At 12.53 2005.05.08 -0400, you wrote:
>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 curren=
t
>>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 relativel=
y
>>fixed output that jumps to V+ depending on the differential input).
>>
>>Some other things I've tied include shorting both inputs to ground whic=
h
>>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 bot=
h
>>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 typical=
ly
>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 temperatur=
e.
>
>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 so=
me
>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 re=
sistors
>at a gain of 200. You need to pull out your quadrille paper and calculat=
or
>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.

That's why using an instrumentation amp seems ideal (Burr-Brown comes to
mind).

Or he could directly use an instrument-amp made just for this purpose:
INA198 (or INA193,INA194,INA195,INA196,INA197, each one with its own
configuration).



>Best regards,
>
>Spehro Pefhany --"it's the network..."            "The Journey is the re=
ward"
>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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