The REF61xx family of voltage references are stable with output capacitor values ranging from 10 µF to 47 µF. At a low output-capacitor value of 10 µF, an effective series resistance (ESR) of 20 mΩ to 100 mΩ is required for stability; whereas, at a higher value of 47 µF, an ESR of 5 mΩ to 100 mΩ is required. The shaded region in Figure 53 shows the stable region of operation for the REF61xx devices.
A capacitor of value 1 µF is required at the FILT pin for stability and noise performance. A low ESR (5 mΩ to 20 mΩ) is easily achieved by increasing the PCB trace length, thus eliminating the need for a discrete resistor. Higher values of ESR (greater than 20 mΩ, but lesser than 100 mΩ) can be intentionally added to increase the output bandwidth of the REF61xx. This higher ESR improves the transient performance of the REF61xx, but worsens noise performance because of increased bandwidth.