Most SAR ADCs, and a few delta-sigma ADCs, switch binary-weighted capacitors onto the REF pin during the conversion process. The magnitude of the capacitance switched onto the REF pin during each conversion depends on the input signal to the ADC. If a voltage reference is directly connected to the REF pin of these ADCs, the reference voltage droops because of the dynamic input signal dependent load of the binary-weighted capacitors. Because the reference voltage droop now has input signal dependance, significant degradation in THD and linearity for the system occurs.
In order to support this dynamic load and preserve the ADC linearity, distortion and noise performance, the output of the voltage reference must be buffered with a low-output impedance (high-bandwidth) buffer. The REF61xx family of voltage references have an integrated low output impedance buffer that enables the user to directly drive the REF pin of a SAR ADC, while preserving ADC linearity and distortion. In addition, the total noise in the full bandwidth of the REF61xx is extremely low, thus preserving the noise performance of the ADC. Voltage-Reference Impact on Total Harmonic Distortion (SLYY097) correlates the effect of reference settling to ADC distortion, and how the REF61xx achieves lowest distortion with minimal components and lowest power consumption.
The output voltage of the REF61xx does not droop below 1 LSB (18-bit), even during the first conversion while driving the REF pin of the ADS8881. This feature is useful in burst-mode, event-triggered, equivalent-time sampling, and variable-sampling-rate data-acquisition systems. Functional Block Diagram shows a simplified schematic of the REF61xx.