Hi Bill, I went to Creatron Saturday, and got a CA3140 opamp, as well as decoupling caps. I thought of using a trimpot for the voltage divider, but indeed, as Eric mentionned, I don't want to load the source.
Hi Robert,
As a follow up on that OPA, if you want to reach the +8V output from the OPA, you must use a supply rail of at least 2 to 3 volts more.
This applies to most common OPA, they need that overhead voltage.
So about +12Vsupply would be OK.
The divider could be a lowish resistance pair on the output of CA3140, that give a down step of +8V to +5V.
The PIC's ADC requires to 'see' a source resistance of 10K or less.
OR
you could use a high resistance divider, megohms range on the NONINV input, to give an input of +5V from +8V.
That means the max output swing would be +5V, so you could use a +9Vsupply to the OPA.
With regard to the minimum voltage output from CA3140, you can get within a few +mV of zero,
when a single rail voltage is used.
Hope you follow that.