Paul Hutchinson wrote:

>
>In addition to the good suggestions others have made here's something I've
>seen often.
>
>If the load you're driving has too much capacitance you can get oscillation
>on the output of most op-amps. With a LM324 type it takes only a few tens of
>pF to make the output oscillate. Some op-amps can tolerate a lot more load
>capacitance. To decouple the op-amp from the capacitive load simply add a
>small (10 to 100 ohm) resistor in series with the output. If the series
>resistance causes too much drop in the output you can move the feedback
>takeoff from the output pin to the other side of the resistor. National Semi
>used to have a good description of this problem and cure in the LM324
>datasheet.
>

        This is quite true. Sometimes you won't even see an oscillation, it
just appears as a DC offset from hell (nothing you do will make it go
away). I've had to use the resistor several times, with the DC feedback
resistor moved to the 'load side' of the series resistor (so that my gain
is not affected). Do not move your compensation capacitor though! You're
asking for trouble if you do, as you are dropping one of the pole
frequencies, reducing phase margin and contributing to the instability.
Keep the capacitor connected between the op amps output and the inverting
input.


        Jim Nagy
        Elm Electronics
  ICs for Experimenters
http://www.elmelectronics.com/

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