5 pA with a room temperature maximum of +/-10 pA. You are telling me that the amplifier is in a gain of +50 V/V in a +/-5 volt system. If the input bias current is pulling through the 10 k resistor you will see a worst case voltage drop of 100 nV (DC). Then at the output of the amplifier you can measure this error as a +/- 5 uV.  Is this the type of error that you are seeing in your circuit?

  • 0
    TI__Guru 62885 points
    Hello,

    Could you share the schematic you're testing on? The OPA140 is a JFET amplifier and while many JFET amplifiers include back-to-back diodes, the OPA140 does not. We'll double-check the design archive to be sure, but something might come to light when we see the schematic.
  • 0 in reply to Collin Wells
    Prodigy 30 points

    Hi all,

    here is the schematic of this detail. I have to correct the amplication which is 10.

    During a selftest I apply voltages from 0.5V to 5V to a selected channel of the mux and read the voltages via the IC23 voltage follower.

    DAC:0.5 Read: 0.497
    DAC:1.0             0.996
    DAC:1.5             1.496
    DAC:2.0             1.994
    DAC:2.5             2.387
    DAC:3.0             2.506
    DAC:3.5             2.908
    DAC:4.0             3.271
    DAC:4.5             3.628

    The DVM-tested voltage at node TP1SB is very close to the DAC-settings.

    Next step will be removing R4 to see where current flows.

  • 0 in reply to Karlheinz Storck
    TI__Guru 62885 points

    Hello,

    The input common-mode range of the OPA140 is being violated in this circuit which is why you're seeing this behavior.  For a +/-5V supply the input common-mode range of the OPA140 is -5.1V - +1.5V.  Therefore, applying voltages to the non-inverting input that are greater than +1.5V will result in undesirable operation, namely significantly increased offset voltages, lowered open-loop gain and bandwidth. Also, it can be seen in the Ib vs. VCM graph that the input bias currents begin to increase significantly once the input common-mode range is violated. 

    If you power the OPA140 with the +/-12V rails the troubles should be resolved.  If you'd like to power the op amp with the +/-5V rails then consider a rail-to-rail input device with similar performance such as the OPA191, OPA192, or OPA197

  • 0 in reply to Collin Wells
    Prodigy 30 points
    Thanks Collin,
    I have overseen this, because it is not the operational range of this section. I was not aware that violating the VCM will result in current.
    Regards
    Karlheinz