> 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.
>

> Mike,
>
>  The equation you suggested is absolutely right -- that's exactly the
> function I want, although the circuit doesn't have to be single supply.
>
>  Further, to clear things up w/r/t an earlier suggested solution:
> Vout isn't necessarily going into an ADC -- it might very well stay analog,
> so the output has to be linear.  Seems like it'd be totally doable with a
> few op amps -- is that true?
>
> Thanks again,
>
> Todd
>
> -------------------------------------------------
>
> Mike Hord wrote:
>
> To clarify:
>
> Given Va, Vb, and Vc, with output Vout and single-rail supply Vdd:
> Vout = [(Vb-Vc)/(Va-Vc)]*Vdd
> where Va > Vb > Vc, right?
>
> Mike H.

>On 6/9/05, Todd Bailey <tm_baileyspam_OUThotmail.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

> 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.

> > >> Given Va, Vb, and Vc, with output Vout and single-rail supply Vdd:
> > >> Vout = [(Vb-Vc)/(Va-Vc)]*Vdd
> > >> where Va > Vb > Vc, right?
> > >
> > > The equation you suggested is absolutely right
> >
> > Really?  I thought you defined B as the bottom voltage and C as the middle.
>
>Perhaps he did, but I "re-labeled" to make it a little more logical for me.
>
>At any rate, I'd suggest using a digital solution, too, if you can make it
>work.
>Not knowing right off what the actual ranges are makes it tough, but if they
>are correct, using (with my nomenclature) Va as the Vref+, Vc as Vref-, and
>Vb as Vadc, the resulting reading of the ADC could be directly sent to a
>DAC of some sort with the end result being a voltage scaled between the
>supply rails of the DAC in exactly the manner you're looking for.  No muss,
>no fuss.  In fact, even if Va and Vc AREN'T within the acceptable range for
>the Vref+ and Vref- on the ADC, you can force them to be with pretty simple
>op-amp circuits.
>
>Since you mentioned that these voltages are "slow-changing" (to me that
>means Hz or even sub-Hz range), this sort of method should work quite
>well.
>
>Mike H.

>-----Original Message-----
>From: RemoveMEpiclist-bouncesTakeThisOuTmit.edu [spamBeGonepiclist-bouncesspamBeGonemit.edu]
>Sent: 10 June 2005 14:18
>To: TakeThisOuTpiclistEraseMEspam_OUTmit.edu
>Subject: Re: [EE] Analog "Proportional" Amplifier Design Question
>
>
>
>Todd Bailey wrote:
>>>Given Va, Vb, and Vc, with output Vout and single-rail supply Vdd:
>>>Vout = [(Vb-Vc)/(Va-Vc)]*Vdd where Va > Vb > Vc, right?
>>
>>The equation you suggested is absolutely right.
>
>
>Thanks for all the replies!
>
>  Based on the above equation, as Spehro and others suggested,
>I've been
>drawing up a circuit using two diff amps to perform the two
>subtractions
>followed by an analog multiplier (the AD632 -- I'd used the
>633 before and
>thought THAT was an expensive IC) to divide the results of those two
>operations.  I expect the final version will need a lot of trimming.
>  My next question is:
>  Is there a way to multiply the result of the division operation by a
>constant (Vdd or some other scaling voltage) without another analog
>multiplier?