[sdiy] need help with a filter

Dan Snazelle subjectivity at hotmail.com
Sun Dec 22 00:28:21 CET 2013


just so you know
attached photos do not show up on the list
could you post on flicker or some other website and post the link?

thanks

Sent from my iPad

> On Dec 21, 2013, at 3:34 PM, "Nugent, Allison C. (NIH/NIMH) [E]" <nugenta at mail.nih.gov> wrote:
> 
> 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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