Just to clarify on the ground thing: a ground in an electronic circuit is the reference point that all other voltages in the circuit are measured relative to. It doesn't mean it has to be connected to an earth ground.
Most circuits that use a single power supply and all voltages are positive relative to ground, will use the negative terminal of the battery or power supply as the ground, and all voltages are positive relative to ground. It's also possible to make a circuit that uses the + side of the power supply as the "ground" and all voltages are negative relative to this. This is a lot less common though.
But op amps work with voltages that can go both positive and negative relative to ground. Thus the need for a split/dual power source, with the ground "in the middle." Assuming 9V batteries are used, and they're fresh enough to be putting out 9 volts, the V+ pin on the op amp will be +9V relative to ground, and the V- pin -9V relative to ground. Voltage from V+ to V- (rails) will be 18 volts. Remember, voltage is always relative between 2 points, so the ground provides a common reference that all voltages in the circuit are relative to. If VG1 is putting out a sine wave at +/- 0.5V, that's its voltage relative to ground. Measuring VG1 relative to V- will give a different reading of 8.5V to 9.5 V. Measuring it relative to V+ would give you -8.5 to -9.5V.
Now the LF353 can only handle inputs beyond 2.5V of the negative or positive rails, so with a +/-9V supply (18V total), it can handle inputs from -6.5V to +6.5V, and the output should mirror this in your voltage follower circuit (once again, relative to ground). More modern "rail-to-rail" op amps can handle inputs and outputs closer to the rails, like within 0.5V. Here's a crude diagram illustrating this:
Code:
V+ Rail (+9V)
|
+2.5V
|
Max Input Voltage (+6.5V)
|
+6.5V
|
Ground (0V)
|
-6.5V
|
Min Input Voltage (-6.5V)
|
-2.5V
|
V- Rail (-9V)
There is a way to use an op amp such as this with a single power supply, but you still run into the "minimum 2.5V to the rail" rule. If you were to use a single 15V supply, with ground on the V- pin, it would work as long as your inputs are within the range 2.5V to 12.5V. Since the op amp doesn't have a "ground" pin, you can have "ground" be anything you want, though it's usually 1/2 the total supply voltage, as long as your inputs (and expected outputs) aren't too close to the rails.
