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'[EE]:Bridge voltage calculation - conclusion!!!'
2001\02\27@085558 by

Thanks for the input on this one. For your info, after some headscratching and a lot of paper, here's
the best answer I've got to, and verifed correct with Excel
<use fixed pitch font>

Given a bridge :  Vin
+----+
R1  R2
+----|--- Vout (differential)
|    +---
R3  Rx
+----+
GND

Calculate Rx given V, V=Vout/Vin

R2
Rx = ------------------
R1+R3
-----------   -1
R3-V(R1+R3)

For my case, where R1=R2, we can also say R2=R2=1 as R3 and Rx can be
scaled to compensate.

This then simplifies to
R3-V(1+R3)
Rx = -----------
1+(V(1+R3))

And can be coded as
N=V*(1+R3)
Rx=(R3-N)/(1+N)

..which shouldn't take too much code, even in C! - any improvements
welcome, though!

Now to figure out where all the fixed points need to go...!

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On Tue, 27 Feb 2001, Mike Harrison wrote:

{Quote hidden}

On Mon, 26 Feb 2001, Wojciech Zabolotny wrote:

>                                     Rx + R2
> (C3) solve(Vout=V1-V2,Rx);
>
>                            (R2 R3 + R1 R2) Vout - R2 R3 Vin
> (D3)               [Rx = - --------------------------------]
>                                (R3 + R1) Vout + R1 Vin

Hmm. Looks the same to me.

Now this is Interesting. I didn't realize Excel had an algebraic symbolic solver
like Maxima! Maybe it's about time I revert back to windows.

BTW, I installed Maxima after Dr. Zabolotny's post. I was thoroughly impressed.
You Linux fans oughta go install this

http://www.ma.utexas.edu/maxima.html

You window's fans will be glad to know the author is working on a port. Or you
can cygwin it now.

(mac fans have to wait for OS X)

Scott

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