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Thread: Non interacting zero and span. how?
face BY : Chris Eddy email (remove spam text)

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.

<39271E30.8CBCCFC3@nb.net> 7bit

See also: www.piclist.com/techref/index.htm?key=non+interacting
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