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    Operational amplifier driving capability test

    Dear friends,

    I want to test the driving cabability of the operational amplifier, mean to test how much maximum current could the op-amp drive (which defines the maximum load that can be connected )

    Thank you in advance

    Regards

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    Re: Operational amplifier driving capability test

    Simply do load value sweep with observing THD or SNDR.

    However load could be capacitive.


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    Re: Operational amplifier driving capability test

    Thank you Pancho,

    I am only intereseted in driving current cabability

    What about sweeping a DC current source the output of the opamp in buffer connection where the input is hold at the VCM. Then the driving cabability is defined at the region where the output is equal to input, is this test right ?

    Thanks



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    Re: Operational amplifier driving capability test

    Load current could be sink and source direction.
    So simple DC current source is not appropriate as load.


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    Re: Operational amplifier driving capability test

    Quote Originally Posted by pancho_hideboo View Post
    Load current could be sink and source direction.
    So simple DC current source is not appropriate as load.
    Thank you Pancho,

    Then how to simulate the sourcing and sinking capability individually, that is what I am interested in indeed



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    Re: Operational amplifier driving capability test

    The described setup (setting a fixed output voltage) doesn't fully test "driving capability", you want to evaluate the available output current over a defined output voltage range.


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    Re: Operational amplifier driving capability test

    There are several output current measurements you might
    like, relating to different concerns.

    Many op amps (especially older medium-voltage bipolar ones)
    have explicit short circuit current protection. This tends to
    be simple, Vbe/R type local feedback at tens of mA. You can
    measure by force-to-rail, measure Ipin.

    However this is a protection function, not a signal function.
    It will "wind up" the front end and distort badly the back end
    if you're up against it.

    A more fidelity-oriented measure, is how much load current
    you can draw while still meeting specs like Vio. This tells
    how much load while still being as accurate as you picked
    the part for. Put the op amp in a classical "servo" test loop
    set up to measure Vio, and sweep the load +/- recording
    the Vio (or its proxy voltage, for post-calculation) and call
    the "violates Vio" point your max load for accuracy.

    You could also apply the same "test logic" to AVOL, PSRR et
    al, depending on what you know about the application "care-
    abouts".


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    Re: Operational amplifier driving capability test

    Hi,

    What happens on overcurrent?
    Is the output weak, an thus the output voltage drops?
    Or is the ouput strong and the Opamp overheats?

    Klaus
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    Re: Operational amplifier driving capability test

    I attached a 1k resistor load on a 741 op amp. Specs say max output current is 10-25 mA.
    I was surprised to see output voltage drop 5 percent. Current draw wasn't near 10 mA.

    However it makes sense in terms of the output impedance. Typical figure is 50-75 ohms. 741 internal schematic suggests a similar amount. So 1k ohm calculates to about a 5 percent load. Hence the 5 percent voltage drop.


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    Re: Operational amplifier driving capability test

    Hi,

    I attached a 1k resistor load on a 741 op amp. Specs say max output current is 10-25 mA.
    I was surprised to see output voltage drop 5 percent. Current draw wasn't near 10 mA.
    I don't expect a 5% voltage drop on a non saturated, feedbacked OPAMP.

    Thus I expect the test setup was either in "non feedbacked" condition, or the Opamp output was in "saturated" condition or both.

    Also it's not clear if this is a DC test, or an AC test. If AC: what frequency and what waveform?
    I also expect that the Opamp behaviour depends on Opamp supply voltage...especially when at or close to saturation.

    *******
    @OP:

    which defines the maximum load that can be connected )
    I don't this is true.
    There are (at least) to test setups to determine drive caoability.
    * with load (typical operating condition), here you have to define THD for AC, or voltage drop for DC
    * or without load = short circuit condition, no typical operating condition. Here the output is externally forced to a given voltage. But since this is the "without load" setup it can not be used to determine the max load that can be connected.

    You need to define your test conditions.
    * supply voltage
    * load (resistor, short circuit, constant current)
    * load connection (to supply rails, to VCC/2, or anything else)
    * limiting condition ( "equal", or "not equal" is no valid condition for an analog system, because you never can expect two analog voltages to be exactly the same. You need to define a deviation.

    Klaus
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    Re: Operational amplifier driving capability test

    Dear friends,

    Thank you very much for your interaction to my post,

    To shortlist what I have understand from you, measuring the driving (sourcing and sinking) is subject to op-amp performance regarding output voltage and THD. I was further reading until I found an explanation brief explanation from the book introduction to cmos op-amp and comparators by Gregorian.

    He was designing a buffered folded op-amp to drive 50 Ohm from 3 V supply voltage, in his design specification he wanted the amplifier to be able drive 2 Vp-p output voltage. As you see from the page picture below he connected the amplifier as an inverter and applied a sin signal in transient simulation. By this way he could notice the drop in the output voltage at this load and the non-linear distortion.

    Should I follow this method ?

    Thank you

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    - - - Updated - - -

    Recently I found this test from a thesis,

    the author is connecting the op-amp as a comparator, he measure the sourcing cabability by making Vin+ > Vin- so the output is positive, then he sweep a connected current source at the output to monitor the drop in the output voltage, then he do the same when the output is low to test the sinking

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  12. #12
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    Re: Operational amplifier driving capability test

    This looks like a CMOS op amp with a current limited
    output. You still need to decide whether you want the
    "ultimate current" (for reliability etc.) or the minimum,
    maximum current (for load driving rough analysis) or
    "the max current with all else still in spec" (for accuracy).


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