OPA192: Basic question of "Precision Reference Buffer"

Takumi Suzuki1

Mastermind 8620 points

Part Number: OPA192
Other Parts Discussed in Thread: TINA-TI

Hi,

Let me ask you a basic question about "precision reference buffer" as below.(Figure69, page34)

I do not understand the behavior of this circuit well.

Will you tell me the behavior?

- Why do you design "2 feedback roops" ?

I suppose that "IN roop" is for high frequency noise, and "OUT roop" is for DC.

- How to calculate RF, RFX, CF, RISO, CL

- Will you tell me "transfer function" of this circuit?

Thank you for your support.

Best Regards,

over 7 years ago

0 kai klaas69 over 7 years ago

Guru 140200 points

Hi Takumi,

you might want to look into this thread:

e2e.ti.com/.../2612170

Kai

0 Thomas Kuehl over 7 years ago in reply to kai klaas69

TI__Guru* 93105 points

Suzuki-san,

Please see the e2e link that Kai suggested. If you need anything more please let us know.

Regards, Thomas
Precision Amplifiers Applications Engineering

0 Takumi Suzuki1 over 7 years ago in reply to kai klaas69

Mastermind 8620 points

Kai-san,

Thank you for your kindnes.

Best Regards,

0 Takumi Suzuki1 over 7 years ago in reply to Thomas Kuehl

Mastermind 8620 points

Hi Thomas-san,
Thank you for your kindness.

Though I checked the thread, I did not undrstand well the behavior, sorry.
I suppose that inside feedback is for phase compensation and outside feedback is for DC stabilization.
Is my idea correct?

I do not understand how to determine the R/C value.
How do you determine the value of Riso(37.4ohm) and Rfx(10K), Cf(39nF), Rf(1K)?
Why did you select 37.4ohm for Riso and 1kohm for Rf?
For example, is it OK to use 10Kohm for Rf not 1Kohm?

And do you have "transfer function" of this circuit?
I think it is a little bit difficult to calculate transfer function of this circuit...
If we have to use TINA-TI to determine R/C value when we try to change the value, it seems not to be very convenience I suppose.

Thank you for your support.
Best Regards,

0 kai klaas69 over 7 years ago in reply to Takumi Suzuki1

Guru 140200 points

Hi Takumi,

the circuit shall be a reference buffer. Such a buffer usually has a big output capacitance. But if you connect this circuit as a voltage follower you will run into trouble, because the open loop output impedance of OPAmp would create a low pass filter with the output cap and a phase lag in the feedback would be introduced. This phase lag would erode the phase margin and the OPAmp would become unstable.

Now a trick is introduced: The output capacitance is a bit isolated from the output by the help of Riso. And from a point in front of Riso a cap (CF) is wired to the inverting input. This cap makes the opposite and introduces a phase lead into the feedback loop and partially recovers the phase margin again.

Rfx is needed to limit the current into the input circuitry of OPAmp, I guess. Riso should be sufficiently high, but not too high, because the output currents will cause a voltage drop across is. And finally Rf must be rather small, because it will create another phase lag in combination with the input capacitance of OPAmp again.

All this is very sophisticated and it's much easier to run a simulation and change the values of the feedback components to see what's happening instead of solving the transfer function. That's why I have suggested you to run a TINA-TI simulation.

Kai