Bob,
One more point. Op amps are really neat. I quoted the case of a static
voltage at the ref pin, but it can also be dynamic, where you may wish to
adjust the output of the motor drive dependant on carriage velocity. You
would then find a tachometer (linear DC generator) and apply that voltage
through another Op amp to the reference pin on the motor driver. If you use
a negative voltage and not ground it is also possible to use the motor drive
as a dynamic brake. You have lots and lots of options here. The data sheet
from the manufacturer is your friend.
Steve

"Bob Engelhardt" wrote in message
...
I'm using a dc motor & a servo amp to run the lead screw on my lathe. The
servo amp is an Advanced Motion Controls model 25A20:
http://www.a-m-c.com/download/datasheet/25a20i.pdf

I'm setting the speed with an analog input on pins 4 & 5 ("REF"). This is
described as "Differential Reference Input (±10 V Operating Range, ±15 V
Maximum Input)". My assumption was that this voltage could go to 15v, but
beyond 10v there would be no additional effect on the output. I was wrong.
The output continues to increase as REF increases to 15v. I didn't check
if it was linear in the 10 - 15v range.

The question is: is it a bad idea to use it in the 10-15v range? It might
be useful to have the extra output in certain special cases. E.g. rapid
reverse.

Thanks,
Bob

PS - is there another NG that might be better posted to?