Bob,
Normally, the differential input is supposed to generate a linear output of
the amp, where the amp output between min and max is a differential function
between 0 and the reference voltage you apply to the ref pin. I believe the
15 volt number you quoted is the max reference voltage you are allowed to
use by the manufacturer, not a must be number. This is how OP amps work. The
amp is linear. You, as the end user, determine the linearity range by
voltage assignment to the ref pin.
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?