Hello, I've looked all the datasheets of the TL081/TL082, and Googled around with keywords like "single supply TL081" and others, but didn't find a schematic or an answer to my question, which in its substance is:
Having gnd and +5V which feed my PIC, if I add a +9V line, can I place a TL081 between gnd and +9V (it would be out of spec of 1V, but it's not the problem I'm asking about) and expect it to work well? I'm using it as a simple voltage follower. Do I need to make a virtual ground (e.g. via two 470 ohm resistors) and make some current flow between it and the TL081 output? (from the datasheet the TL081 doesn't seem to require any current flowing between its output and ground, but there's no mention to a circuit like this anyway). The opamp + input would receive voltages between +2V and +5V, i.e. (relative to the midpoint of its gnd/9V power) -2.5V to +0.5V. I need to attach its output to the PIC's ADC, where I'll read from 2V to 5V. Of course I can't use a TL081 on the gnd/+5V power bus, and a dual -5V/gnd/+5V would force me to modify my already built PSU and, more than that, wouldn't give me the
2V..5V range into the PIC ADC that I need, since the TL081 is not rail-to-rail. Yes I know there are nice rail-to-rail opamps, even FET-input ones, but I've a lot of TL081 and if the circuit above has no problems working, why not use it? It would take me many days to order another opamp, I'd have to throw the current prototype to make another one from scratch, etc..The circuit I'm thinking of is this:
Gnd | \ / 470 +9V +5V \ | | | 470 | |
--------------*---/\/\/----* | - | | input | | + | |
---------------------- + | | TL081 --*--------- ADC PIC .-- - | | | | | | | | Gnd | Gnd | | '-------------'
input ranges from -0.5V to +2.5V, I expect to get on the PIC ADC +5V to +2V in response to this input range.
Greets and Thanks a lot in advance for your help! TPM