I need more information regarding the dropout voltages at different load condition varying from 1mA to 20mA( If possible).
ADR425 data-sheet page 13 figure 22 shows Load Current Vs Differential Voltage (Vin- Vout) plot. Please note based on ADR425 load regulation specification, ADR425 can only source up to 10mA.
If requirement for VREF is to source 20mA and support low dropout/differential voltage I would suggest to please take a look at LT1461-5 VREF.
In order to help you provide more relevant VREF option to meet your application requirements, can you please share specific parameters of VREF which are required? E.g. noise, accuracy, minimum dropout voltage, TC.