Just got bitten. Or rather, just found out after over two years I'd made a booboo. I had used a Microchip MCP618 as a unity gain buffer for an ANLC with the sensor unit located a remote location. I was upgrading the load-control part of the circuit yesterday and while monitoring the output of the op amp at between sunset and dusk I was shocked to see the voltage jump from from VDD to around 4.80V. (Thereafter it decreased in a smooth continuous gradient). That sent me into a near-panic. Questions raced through my head. Was the long length of cable--some 20 meters--from sensor to control circuit responsible for this behavior? Do I need to add a high value load resistor? Is there some soldering/board defect at the sensor?
Eventually I zeroed in on the fact that the 618 is not a full rail-to-rail op amp. Rechecking the datasheet and refreshing my memory, indeed while its output can, its input does not swing rail-to-rail. So to test whether the 618 is cause of the problem I bench-tested it using a 100K pot as input to the unity gain buffer. Sure enough as I ever so gradually turned the pot down from maximum, the buffer output jumps from 5.18 (VDD) to around 4.90V.I tried this several times just to make sure its inherent. Well, it doesn't miss a beat. The same discontinuous output occurs every time.
I then popped the 618 off the breadboard and dropped in a pin-for-pin compatible MCP6273. This op amp has true rail-to-rail input and output. As I was hoping, as the pot is wound down the output voltage does not exhibit the sudden drop but instead smoothly and continuously decreases from VDD.
Whew! I'm just glad it isn't some much more major error or defect.
Moral of the story: Make sure the op amp has rail-to-rail input and output if the application requires the entire range of values.