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'[EE] Non interacting zero and span. how?'
2000\05\21@002347 by

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.

<x-flowed>At 07:22 PM 5/20/00 -0400, Chris wrote:
>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?

1. What is your current circuit?

2. What type of op-amp are you using?

3. Is an inverting configuration acceptable?

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: rallcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

</x-flowed>
Chris Eddy wrote:

> 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.

Unfortunately, it works the other way about.  Assuming the zero bias
potentiometer has a low impedance, the two controls do not interact much
at all.

Your problem is that you are *not* zeroing it in the first place, but
setting an offset.  The offset of course, represents the real offset
multiplied by the scale (span) figure; of course it depends on both.

The procedure you want is in three steps.  Set the zero control for
*zero* output with zero input.  Apply a 5V input and set the scale pot
for 4V output.  Set the input back to zero, and adjust zero bias for the
desired 1V output.
--
Cheers,
Paul B.

At 07:22 PM 5/20/00 -0400, you wrote:
>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.

Try a pot connected as a rheostat in *series* with the signal (summed
to the non-inverting input) and adjust the 1V level by summing to the
inverting input (fixed feedback resistor).
You'll need a -ve reference for this to work.

If you want to use a +ve reference it might be easier to buffer the
zero adjust voltage with an additional unity gain op-amp and use your
original circuit, but with the span adjust pot again in series with the
signal to the non-inverting input.

Best regards,

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

{Quote hidden}

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

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