[sdiy] generate 0v from out of nothing..

[Tony Clark]

> Isn't there a old analog-guru trick to generate 0V from out of nothing? This
> would help me a lot because my biggest problem with the pcb is the
> ground-signal (which is used almost everywhere)-
> 
> I think about using a free opamp in my TL074. Just generate a voltage near
> 0V from V+ and V- with a voltage divider from the rails, and connect them
> with the positive and negative input of a opamp. In theory this should give
> me a nice 0V at the output, shouldn't it?

   You don't mention a feedback loop and that may be your problem.  Since 
the open loop gain of an op-amp is HUGE, any small deviation from the 
positive to negative input pins will cause all sorts of wonderful 
oscillations and what not.
   The best thing to do is have your two resistors (one from positive 
supply, one from the negative supply) tied to the positive 
pin and then tie the negative pin to the output, standard voltage 
follower circuit.  Even better would be to have a trim pot between the 
two resistors so that you can trim out any small offset that may be 
present due to errors in resistor values and supply voltages.

> I've never did it.. maybe that thing begins to oscillate or so.. or it
> outputs anything but not 0V.. Is this practical? (I don't put a big load at
> the 0V. maybe 1 or 2 mA - just need it as a reference for other opamps).

   It should work fine.  Most op-amps can easily source (or sink) 10 
times that.

   Tony

-----------------------------------------------------------------------
  The E-Music DIY Archive - http://aupe.phys.andrews.edu/diy_archive
-----------------------------------------------------------------------

"We wouldn't want to ship something that doesn't work"
   - Carl Stork, general manager, Microsoft Windows division
     Excerpt from EE Times April 2, 2001