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'[EE] Instrumentation Amplifier'
2006\05\12@153708 by Mohit M. \(Lists\)

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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.



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2006\05\12@160423 by Bob Blick

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> 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?

Hi Mohit,

R3 and R5 will influence the common mode, not R1 and R2.

Also, don't use a large value pot, use a small one in series with a fixed
resistor. For instance, if R3 is 10K, for R5 use a 1K pot in series with
9500 ohms. Make 9500 from 10K in parallel with 220K.

Cheers,

Bob


2006\05\12@160704 by Spehro Pefhany

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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"
spam_OUTspeffTakeThisOuTspaminterlog.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


2006\05\12@162301 by David VanHorn

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Hand-match your resistors using a DVM. Sort through a bunch and find the
closest you can, then trim up as needed.  You can usually do 1% or better in
a given lot of 5% resistors.





--
Feel the power of the dark side!  Atmel AVR

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