Hi All,

I am considering using an OPA2228 op. as a unity gain input buffer/driver in a summing amp as a virtual earth mixer, but understand these chips should only be used at a min.gain of x 4-5. as they are unstable below this point & hence will oscillate!

Q; would putting a low pF cap across the feedback loop solve this problem? What value should this be then? Is there any other workabout to make it stable?

I then found out that OPA2227 is a more suitable chip for my purpose, but I've already bought the opa2228!...haste is waste!

What are the differences between OPA2227/8 with suffixes P/PA in their characteristics. Couldn't find much info Can someone throw some light on this please.

Thanks.

 

All opamps have a "frequency compensation capacitor" inside. Most are stable when their gain is set to 1. Some have a smaller capacitor inside so that their high frequency response is better but they are not stable at low gains.

adding a capacitor axcross the feedback resistor makes the gain at high frequencies 1 and will cause some opamps to oscillate and will not fix the oscillation of a de-compensated opamp.

 

P is for PDIP, I think.
The "A" type has a bigger input offset voltage, see specs.