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<div>David said, Sun Nov 15 06:33:27 CET 2020:<br>
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<div>“After many years of building circuits, one acquires a basic sense of where stability caps may be required. Often, I discover that an opamp circuit is RINGING A BIT WHEN I GO PROBING AROUND WITH THE SCOPE. I tend to lay out the pads for stability caps
on most inverting opamp circuits, but often don't stuff them unless I find that I have to. I keep a big stash of ceramic 101s for just that purpose.” [Emphasis added]</div>
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<div>Is this “ringing” (ringing being a DAMPED oscillation following a discontinuity) or is it rather an actual oscillation (usually low level and HF looking like an out-of-focus trace ----caused by the scope cable capacitance? The problem is caused by the
scope cable and goes away when you stop looking! Then how can you know! A series 1k resistor on the scope cable is all you need. See my MW comments of Oct 20: </div>
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<div>“The second useful function of the 1k series resistors is that they “decouple” capacitive (typically shielded cables at perhaps C=100 pfd/meter) loads from op-amps thus preventing high-frequency (MHz) oscillations WITH ASSOCIATED DC SHIFTS. Connecting
a cable (even to a scope) directly to an op-amp output forms an RC low-pass (R being the inherent op-amp output resistance of perhaps 100 ohms). Such an oscillation is HF but low level (slew limited) and non-symmetric (non-symmetric slew limiting). The result
is a fuzzy looking scope trace (looks out of focus) and is only there when a cable is attached, and as a VCO control can cause a small but noticeable pitch shift.</div>
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<div>The oscillation occurs because the R is internal to the op-amp and the RC is INSIDE any feedback loop and contributes excessive phase shift. With the 1k series the RC (R now 1k) is OUTSIDE the op-amp’s feedback loop. Fuzzy trace and pitch shift gone.”</div>
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-Bernie<br>
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