piclist 2000\05\21\071201a >
Thread: Non interacting zero and span. how?
face BY : paulb@midcoast.com.au

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.
       Paul B.

<3927C443.49F2531@midcoast.com.au> 7bit

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