INA138 - Hi side current shunt monitor.  Designed specifically for this 
application.  We use them regularly to good effect.  Ideal for one-off 
or low volumes.  If you insist on rolling your own, take a look at the 
schematic on the front page of the datasheet  < 
http://focus.ti.com/lit/ds/symlink/ina138.pdf > and pay attention to 
common mode range as others have suggested (a rail to rail amp should 
keep you out of trouble).  Taken literally, that schematic errs by the 
base current of the transistor and minor opamp mismatches but you won't 
even notice that compared to trying to match resistors for a homebrew 
differential amplifier (with x100 gain).

Dave

Electron wrote:

><snip>
>That's why using an instrumentation amp seems ideal (Burr-Brown comes to
>mind).
>
>Or he could directly use an instrument-amp made just for this purpose:
>INA198 (or INA193,INA194,INA195,INA196,INA197, each one with its own
>configuration).
>  
>


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