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Originally Posted by Horse /img/forum/go_quote.gif
How can there be such a problem with such a stable opamp as the AD8620?
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Even slow, docile chips can be made to oscillate, and 25 MHz isn't slow.
It appears from further study of the datasheet that the bandwidth of this particular op-amp is probably what's to blame here, not some special thing about the chip itself. Another 25 MHz chip might behave the same way.
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what if one wants to use a better (and bipolar) opamp such as the LT1028? Or one of the LME duals? Or an LT1358? This could be easily done if that jung multiloop thingy was left out, one and for all |
It's easy to convert the PIMETA v2 to the standard feedback configuration: leave R6 out, and short R5. Balance the input offset currents, and there you are, bipolar-input based amp.
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Originally Posted by NelsonVandal /img/forum/go_quote.gif
Tangent, since you're not using the 1 nF cap bypassing the 10 k input resistor suggested by National I don't think 10 k is an appropriate value.
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Ah, I clearly misunderstood what they were doing there. And page 15 is suddenly so clear now. It all but told me this would happen.
Luckily the current board design doesn't have to change to fix it. Not that that would be a blocker. There are already changes coming in the next version. I just don't want to try to make room for more parts if I don't have to.
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I've read about instablity issues that's been cured by a 100R resistor. |
Yes, page 15 of the datasheet covers this, in the Bandwidth and Stability section.
Some buffers, like the BUF634, have this input resistor already. If you go back to the META42, it had this resistor, because the sadly departed EL2001 and 2002 don't have internal input resistors.
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Aries' real strength is in the breadth of their offerings. BrownDog doesn't have nearly as many adapter types.
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Originally Posted by Horse /img/forum/go_quote.gif
BTW maybe one could have chosen the LME49600 for the buffers.
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Or maybe one could not tolerate a footprint 50% bigger than a DIP-8 on a board that wasn't allowed to get bigger just to make room for it.
If they made it in a DIP-8 or SO-8, I'd definitely have tried it. Then again, if it were in that package with the same pinout as the BUF634, maybe PIMETA v2 wouldn't exist. Constructive destruction.
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Originally Posted by NelsonVandal /img/forum/go_quote.gif
Just to illustrate what happens in this circuit
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Awesome, thanks for that. The 10K goes.
The schematic won't change for a while yet, as my web host seems to be having network problems. (Ain't my server, really!)
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with an added 1nF cap in parallel, but I can't see why one would want capacitor coupling |
You might be forced into it with some chip combinations. Page 15 again.
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with no or a 100R resistor and a 3pF cap in parallel with R6 |
Beautiful, that's almost exactly what I see in the real world with 10pF across R6, only with somewhat rounder corners, as you'd expect.
I have some 1s, 2.2s and 4.7s coming, so we'll see how little capacitance we can get away with here.
So the next question is, do I shove parts out of the way to make room for the new C6L/R, or do I just have people use SMT caps with leaded R6es, and leaded caps with SMT R6es? It's fine with me, but maybe there are good reasons not to do it?
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You could have SMD 0805 pads on the bottom, under the R6 resistors. |
Almost every thru-hole part on the PIMETA v2 board has SMT pads on the bottom side of the board already. They're mostly 1206es, though. Nothing smaller than that.
The only place this isn't done, where it might have made some sense, are the transistors, which could probably accept SOT-23s on the bottom side. Not that explicit pads are really necessary.