At 07:29 PM 3/30/2010, ivp wrote:

>Point A is at 10.04V and Point B is at 9.96V. A difference of 80mV, which
>equates to 80/210 * 1000 = 380rpm, and that's what the needle indicates

I would try buffering each input.  I suspect that the meter driver 
has significant output impedance and the unequal input impedance of 
your diff amp is loading the meter driver un-equally.

Note: input impedance on the (+) input is 20K, input impedance on the 
(-) input is 10K.

Its quick to try and should solve your problem.

dwayne

-- 
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
www.trinity-electronics.com
Custom Electronics Design and Manufacturing

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist