At 10:22 AM 5/8/2005 -0600, you wrote:
>
>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

You need to think about the common-mode range of the inputs and the
range of the output. This circuit should work if you use +/-5V supply
on the op-amp, but also note that the input common mode range is
not guaranteed to include the +ve supply on the LF351 (though it typicall=
y
might, unlike many other op-amps). So, if you're trying to measure the
current on the 5V supply, it's marginal at best, even at room temperature.

With a +/-15 V supply, the input common mode range is only guaranteed
to be +/- 11V over temperature (0 <=3D Ta < =3D 70=B0C), also the output =
voltage
swing in only guaranteed to be +/-12V. That 3-4V difference can cause som=
e
major difficulties.

Also, the schematic you show only has a gain of 10, not 200. If you want
decent common mode rejection you will need very close matching on the res=
istors
at a gain of 200. You need to pull out your quadrille paper and calculato=
r
and figure this out. The offset voltage of the op-amp will enter into it =
too.
But none of this much matters until you've dealt with the gross problems =
I
mentioned in the preceding paragraph.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rew=
ard"
speff@interlog.com             Info for manufacturers: http://www.trexon.=
com
Embedded software/hardware/analog  Info for designers:  http://www.speff.=
com





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