Voltage references are key components in data conversion systems which enable the ADC and DAC to read accurate values and are used in various sensing applications. While they are simple parts which often may only have two or three pins, there are numerous parameters that affect the performance of a reference, and careful consideration of all the parameters are required to select the proper one. This paper covers the differences between the shunt and series architectures, explains key parameters and special features, and shows how to properly calculate the error for a given reference in a given operating condition.