On 5/8/05, ChrisEGroup@rocklizard.org wrote: > Hi Everyone, > > Thanks for the feedback. I'm getting some good information. > > Below are answers to all the questions posted plus some additional > information. > > * I made an error on the schematic. The gain is 100x. I am using 10K/100 > Ohm resistors in each OpAmp voltage divider. > > * The circuit OpAmp is being powered from +5v/0v. From the posted > comments it looks like this is too low for the range I need. I have a > +12 volt supply available on my "real" circuit for a greater range. > I don't think the LF351 includes the - supply in its common-mode range. so +12 and 0 won't work. +5 and -5 might. > * The OpAmp inputs range from 0V TO 50 mV - My goal is to measure > current in the 0mA to 500mA range (possibly lower since I see the issue > with such a large range) with the 0.1 ohm resistor at 5 volts this is a > differential measurement range of 0V to 50 mV. I want to scale this to a > 0V to 5V range (or as close as possible) so that I can use a PIC A/D to > measure the current flowing into the circuit. The LF351 has a 10mV offset voltage. This is really going to affect linearity when your signal is 0 to 50 mV... that's 20% of full range! I suggest you get a better-suited op-amp, with low input offset voltage and the negative supply in its common-mode input range. (input offset current isn't critical). And get one rated to run from a single 5V supply. A bipoplar-input amp is more suited to this circuit than a JFET amp, in my opinion. Regards, Mark markrages@gmail -- You think that it is a secret, but it never has been one. - fortune cookie -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist