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. * 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. *** So What Is This For? *** This OpAmp circuit will be used in a PIC controlled power supply. I'm using a PIC to control an adjustable voltage regulator and I figured that since there is going to be a PIC in the power supply why not add voltage and current monitoring. My power supply design has 2 5V @ 1A outputs and 1 adjustable output (0V to 12V @ 1A). I have planned an additional regulated output to power the PIC and the OpAmp. I will be monitoring all 3 voltage outputs and the current output of at least one of these supplies. I may decide to monitor all 3 or to provide a switching mechanism to monitor one of the three. I'm sill experimenting and designing so everything is open to change at this point. This is a "junk box" supply. I'm using a surplus switcher that outputs a clean 14V at 3A. I feed this output - with additional filtering - into the three voltage regulators. The PIC is for control of the variable supply and for monitoring the outputs. *** Some additional information and questions: *** * The circuit we are discussing in this thread is not meant to be my final circuit. It's sole purpose is to help me understand the design issues - I'm trying to keep it as simple as possible at this point. Once it's working I'll use all I've learned to build the real thing. * I'm using this OpAmp because I had it on hand and I'm using these resistor values because the math is simple. I do have other OpAmps - including some LM358s. * The OpAmp does need to be driven from a single sided supply since my power supply design does not have any negative voltages available. I can power the OpAmp with 12 volts instead of 5 volts to increase it's linear range. * My desired current measurement range is 0A to 1A total. I figure I will need to use multiple OpAmp circuits to acheive this range and that's not a problem. * The unused OpAmp pins are floating. *** Questions *** * I did some research on current measurement and I'm wondering if it would be better to use a Hall Effect current sensor? I don't know anything about these and I'd like to know if anyone has used these before. What do they cost, where can I get them and how would I buffer the output? From the bit of reading I did it looks like a simple non-inverting OpAmp would do the job just fine. If these sensors are not too expensive and work well I might just take that route. * Is a differential amplifier the only way to scale the output from the sense resistor? I was thinking that a standard non-inverting amp with the input on the high side of the resistor and the ground on the low side of the resistor might work. I'm building one now to take some measurements. I've just broken out my "Art of Electronics" book and I'm going to read the OpAmp chapters again. Thanks for all the help so far. Please keep sending feedback! -Chris -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist