A couple of fairly obvious things come quickly to mind:

1) Your differential gain is going to be 10x, not 200x

2) I assume your power supply is a single +5 volts and
ground.  An LF351 won't have much space within its I/O
constraints with which to operate in a linear fashion.
You'll need at least an LM358/LM324 type opamp here;
better yet, one with rail-to-rail inputs and outputs.

3) What is the common-mode potential at your current sense
resistor?  If it strays outside of the 0 to +5V range,
you'll be out of luck!

Brian Aase

> 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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