On 6/9/05, Todd Bailey <tm_bailey@hotmail.com> wrote:
> Friends,
> 
>   So I have this sticky analog design problem which I don't think should be
> that complicated, but I just can't find a good reference for it.  I suspect
> the circuit I'm looking for is related to a diff amp with some kind of level
> shifting, and I know this list is about microcontrollers, but I thought
> maybe someone would know the answer to this question just offhand.
> 
>   It's especially annoying because I can't think of what the appropriate
> name of this circuit would be.
> 
> Here goes:
> 
> The input to this mystery circuit has three slow-moving positive voltages
> (essentially DC) which are all linear and floating.  Voltage A, although
> floating, will always be the highest of the three.  Voltage B will always be
> the lowest, and Voltage C will move around between the two over time.  I
> know somewhere out there is a circuit which takes those three inputs and
> gives an output voltage which is proportional to the floating input voltage,
> but over a fixed range (like a rail-to-rail supply).
> For example: Voltage A is 1.5V, Voltage B is 1.0V, and Voltage C is 1.25V.
> If the rails on the circuit are 5V and ground, the output would be 2.5V.  It
> would have exactly the same output (2.5V) if A, B, and C were, say, 1.2V,
> 0.8V, and 1.0V, respectively.
> 

Mathematically, your desired output voltage is (B-A)*5/(C-A)

Subtracting A from B is easy.   Scaling is harder.  You need a
variable gain device. If you are just going into a ADC, you can do A-B
and 5*(C-A) in op-amps, then feed the A-B into the ADC with 5*(C-A) as
the reference.   This makes the ADC do the scaling.

Regards,
Mark
markrages@gmail
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