How to calculate gain of wheatstone bridge and amp?

Tube or solid-state, this section goes to eleven!
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Vivian01
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Post by Vivian01 »

I have the following circuit:Image
have a hard time determining the mathematics of this circuit. If we ignore the Wheatstone Bridge to the left, focusing on the differential amplifier, i know that the amplification is given by:

(V2-V1)*(R6/R5)

Given that R6=R8 and R5=R7. But what happens when I add my Wheatstone bridge, which doesn't have the same resistor values on the two sides of it? (10k ohm at the left side and 1k ohm on the right side).

In the picture i have managed to balance the R5 and R7 so that the output is ~0V when there's no difference in the bridge - but this is just by lucky guessing. I would like to understand the mathematical formulas for the circuit :-)
R5 is not same as R7. Which is correct because R5 is connected to R1 and R2 while R7 is connected to R4 and R3. Those two nets must equal the same resistance somehow? My problem is that I don't know how to calculate R5 if I change R1 and R2 for example.
Then I figured a way to state this up front - a differential amplifier made from an op-amp and four resistors is perfectly fine when (and only when) the input sources are much, much lower impedance than the resistors used. You will not get this when a bridge is attached because, as the bridge is off-balanced by whatever it is you are measuring, its output resistance changes, generating significant errors in a diff amp configured for a gain of 50.

The junction of R4 and R3 sees a high loading impedance - basically it's R7 in series with R8 and of course the non-inverting input impedance of the op-amp can be ignored. On the other hand, the loading impedance seen by the junction of R1 and R2 is just R5 because the inverting input is forced always to be at the same potential as the non-inverting input. This is natural for op-amps in linear applications like this.

If I don't use an INAMP then use two op-amps LTC6081 bought from http://www.kynix.com/Detail/580552/LTC6081CMS8.html wired like an INAMP:
Image
What do you think? Is there other way to clarify this? Any suggestions will be appreciated!
Best regards,
Vivian

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Dirk_Hendrik
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Post by Dirk_Hendrik »

With the best willingness I fail to see what is your problem as well as the actual question. As well i'd like to know the supply voltage for the opamp and the input voltage for that balanced bridge.
Sorry. Plain out of planes.

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