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OPA454: offset voltage depended on output current

ttd
Mastermind 7660 points
Part Number: OPA454

Dear Technical support team,

I have some question about OPA454.

The configuration is voltage follower and it shows offset voltage when output current is increasing.

0mA ：offset voltage 0.1mV

50mA：offset voltage 7.7mV

My test result and the detail of schematic is attached below.

result.pdf

I'd like to confirm that this offset voltage is either OPA454's behavior itself or not. 

And do you have a Vo(Voltage output swing from rail) on 50mA?

Datasheet has only Io=26mA.

Best Regards,

ttd

  • 0
    TI__Mastermind 23050 points

    Hi,

    Figure 10 in the datasheet (shown below) is intended to demonstrate how the output voltage swing changes with output current. Usually this figure is referenced to the supply rails, but it appears in this case it was only tested at maximum supply. 

    However, I do not believe the behavior you are observing is related to the output limitations, as you are more than a few volts away from either rail. This behavior is actually something you could see with any op amp. If you notice in the table you attached, as the output current increases, the Aol at which the measurement is specified also decreases. This is because the gain of the output stage decreases as you drive more current. You can think of Aol as a changing offset voltage with a change in output voltage, it is defined as Aol = Vout/Vos. As Aol decreases, to maintain a given output voltage the offset voltage must also increase. If for example the Aol at 50mA has degraded to approximately 60dB, then this would correspond to 1/10^(60dB/20) = 1mV/V. So for every volt at the output, you will have an additional 1mV of offset. Here is an article that explains this relationship in further detail if you are interested: 

    Additionally, the transfer function for a closed loop op amp is Vout = Vin * Aol / (1 + AolB), where B is the feedback factor. When Aol is large, this effectively reduces to 1/B, which yields the familiar gain equation of 1+Rf/Ri. As Aol decreases though additional gain error is introduced. If for example you are in a buffer configuration and Aol is at 60dB, this will introduce an additional 1mV of error. 

  • 0 in reply to Zak Kaye
    Mastermind 7660 points

    Hi Zak Kaye,

    Thank you for your reply.

    I understand that the increasing output causes offset on OUTPUT.

    I couldn't reproduce the this behavior between 0mA and 50mA with TINA TI.

    OPA454 simulation model doesn't have it , right?

    Please refer to  attached my simulation file.

    OPA454.TSC

    Best Regards,

    ttd

  • 0 in reply to ttd
    TI__Mastermind 23050 points
    Hi,

    Unfortunately the OPA454 is an older model and does not accurately model the effects of output loading as shown in the datasheet curve above. Additionally, our models do not currently include the effects of Aol degrading with increased output current, so you will not see the offset I described above in sim. We are constantly striving to improve our models though and hope to include these kinds of effects in the near future.