I'm using the op295GP opamp. In the datasheet on the 1st page it's says it has a wide single supply operating range 3 to 36v. What's the difference between the supply voltage ( pg5) and the single supply operating range 3 to 36v?
I would have preferred that they wouldn't say 36 V on the front page. That 36 V is Absolute Maximum Ratings (AMR).
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. (page 5)
You should never operate a device continuously at AMR. For typical power supply values you have to check Electrical characteristics on pages 3 and 4. The information is a bit hidden, but you'll find that most parameters are given under +/-15 V power supply. Stick to that, and forget you ever saw 36 V.
By the way, the 36 V is the same as +/-18 V. That's because an opamp doesn't know the concept ground, it only knows V+ and V-, and the difference between those. If you would look a an opamp's schematic, you won't find any net that's ground, as half-way between V+ and V-.
So, maximum 30 V single supply, or +/-15 V dual supply.
This means that you can either have the +rail be 3 to 36V, and the -rail be GND, or you have positive and negative voltage rails, such as 10+/10-. You need more hardware to generate a second (negative) voltage rail, so having a single voltage supply op amp has plenty of uses.