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In Reply to: RE: decoupling an LM4562 op amp posted by Wally on May 05, 2009 at 09:44:26
Usually it's power supply pins to ground
Follow Ups:
The consensus on here to stop oscillation is a cap directly between the v+ and v- pins as close to the point they leave the chip, rather than bypassing to ground! That's what I've done with other chips like the 6172 over the last 6 or 7 years. I'm just asking for anyone's particular experience with the 4562.
The consensus on here to stop oscillation is a cap directly between the v+ and v- pins as close to the point...
....My experience, and my instinct, is that that would make oscillation worse. But I can't prove anything.
Thanks for the reply. I suppose it might do that in some situations. I wonder what the theory would be to explain it.
Well, my take is that if the capacitor connects the rails at ac, when one rail goes up it will want to pull the other one up. When it goes down it will want to pull the other one down. So ground is staying where it is and the whole amplifier is jumping up and down relative to it. If the positive input is referenced to ground, well, it's seeing [what we think is ground] jumping down and up, and it wants to amplify the jumps. Because it's an amplifier and it wants to amplify things that are applied to its input.
This can't be good. You want the amplifier to be on a secure platform, if for no other reason than decorum.
M
I suppose in theory both rails will be shorted to ground by decoupling caps earlier on, but any inductance from the tracks to the point where the rail connects to the IC will mean that the decoupling will become less effective as frequency increases. I think the stability of the opamp circuit is also affected by capacitive negative feedback (through actual capacitors and stray capacitance). So I think some circuits and opamps will be much more affected than others.
What cap and value do you use for v+ to v- for 6172? I recently added Rubycon ZLH 2200uf from pin 4, 8 to ground and it has good result. Never tried 4562 but it should be the same, I guess.
I use 2 x 0.47uF BG Nx-HiQ caps connected in an L cancelling pair. The 6172 is much faster and has a wider bandwidth so is more likely to oscillate than the 4562. The newer chip also has better noise rejection, so in theory it should not be quite as touchy as the 6172. Whether practice is the same as theory is another matter though, hence my request for people's experience. I suppose I'll just have to suck it and see.
...can't believe nobody's got any experience to share.
If current is taken from ground you should of course have caps between supply voltage and ground. A good start is to have 100 nF /63 V polyester, ceramics etc, physically small caps, between V+ and gnd and V- to gnd. Avoid big caps since their resonance frequency become lower => works as inductance over the resonance.An another good idea is like someone else point out is to read the datasheet for the opamp in mind.
Edits: 05/09/09 05/09/09 05/09/09
Thanks for the reply. Yeah, reading the datasheet was the first thing I did. Just wanted some real world experience.
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