3 The impact of the voltage reference on THD

Repeated sampling of the voltage reference pin can cause current transients to appear that may only be separated by a few nanoseconds. However, for an ADC, the external reference must settle or recharge by the end of the sample phase in order to avoid a large gain error. Slowing the sampling speed could fix this issue, but that is not always an option. Typically, the more precise the ADC, the more current draw required on its reference input. If a voltage reference does not have high-enough bandwidth, or has too high of an output impedance, it will not be able to recharge the reference input of the ADC. This will cause a voltage droop, leading to gain error and lower ENOB.

For this reason, a high-bandwidth, low-output-impedance buffer external to the voltage reference is sometimes necessary to increase the THD of the ADC and meet the data-sheet specifications for distortion and ENOB. Some ADCs have an internal voltage reference buffer, but not all do. Figure 2 shows where to add an external buffer to increase the THD of your circuit.