Often, an AC signal needs to be conditioned or captured by an ADC, but the design only has positive voltage supplies. For reasons of cost, noise, and the complexity of adding negative supplies, dual supply op-amp circuits are not as commonly used in modern designs. This has necessitated the use of a reference voltage, commonly known as a "virtual ground", typically at the mid voltage of the analog supply rail.
There are several ways to create this reference voltage, but some of them can create problems in the circuit. The simplest method is by using a resistive voltage divider to provide the reference voltage. While this is the least expensive method, it is also the least accurate and can be unreliable if the reference does not feed into a high-impedance input. Any currents flowing in or out of the midpoint node will result in offsets and a change in the virtual ground potential.
Dedicated voltage references can also be used. They are more stable over temperature, and more accurate than a resistor divider, but typically can either only source or sink current, and therefore do not regulate well in circuits where a return current flows back into the reference. The Analog Devices REF19x series precision band gap voltage reference is one example of this kind of reference. A load resistor can be added to the output to provide some ability to sink current. In the example circuit below, R600 was added to sink roughly 50% of the max output current of the reference.
The best methods are to use either a dedicated "virtual ground" IC like the TLE2426 (check the stability versus output capacitance chart!), or to use a resistor divider or dedicated voltage reference along with a voltage follower. One critical parameter for the voltage follower selection, is the ability to both source and sink more than the amount of current required by your circuit.
Most op-amps cannot drive directly into the capacitive load, which is typically used to bypass the reference output (0.1uF, give or take a few orders of magnitude). Without proper care, the reference can be noisy, or even worse, oscillate. In this case, a small series resistor (10-100 Ohms) in series with the output can be used to prevent a direct capacitive load. The feedback is often taken from the reference point for better tracking. The TI LM8261 is one good example of an op-amp that serves very well as a virtual ground, with the added benefit of unlimited capacitive drive ability. No output resistor is required to ensure stability.
The examples mentioned here offer a few tested and simple methods to get a solid virtual ground reference voltage.