OPA4244: Output impedance

BenoitBosquet

Part Number: OPA4244

Dear all,

I'm interested to have more information on the graph posted by Paul in the following thread https://e2e.ti.com/support/amplifiers/f/14/t/786914

Can we consider Zo@DC equal to Zo@10Hz (=727,7 Ohms)?

Could you please confirm that data are related to open loop output impedance? To get OPA4244 closed loop impedance is it correct to divide Zo(Aol) by the following factor: Aol/Acl?

ex: @10Hz , op amp OPA4244 (Aol=106db) configured as a voltage follower (Acl = 1) then Zo(cl) is about 727,7/199526/1=0,0037Ohms.

Thank you and best regards,

Benoît,

over 4 years ago

0 Paul Goedeke over 4 years ago

TI__Mastermind 30600 points

Hi Benoit,

Yes, you are correct - the closed loop output impedance is Zo/(1+AolBeta). This video discusses this in some detail:
training.ti.com/modern-op-amps-output-impedance

Let me look closer into the data for low frequency and get back to you on your question regarding estimating the DC Zo.

Thanks,
Paul

0 Paul Goedeke over 4 years ago in reply to Paul Goedeke

TI__Mastermind 30600 points

Hi Benoit,

I looked closer at the data and our test set-up and the roll-off at low frequency is due to the design of our test PCB, which is AC coupled. You're seeing the high-pass roll-off, which is reducing the results.

For an estimate of the DC Zo, I would simply take the peak value given in the plot of about 1400 ohms. Is this value important to your design?

Thanks,
Paul

0 BenoitBosquet over 4 years ago in reply to Paul Goedeke

Prodigy 210 points

Hello Paul,

Thank you for your help on this. We want to use this part to drive the ref pin of an instrumentation amplifier. INA datasheet states that for best performance it is recommended to maintain a source impedance to the REF terminal that is below 1 Ohm.

The ref pin is not intended to be updated on a regular basis, it is why I'm interested to know output impedance at DC. I think you are right, an estimate should be enough. So if 1400 Ohms can be considered as the worst case value then we are below the 1 Ohm specified by the INA datasheet.

I'm curious to know what is your test configuration. Can you share this? Thanks.

Benoît,

0 Paul Goedeke over 4 years ago in reply to BenoitBosquet

TI__Mastermind 30600 points

Hi Benoit,

We use a custom PCB to measure the Aol and Zout of an amplifier and then calculate the Zo from those results. I can't share the schematics with you, but there are several good documents floating around online which discuss measuring op-amp parameters. The key is to use a false summing junction and buffer amplifiers to get the results.

www.eetimes.com/author.asp

e2echina.ti.com/.../_5E00_Op_2D00_Amp-Test-2008_5F00_04_5F00_23_5F00_0900_5F00_3_5F00_DavidBaum.pdf

Thanks,
Paul

0 BenoitBosquet over 4 years ago in reply to Paul Goedeke

Prodigy 210 points

Thank you for the links.
Best regards,
Benoît,

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OPA4244: Output impedance