On caps and op-amp feedback...

[gstopp at fibermux.com]

     Were you using one of those white poke-the-components-in-rows-of-holes 
     prototyping boards? If you did then that may be it - there's lots of 
     capacitance between adjacent rows in those things.
     
     - Gene


______________________________ Reply Separator _________________________________
Subject: On caps and op-amp feedback...
Author:  Christopher List <Christopher_List at sonymusic.com> at ccrelayout
Date:    4/17/96 11:48 AM


Howdy DIYers - 
     
A few questions...
     
I built the 2164-based mixer circuit I've been designing on a breadboard - 
worked fine. I moved it to a protoboard, soldiered it up, tested a channel and 
when the signal got above a certain level it would start to get all wacky at 
the peaks (tested with a triangle wave). Reaching into my 
"the-masters-do-this-so-give-it-a-try-even-though-it-seems-like-voodoo" bag of 
tricks I threw a 100pf cap in series with the feedback resistor on the current 
to voltage converting op-amp. Viola - it worked fine. Now this didn't take any 
great brain skills on my part to figure out 'cause it was shown this way in the 
data sheet for the 2164 - what I'm wondering is: 
Why was it stable on the breadboard w/o the cap and not on the protoboard? 
The data sheet for the OP176 op-amp had a terse explanation - something to do 
with "the source capacitance and source resistance create a pole with the 
feedback resistance and op-amp internal capacitance at the summing node (- 
input)". What do they mean by "pole"?
Is there something in the way I built it that I should look out for? 
This is one of those weird "stray capacitance's" things - isn't it? 
The 100pf cap acts as a sort of low-pass filter - right?
     
I should have a schematic for this guy ready in a day or two (based on my 
questions you're probably not interested!) - but trust me, it's a really cool 
module...
     
- Thanks,
Chris