Servo amp question

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?
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Thanks for all the responses. To clarify what I'm doing: I had set the gain on the servo to give a maximum feed rate of 1/2" / second, with a 10v input. I have an op-amp controller feeding the servo 0 - 10v. But I can get an out-of-bounds condition where the op amp output is 15v.
So I think that I'll put a 10v zener clamp on my controller output for the forward feed, but use -15v for the more-rapid reverse.
Bob
BTW - this is turning out very nicely & I expect to be writing it up for the DropBox.
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