TLV521: What value of isolation resistor should be used

Viktorija Cecil

Part Number: TLV521
Other Parts Discussed in Thread: OPA320

Team, customer plans to use TLV521 op amp in thier design as a buffer for a voltage reference. As such, they plan to use it as unity gain follower with a 10μF filter capacitor. Their question is: What value of isolation resistor I should use for this configuration in order to keep the op amp stable?

Thanks

Viktorija

over 5 years ago

0 Caro Walter over 5 years ago

TI__Mastermind 22312 points

Hello Viktorija,

The capacitor value that the customer is trying to drive is very large. Therefore I believe that the circuit will have to be "slowed down" significantly to drive it, especially in unity gain configuration. This forum post goes into further detail:

Taming Oscillations—the capacitive load problem

Other Parts Discussed in Post: OPA320 We’ve been looking at stability of op amps, considering how phase shift (or call it delay) in the feedback path can cause problems. Picking up from last week, stability…

By Bruce Trump over 13 years ago in Archives > The Signal

4 comments

As for the calculation of Riso, the customer may follow the guidance outlined in this ppt (also linked in the article above): https://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/01-864-00-00-00-66-32-73/Solving_5F00_Op_2D00_Amp_5F00_Stability_5F00_Wells_5F00_6_2D00_5_2D00_12-_2800_2_2900_.pdf

Specifically, Slides 34 to 59 provide two methods to calculate Riso.

All the best,
Carolina

0 Former Member over 5 years ago

Hi Viktorija,

I agree with what Caro said, but would just like to add that the isolation resistor is typically on the order of 10 to 100 Ohms. If you have a stability simulation set up, you can simply modify the isolation resistor with these types of values until you find an acceptable stability margin.

This is just a shortcut and rule of thumb for when you need to do this. Regardless of how you pick the value, it is a good idea to verify the stability margin before advancing to production.

Regards,
Daniel

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TLV521: What value of isolation resistor should be used