----- Original Message -----
From: "Chris Eddy" <ceddy@NB.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, May 20, 2000 5:22 PM
Subject: [EE] Non interacting zero and span. how?


> I have a question that is just burning me up.  And when I try to solve
> it I get a mental block.
>
> To simplify the problem, say I have an op amp circuit with an input
> signal of 0-5VDC and a desired output of 1-5VDC.  I have a zero and a
> span pot.  Normally, I build this circuit with the zero pot and the
> signal in question summed with two resistors into the + pin of the amp.
> Then wrap the output back to the negative pin with a pot for the span.
>
> The big complaint form customers is that the zero and the span interact
> a lot.  My quest is to change the circuit so that the zero pot
> accurately sets the 1V level at 0V in, and then adjust the span pot for
> 5V out at 5V in.
>
> By the way, the application is converting to 4-20mA.  Any output where
> the ratio of low to high is 1:5 is acceptable.  It does not have to be
> 1V and 5V.
>
> The application in mind has split rails, but it would be nice to have a
> single supply solution for those other jobs too.  Any grand ideas come
> to mind?
>
> Chris Eddy
> Pioneer Microsystems, Inc.

You could use an inverting summing configuration with a fixed resistor in
the feedback loop and 2 rheostats connected to the inverting input. One
rheostat can connect to a DC source and the other to your signal. The output
will be inverted so you may have to use another opamp invertor to get the
correct polarity.

Dan Mulally