Timeline for OPA211 weird frequency response --- why?
Current License: CC BY-SA 3.0
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| Feb 7, 2018 at 23:50 | history | edited | D.A.S. | CC BY-SA 3.0 |
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| Feb 7, 2018 at 23:12 | history | edited | D.A.S. | CC BY-SA 3.0 |
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| Feb 7, 2018 at 23:04 | comment | added | D.A.S. | rise time 10~90% = 0.35/BW. How much BW you need depends precisely on rise time. Example 1ns rise time for 350MHz into 50 Ohms 10Vpp = 200mA/ns, 10V/ns is only 5% of 50MHz square wave to get 7th harmonic slightly attenuated .. like this? See my related answers electronics.stackexchange.com/questions/352304/… this will do 1.2Vpp with 1200V/us. Current is the problem G = +2, < 0.1 dB Peaking, Rf = 750 Ω 350MHz min 440 MHz typ 100R load OK with 50 source then coax then 50R term | |
| Feb 7, 2018 at 21:54 | comment | added | Cal-linux | It's not so much a case of "I want 50MHz", it's more like "I need to see edges on squarewave differential signals, and I want those to be as sharp/fast and clean as possible".... I know that specs like "as ___ as possible" are dangerous --- one could always get to a $200M solution, which yes, is more ___ than the others! :-) I'm looking at alternatives such as the OPA847 (because it's pin compatible) and the THS4509... Now, you mentioned "current feedback" as a potentially better alternative. Could you comment on those vs. the couple of options I'm entertaining? | |
| Feb 7, 2018 at 20:33 | comment | added | D.A.S. | If you want a 50MHz linear gain, just say so and include all other specs | |
| Feb 7, 2018 at 20:28 | comment | added | D.A.S. | Zout=Zo/Av and when Av open loop =1 Zout=Zo and the stray pF matter when R feedback is too high. which is why they chose something < 1k.. If you learn how to compute ratios and intersect breakpoints, measure Q etc.etc on my log curve , is one way to look at it. | |
| Feb 7, 2018 at 19:30 | comment | added | Cal-linux | Couple of clarifications: (1) By "stray positive feedback (p3) being more than the stray negative feedback (p2)" you mean stray/parasitic capacitance from pin 6 to pin 3 is higher? (that seems sort of unavoidable, in that pin 3 is physically closer to pin 6 than pin 2). (2) Why does the open loop Zo matters? Once you use feedback, the output behaves as a nearly-Zo=0, but it doesn't matter, since it is the source of the wave; as for reflected waves coming back to the output, the 50-ohm Rout helps dissipate energy. Is my understanding flawed? | |
| Feb 7, 2018 at 18:53 | history | edited | D.A.S. | CC BY-SA 3.0 |
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| Feb 7, 2018 at 18:44 | history | edited | D.A.S. | CC BY-SA 3.0 |
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| Feb 7, 2018 at 18:37 | history | edited | D.A.S. | CC BY-SA 3.0 |
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| Feb 7, 2018 at 17:58 | history | answered | D.A.S. | CC BY-SA 3.0 |