[sdiy] while we're talking about scaling & biasing etc

[Ingo Debus]

Am 25.05.2009 um 20:01 schrieb Jason Proctor:

>> Which opamp are you using?
>
> thanks for the response.
>
> mc34072. i picked it because it was rail-safe.

I've got no experience with this one.
However, the data sheet doesn't say that there are some special  
tricks required to keep it stable (at least I don't find any hints).  
Figure 26 (Phase Margin and Gain Margin versus Differential Source  
Resistance) indicates that this opamp is happier when connected to  
lower resistances. Perhaps you can lower the resistor values of your  
circuit? But perhaps I'm on a completely wrong track here.
Here's a quote from the data sheet: "As with most high frequency  
amplifiers, proper lead dress, component placement, and PC board  
layout should be exercised for optimum frequency performance. For  
example, long unshielded input or output leads may result in unwanted  
input–output coupling." But this surely applies to many other opamps  
too.

It would probably be helpful if we knew more about the "fuzziness" of  
the opamp's output. Is it noise, or a single frequency? Can you  
trigger your scope on that signal?

Ingo