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OPA171 large input offset current at differential voltage > 0 Volt

Eric Brocke
Prodigy 20 points
Other Parts Discussed in Thread: OPA171, OPA141, OPA140, OPA343, OPA314, OPA365, OPA377, OPA376, OPA337, OPA347

I use the OPA171 in current limiter of an input circuit. The circuit operates in two modi: a closed loop and an open loop. Closed loop no problem. In open loop there is also an large differential input voltage which caused a current (IOS) trough the feedback circuit (see simplified circuit below), and gives an extra voltage offset on Um. Ur is 2 Volt and Um can be from 0 upto 2 Volt.


Although not explicitly stated in the datasheet, i guess, the test condition of input offset current IOS of the OPA171 is at zero differential voltage. Is there extra data of the IOS available at higher differential voltage (in my case up to 2 Volt)?

Is another opamp available without this kind of problem. Specs: power supply 12 Volt, VCM including GND, package SOT553 (if not available SC70 or SOT23).

Thanx,

Eric

  • 0
    TI__Guru 63750 points

    Eric,

    The default test conditions used to test OPA171 parameters are shown at the top of PDS table - see below: Vs=+2.7 to +36V, Vcm=Vout=Vs/2, Rload=10k.

    Of course, any op amp MUST be in a close-loop configuration to test any of its parameters (that is the only way ANY op amp should be used), which results in a virtual short between the input terminals.

    Additionally, since the OPA171 has an internal back-to-back protection diodes between the input terminals, pulling the inputs more than 0.5V apart without limiting the input current to less than 10mA will destroy the part.  If you have to use the op amp in an open-loop configuration, please take a look at OPA140 and OPA141 PDS's - their input stage (though NOT recommended) can withstand an input differential voltage of up to 40V but the input common-mode voltage range is limited to (V-)-0.1V<Vcm<(V+)-3.5V

  • 0 in reply to Marek Lis
    Prodigy 20 points

    Marek,

    Sometimes the maximum differential input voltage VID is specified on the 'Absolute Maximum Ratings' table. In the case of the OPA171 it is not. Perhaps it is a good idea to specify it on all datasheets (in future).

    Thank you for your suggestion (OPA140) although it comes in an SOT23 package. Because of the many input circuits the size may be a problem. Are there perhaps other alternatives (in a SC70 package), with some other specs (power supply). It might be possible for us to reduce the powersupply voltage to a lower value.

    Can you indicate which opamp families allow a large differential input voltage? Then we can search ourselves for a suitable candidate.

    Regards,

    Eric Brocke

  • 0 in reply to Eric Brocke
    TI__Guru 63750 points

    Eric,

    The main reason we do not always specify the maximum input differential voltage, Vid, in our op amp datasheets is that, unlike comparators, op amps are meant to be used only in a close-loop configurations where feedback drives the inverting input to a virtual short with a non-inverting input; however, I do agree that it would be good to consistently specify the max Vid in all of our PDS's just in case customer intentionally or under fault conditions applies a large differential voltage between the inputs or allows the op amp to be used in an open-loop configuration.

    Besides, OPA140 and OPA141 previously mentioned, there are few other op amps with supply range above 12V that allow a large max Vid - for example TLC07x family (see attached).  Also, most of our low voltage CMOS op amps (with max Vcc=5.5V) like OPA337, OPA343, OPA365 including OPA376/OPA377, OPA314 and OPA347/348 in SC70 package can handle a full supply voltage between their input terminals.