OPA227: Closed Loop Output Impedance

Art Mecina

Genius 16505 points

Part Number: OPA227

Hello Guys,

Good day.

Our customer wanted to know the closed loop output impedence of OPA227U.

Is this available?

Thanks in advance!

Art

over 3 years ago

0 Michael Steffes over 3 years ago

Guru 47545 points

The closed loop output impedance is the open loop impedance divided by 1+loopgain. That loop gain will depend on the external ckt. This will simulate easily. The datasheet does show an open loop output impedance plot - I find its flatness over frequency questionable.

And there was a model update in 2019, that one does simulate flat at 26ohms. Having done a number op amp output stages, that is kind of rare to be that constant.

0 Raymond Zhang1 over 3 years ago in reply to Michael Steffes

TI__Mastermind 48336 points

Hi Art,

Thanks Michael for the explanation and simulation.

The open-loop gain is very high in this part. If loop gain is at 100dB at 10Hz, then the close loop output impedance at 10Hz is 26Ohm/(1 + 10^(100/20)) = 260 uohm.

Please let us know if you have additional questions.

Best,

Raymond

0 Michael Steffes over 3 years ago in reply to Raymond Zhang1

Guru 47545 points

But again, it is really the Loop Gain over frequency (which will be decreasing) not the Aol. So you need the external ckt.

The DC closed loop output impedance would be 260uohms at gain of 1 - if you are operating noise gain of 1000, it will be 0.44ohms.

0 Raymond Zhang1 over 3 years ago in reply to Michael Steffes

TI__Mastermind 48336 points

Hi Michael,

Yes, the customer needs to show the circuit. I was meant to show a hand calculation as an example at Gain=1, which I did not make it clear.

Thanks,

Raymond

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OPA227: Closed Loop Output Impedance