Always be wary of schematics
Posted by Matthew
Ladyada has an interesting rant on her new blog. In linear circuit analysis, we tend to ignore the difference between the positive and negative inputs. However, in practice positive feedback results in a bistable circuit. (Also read up on negative feedback.)
Usually everyone ends up remembering this detail the hard the way. While designing a tape head preamplifier she was utilizing this circuit from an application note:
After a few hours of staring at the circuit and debugging and wondering “man why the hell is this railing?” I finally look back at the datasheet and realize: oh its in positive feedback, of course its railing.
Just a reminder that the textbook (or pdf) isn’t always right!
You MUST name the text/reference book this came from! WE MUST KNOW! The author and publisher must be publicly shamed.
It actually comes from an application note. I am unsure of the specific source because she did not reference it.
Either way, it is an easy mistake to make and easy to correct.
It’s from the Analog OP27/37 http://www.analog.com/UploadedFiles/DataSheets/5021673OP37b.pdf
page 13 for those who want instant gratification
Was just reading through this and had to chuckle. I ran into a problem of a schematic for Boss DS-1 simply having the inv/noninv inputs marked backwards… however, they still had the pins numbered correctly, and after confirmation with a couple other partial schematics, managed to build the correct op amp stage that didn’t rail at what sounded like maybe 4-5 gain at the most, heh!