Another op-amp stability question (LT1677)

John Miles, KE5FX · 2017-03-29T07:37:32+00:00

Tim W.'s question about the influence of resistance in the feedback path of a voltage follower reminded me of a similar problem I ran into a while back. I've used LT1677s as an integrator in several low-noise PLLs in the past and been satisfied with its performance. It's an old part and a fairly expensive one, but it's quiet and well-behaved in loop filters, with RRIO operation and guaranteed freedom from phase reversal. It's slow (GBP=7 MHz) but not slow enough to be a problem in most RF PLLs. You could ask for lower input bias current, I guess. But overall the LT1677 is a good "jellybean" opamp for use between a low-voltage charge pump and a high-voltage VCO. It's also characterized explicitly for use at Av=+1; see page 13 of the data sheet at . At one point, I needed a voltage follower for something I was breadboarding, and an LT1677 was already lying around on the workbench. When I put it in the circuit, I observed a nasty oscillation that didn't make any sense. It's easy to reproduce the setup that misbehaves. Tie pin 2 to pin 6 as usual, put a scope probe on pin 6, and feed a steady DC level or an LF tone to pin 3: When the input (and output) level approaches 1.13V, I see this: (when fed with sine wave) (when fed with DC from trimpot) Things that don't have a significant effect on the symptom: - Load resistance, either in series or parallel with the scope probe - Load capacitance (lots of C will cause instability as expected) - Source impedance (4.7K in this test bed, but doesn't matter) - Supply voltage (might be a little less prone to occur at >10V) - Resistance in feedback path (too much causes instability as expected, and it might get a bit better with some values of resistance, but I can always make it happen) - The usual suspects like power supply impedance and bypassing, layout, etc. Things that do have an effect: - The frequency of the input signal. If I increase the frequency from the function generator beyond about 1 kHz,...

Mar 29, 2017 26 Replies

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John Miles, KE5FX 9 years ago

Yep.

I routinely play with 10 GHz GBW parts. It's been a while since I stubbed my toe on a 7-MHz one.

Not in the data sheet I linked to, they don't.

If a data sheet from Linear Technology tells me that a part can be used as a unity-gain buffer, I'm inclined to believe it. I agree with the subtext behind Win's comment, in that ignorance of fundamentals is no excuse, but I don't agree that this is a "fundamental" topic. I believe that 99 out of 100 working EEs would have made the same ass-umption, so there needs to be an explicit warning in the data sheet.

-- john, KE5FX

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tabbypurr 9 years ago

Found it.

formatting link

page 6

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John S 9 years ago

There is an error in the schematic on that page. I could tell you where it is, but then I'd have to kill you.

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John S 9 years ago

If he can spell those high-power words correctly, he's just fine.

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krw 9 years ago

A dot connecting the wire crossing the +15V line. How long did it take you to fin it?

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John S 9 years ago

A second or two. It was immediately obvious.

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John Larkin 9 years ago

(If I don't make sense, it's because I just had an endocscopy and I'm still a little drugged)

John Larkin Highland Technology, Inc lunatic fringe electronics

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