Hi,

I'm working on a current measurement circuit. I'm using a 0.1 ohm
current sense resistor in-line and measuring the voltage drop across
this resistor. Eventually I want to use a PIC A/D to monitor the current
so I am attempting to build a differential amplifier using an LF351
OpAmp.

The problem is the circuit is not working correctly. I expect to see a
gain of about 200x and a linear output relative to the differential
input. I've simulated my circuit using a spice tool and it's working as
expected in simulation.

** During testing I used a multimeter for all measurements. There is no
PIC on my breadboard at this time. **
 
In the real circuit I am seeing a relatively fixed voltage output that
jumps to about 5V when the differential input reaches some value (I can
graph this if anyone is interested). 

To simplify things for testing, I built another amplifier circuit and I
am feeding the differential inputs using two variable resistor based
voltage dividers. I'm seeing the same inconsistent results (a relatively
fixed output that jumps to V+ depending on the differential input). 

Some other things I've tied include shorting both inputs to ground which
shows about 1.1 volts on the output (probably due to variations in the
feedback resistors in the OpAmp circuit). I've also tried adjusting both
inputs to get a zero volt output but it's not possible.
 
A schematic of my amplifier can be found at:
http://www.rocklizard.org/webdownloads/difamp.bmp. This is the *exact*
circuit that I am testing. The inputs are fed from 2 adjustable voltage
dividers. I monitor the inputs and the output with a Fluke 75
multimeter. 

It's been a long time since I've had to design and build with OpAmps so
I'm assuming that I've forgotten something here. Does anyone have any
ideas or suggestions?

Thanks!
Chris



 
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