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.

  Another way to think about the inputs would be like the Vref inputs on an 
A/D converter -- there are positive and negative reference voltages which 
are used to set the scale for the input signal.

So my question:  What's this circuit called?  I've had the hardest time 
looking it up in H&H or online or anywhere.  It's sort of like a scaling 
amplifier and sort of like a level shifting amplifier, but not either one of 
those two.  I'd like to find a reference before I go drawing up some sort of 
frankenstein.

Any advice you guys had sure would be great.

Yours, and thanks so much in advance,

Todd Bailey


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