[sdiy] need help with a filter

[Nugent, Allison C. (NIH/NIMH) [E]]

One more thing I can add - in addition to the oscillation when there's no input, it also appears to not actually be filtering when I do give it input.  

Allison Nugent
MRI Physicist
Experimental Therapeutics and Pathophysiology Branch
NIMH/NIH/DHHS
Ph 301-451-8863
________________________________________
From: Gordon JC Pearce [gordonjcp at gjcp.net]
Sent: Saturday, December 21, 2013 2:41 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] need help with a filter

On Sat, Dec 21, 2013 at 06:33:45PM +0000, Nugent, Allison C. (NIH/NIMH) [E] wrote:
> Hello,
>
>
> I don't have a lot of experience building stuff like this, and problems that are obvious to more experienced folks are not terribly obvious to me.   But I really want to get this thing finished...  if you have any ideas, please let me know.  I'm happy to post a schematic, if that would be helpful.
>

If you could draw out a diagram of exactly what you've built and how you're creating the virtual ground, that would be helpful.

Quite often opamp circuits get right uppity about being run with the inputs and outputs floating, especially if there's some feedback with a bit of phase shift around them.  Like, oh for instance, a filter...

Check your virtual ground is properly decoupled, and try sticking a resistor to that point from both your output and input to ensure they're not floating.

Have you got an oscilloscope?

--
Gordonjcp MM0YEQ

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