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OPA735 Zo measurement showing wrong values

Other Parts Discussed in Thread: OPA735, OPA734, OPA333, TINA-TI


I am following Tim Green's Operational Amplifier Stability - Part 10 of 15: Capacitor Loop Stability: Riso with Dual Feedback (acqt_092407.pdf) steps to determine best dual feedback setup for OPA735 as reference driver and ADC input driver.

OPA735 is just as OPA734 used in his example, so I found pretty much the same results as in the paper.

Paper curve:

1) I managed to complete all steps some weeks ago, but I just tried to repeat the simulations and the 3rd step - Determine Ro (pg. 24 of pdf file) - gives a wrong result.

The weird is that I have used the same circuit previously, copied exactly from the Tim Green's material, and I had the right curve (as attached, Ro = 130,15 ohms). Same AC sweep as before, from 1Hz to 10MHz.

Something has changed, I can't tell what it is! Now, TINA tells me Ro = 42,29 ohms, although fz remains good at 91,84 Hz.

2) I tried the same circuit with OPA333 and also can't get the right Ro: TINA tells me 71.72 ohms and fz = 2.24Hz vs. ~ 2kohms, as per datasheet.

3) Yet a third weird thing is that, when I simulate AC sweep from 0,1 to 10Mhz, results get messy: Ro from 0,1 to about 0,2 show a strange curve and phase diagram gets completely off over the entire freq. range!

I have reinstalled TINA (Version SF-TI) in another VM, but have the exact same results. Have also redownloaded OPA735 and 333 TINA Pspice models, thought maybe corrupted files, no success. Attached all curves and circuits mentioned above.

Tim Green's papers series previously in:

seems to be offline right now, attached anyway.