THS3491: Zol test and parasitic capacitance of noninverting terminal questions

you would have to show the full circuit including load. Also, you imply oscillations, please show that. 

Hi Michael,

There is no load. And I thin the board is too thin, so the noninverting terminal is too close to GND causes a big Cin. So I want to add a Cf parallel with 300Ω to compensate Cin. It would be very convenient to know the Zol curve and Rf+NG*Ri curve. How can I do this?

Thanks for your help!

B R,

Owen Yan

Well you never use a feedback Cf with a current feedback amplifier, also you need to isolate the SMA capacitance through a series 50ohm out - usually to a cable then into a 50ohm scope or spectrum analyzer. 

Hi Owen,

current feedback OPAmps become instable when the feedback resistance falls below a minimum value. Because of that, exactly follow the feedback resistance recommendation given in table 2 of datasheet:

As you can see 300R is way too small for stable operation.

And don't forget the isolation resistor or series termination resistor at the output of OPAmp. Never directly touch the output of an HF-OPAmp with the scope probe.

Finally, you should not build up the circuit on a board that might not be optimized for a GHz-OPAmp. To guarantee success, only take the original THS4391EVM to fabricate your demo

Kai

Hi Kai,

As you said, I built the same circuit with THS3491RGTEVM and the thin board. The Rf of both of them are 300Ω. While EVM is stable and thin board is not. So I think there is two possible scenarios to explain why the thin board is instable.

As in the first scenario, Rf is too small to make the PM＜0. And in the second scenario, Cin added a zero to make PM＜0. Experiment results show that EVM is stable with Rf=300Ω, and thin board is in stable. In this way, I think Cin is the key factor that makes the THS3491 instable cause the thin board is too thin while EVM is much more thicker.

Am I right?

Finally, could you please offer the Zol of THS3491 in different package? Thanks a lot for your reply!

B R,

Owen Yan

Hi Michael,

I see the loop gain of THS3491 in the datasheet, but not the open loop gain Zol. Did I miss understand something? Could you please give some guidance about which page shows that?

Thanks a lot for your help!

B R,

Owen Yan

Hi Kai,

Thanks for your help! You are right, I have some misunderstanding of stable. It's not stable when the output is around 0 with no input.

B R,

Owen Yan

Hi Michael,

Thanks for your help! Your article is very helpful!

B R,

Owen Yan.

Hi Michael,

Could you please give some guidance about how to set Davis KLU matrix that is mentioned in your article. Thanks!

B R,

Owen Yan

Well Owen,that is an analysis option that sometimes help convergence in TINA V11, I found out later that option is not available in the V9 available free from TI.

Hi Michael,

I followed your method in the article to build this circuit:

The Ri and L are calculated to be 4.2ohms and 73.9nH which is similar to yours. But as you can see in the next curves:

PM=45.76°  fxover=27.88MHz which is said to be 43Mhz in your article. Could you please help explain? If it's related about the version of TINA? Besides I downloaded the OPA684 model at ti.com.

OPA684Ri and L - Copy.TSCThis is the TINA simulation files I built. Please give some guidance. Thanks a lot for your help!

R3 is in the wrong place on the THS3491 sim, hard to read, can you insert file

3250.OPA684Ri and L - Copy.TSCYes, you are right Michael. I changed the R3 but the results still couldn't match.

Ok, here are some V11 sims, your file first shows the numerical chatter I see sometimes, 

Now try going to different analysis options (very bottom key under Analysis), available in V11, not V9 apparently, 

Yes, that looks better, 

I don't recall what I put into that article you are looking at, is this closer now? From the plots in this article, I would guess the closed loop F-3dB would be about 1.6*57MHz = 91MHz with about 4.2dB peaking? 

https://www.planetanalog.com/stability-issues-for-high-speed-amplifiers-introductory-background-and-improved-analysis-insight-5/

Sorry you don't have these analysis options, there might be other ways in V9 to squelch that chatter. 

Thanks a lot Michael! My final question, can I get the access of TINA V11? Give some guidance please.

Well I bought V11 some years back, they have newer versions now for sale from DesignSoft. Not very much money I recall. Here is your closed loop sim, little off as the loop gain is >2nd order, 33.94dB DC gain, about 2.8dB peaking and 74MHz F-3dB. You can flatten these out scaling the Rf and Rg up. 

Hi Michael,

Michael Steffes said:

Here is an article on how to test phase margin for current feedback, 

https://www.planetanalog.com/stability-issues-and-resolutions-for-high-speed-current-feedback-op-amps-insight-7/

Clever!

Kai

Thanks Kai, this is just adapting the VFA approaches to CFA. 

Hi Michael,

I am trying to derive the bandwidth extension ratio but failed. And I verified the 1.57*fx=f-3db in TINA simulation, but I want to understand it from deriving. So please give some materials about it.

B R,

Owen Yan

I have not published that, and currently do not intend to