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OPA2171 excessive input bias current

Brendon Slade
Prodigy 20 points
Other Parts Discussed in Thread: OPA2171, LM258, OPA141, OPA192, OPA625

Hi.

Please refer to the attached screenshot of the simplified circuit.  This is used in a DC application.

Notes:

1. The negative input is not necessarily at circuit ground potential and the output FET driver has been simplified to just Q1.

2. Not shown is that Q1 can be disabled and has been for these voltage measurements.

3. I cannot see any signs of instability on my 'scope, and the presence of C11 doesn't affect any results.

4. I originally used an LM258 which worked fine except it suffered from input phase reversal so I opted for the OPA2171 which appears to have other issues in my circuit.  I'm wondering if the input phase reversal protection in the OPA2171 has anything to do with this.

I'm using both devices in the OPA2171.  The first one (U3A) is a simple differential amp.  This part of the circuit is working without issue.

The second amp (U3B) is causing me to scratch my head a bit.  If you examine the various voltages you can see there is current flowing through R18 and R17 which is affecting operation.  It appears there is a current path between the inverting and non-inverting inputs.  Am I over-loading these inputs in any way?  The LM258 doesn't present this problem.

Any help would be appreciated.

Regards,

Brendon.

  • 0
    TI__Guru* 93105 points

    Hi Brendon,

    The OPA2171 does have back-to-back protection diodes across the inputs. That would explain the current path between the inputs. The voltage drop across R18 isn't equal to the drop of R17 so some of the input current flowing through R18 is going elsewhere. The OPA2171 cannot be used here because of the diodes.

    You may want to consider the OPA141 JFET, or OPA192 op-amps. They do not have input claming diodes. Find their data sheets here:

    http://www.ti.com/lit/ds/symlink/opa141.pdf

    http://www.ti.com/lit/ds/symlink/opa192.pdf

    Regards, Thomas

    PA - Linear Applications Engineering

     

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    Thanks for your reply Thomas, I'll look into those two devices you suggest.


    What you say confirms some additional measurements I made after my original post, where there appeared to (loosely) be a single diode drop between the input pins, no doubt affected by the internal diode protection resistors.

    This is what had me wondering because, unless I'm missing something, nowhere that I can see in the OPx171 datasheet (Sept 2012 from TI website) are the input clamping diodes mentioned, either by spec or diagram.  If this was specified in the datasheet then I wouldn't have selected this device for this application.  Compare that to the OPx172 datasheet (Dec 2013) for example, where it is clearly specified: Absolute Maximum Ratings: Differential input voltage +/-0.5V; diagrams: section 8.2 and figure 46.

    Regards,

    Brendon.

  • 0 in reply to Brendon Slade
    TI__Guru* 93105 points

    Hi Brendon,

    I agree that the diode information would have helped you and me as well. I had to locate the OPA2171 internal schematics, and then verify with one of the product designers that the diodes were in place across the inputs.

    Our amplifier data sheets have been going through an evolution and even a year difference can mean including the diode information included, or not. To that end, we have standardized on a more inclusive, and hopefully more useful format, for our product data sheets. You can see as an example the just released OPA625 data sheet. It uses the new format and Fig. 69 includes the internal diode information:

    http://www.ti.com/lit/ds/symlink/opa625.pdf

    We will be updating many of our data sheets to use the new format. This is a big undertaking as one can imagine.

    Regards, Thomas

    PA - Linear Applications Engineering