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Subject: Re: OPAMP buffer instability

Date: 03/29/01 at 8:05 AM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: OPAMP buffer instability posted by ordepat on 03/27/01 at 12:26 PM:

Hello, Ordepat.

Several things come to mind.

1) It's a truism that Vcc bypass capacitors are very important. I suggest a 0.1 F from each supply voltage pin to ground. However, there is a very interesting applicaiton note by Analog Devices suggesting a bypass capacitor from the +Vcc to ground and the second 0.1 F from the -Vcc pin to the +Vcc pin and NOT to ground! From memory, I think (I'm not quite certain) that the item is in their Application Note AN-202.

2) I never allow the deliberate use of a capacitor tied directly across the two input pins of an op-amp. The unavaoidable loss of phase margin can clobber an op-amp's stability!

3) Many op-amps are intolerant of capacitive loads on their outputs. Many op-amps will exhibit parasitic oscillations during their most negative output voltage swings when capacitively loaded. If you are driving a fairly capacitive load, some resistive buffering may be required.

4) Finally, just to state the obvious, do make sure that all leads are very short and also be sure that your chosen op-amp is rated for stability at your intended gain. This last point shouldn't be a problem really as most op-amps are rated to be stable at unity gain.

Good luck.

John Dunn - President
Ambertec, Inc.

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