At 01:14 AM 5/13/2006 +0530, you wrote:
>Hello everybody,
>
>I am trying to make an instrumentation amplifier (INA) out of 3x LF356 
>JFET opamps. And cannot use a integrated INA as I don't have any and 
>samples may take a few days to reach me.
>
>Please see the attached gif of an INA. To make things easy: R1=R2 and 
>R3/R4=R5/R6.
>But if R1 is not exactly equal to R2, can I use a trimpot as one of 
>R3,R4,R5 or R6 to trim away any common mode gain? Which one of these four 
>resistors should be replaced by it?
>
>Thanks,
>Mohit.
I suggest R1 or R2, but don't replace the resistor with a trimpot. Rather,
use precision resistors for R1..R6 and put the trimpot in series with a
slightly lower value for the adjustable resistor (preferably connect the
trimpot to the output). There are even better ways to connect it, but
that should suffice for your purposes.

For example, if R1, R2 are 20K 1%, you can could parallel the 20K 1%
with 820K (5% is okay)  and  put a 1K trimpot in series with the pair.

Nominal resistance then (trimpot centered/centred) is 20.02K with about
a +/- 2.5% adjustment range.


>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff


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