hi, this is probably an often repeated subject. i thought i knew the difference between the two but now have doubts....
I realized today that the op-amp I am using is classed as "single rail". However I am actually feeding 3.3V into Vdd and -3.3V into Vss (from an inverting charge pump). the spec says Vdd-Vss <= 7V - which i am satisfying (= 6.6V), and as far as the op-amp is concerned it thinks the -3.3 is it's ground as it has no other reference. which leads me to the question - what actually IS the difference between single and split supply op amps? in both cases they swing between their two supply voltages. just that for single rail it happens to be ground. but couldn't it equally be 1V, or any value less than the upper supply voltage?? my circuit appears to be working fine - i'm inputting ground biased a/c, amplifying, and getting the expected ground-referenced output range, e.g. 1V -> 2V, -1V -> -2V. but should i really be using a split rail op amp??
There really is no difference between single-supply and split-supply op amps. Either can be used in either situation, but split supply op amps are generally used when large voltage swings above and below ground are expected, and single supply op amps are generally optimized for 3.3 or 5 V only operation.
In the later situation, what is usually much more important is whether the op amp inputs and outputs can swing "rail to rail". You should look carefully at the specs to see if they meet your needs and intended application.
It depends how you have defined the ground and the way you feed your input. By setting the ground properly(as you did) you can use a single rail op-amp as dual rail.
The relevant parameters of an op-amp are how close to positive or negative supplies
the inputs and output can go. For rail-to-rail devices they can go very close (inputs
can go beyond in fact), for old fashioned opamps the inputs and outputs cannot
work closer than about 1.5 to 3V to each supply (but they were designed for
+/-15V supplies).
The opamp has no notion of ground, that's your circuit's business - often these days
a synthesized analog ground rail is generated using one of the 2 or 4 opamps in a package,
by using it as a voltage follower from a resistor-divider.