On 3/5/2010 10:01 AM, Marcel Duchamp wrote:
>
> As far as reading ORP signals from 200mV to 800mV, you MUST connect the
> probe such that it presents a negative voltage to the BNC input jack.
> This is because of the inverting second stage.

Looking back on this, I may be incorrect about the polarity.  It all 
depends on where you set your offset pot.

If your probe puts out +800mV to the input, the first stage will turn 
this into +4V.  If the offset pot is set to +2V, you will get 0V at the 
output.  Now if the ORP voltage drops towards +200mV, the output will 
indeed rise to 3V.  This is probably what you want.  The total circuit 
gain at DC is +5. Your input span is 800mV -200mV = 600mV; .6V * 5 = 3V.
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