Voltage follower WTF?

[int2str]

I put a simple voltage follower circuit on a breadboard using the TL972 "rail to rail" op-amp (datasheet).
Powered the op-amp using 5V and put a 0-5V on the input. The result was the attached chart. Blue is in, yellow is out.

Looks like the op-amp is NOT rail-to-rail?
Is this normal? I can make the top go away by supplying more power. But how do I fix the bottom?

[Galaxyrise]

Yep, that's normal.  You have to read the datasheet to know what "rail to rail" actually means, but I don't think it will ever mean you can actually go to both rails on the inputs and get all the way to the rails on both outputs.  Relevant parts of that datasheet:

Rail-to-Rail output swing: +-2.4V at Vcc=+-2.5V

Common-mode input voltage    Vcc- + 1.15 to Vcc+ - 1.15

You "fix" the bottom by giving the opamp a negative supply that is sufficiently lower than the lowest you want it to output in this case, same as how you "fixed" the top.  You can do that by lowering the negative supply or raising the input voltage.  Suppose the opamp was supplied with 0V and +10V and your input came in at 3V to 8V, for example.

[cyr]

The claim is "output rail to rail", which in reality is always going to mean "really close to the rail".

You usually have some gain, so not having the inputs go rail-to-rail is less of a problem. If you really need that, get another part.

[Zero999]

This is known as phase inversion and occurs when the common mode range of the input is exceeded. The inputs exchange functions, so the inverting input becomes the non-inverting input and vice versa.

[suicidaleggroll]

As mentioned above, that op-amp has rail to rail outputs, not rail to rail inputs.  For what you're trying to do, you need both.

[macboy]

Not trying to beat the dead horse here (the consensus is of course, R-R outputs does not imply R-R inputs), but if you look at the Vout vs Vin in section 9.1.2 of the datasheet, you will see a curve that demonstrates the exact behavior that you see when the input reaches within ~1 V of either rail.

[int2str]

Awesome! Thank you guys for really helping me understand and co-relating the datasheet!
Much appreciated!