Long-term stability is a key performance parameter for series voltage references in all precision applications. This is defined as variation of reference voltage over time. The long-term stability value is tested in a typical setup that reflects standard PCB board manufacturing practices. The boards are made of standard FR4 material, the board does not have special cuts or grooves around the devices or go through burn-in process to relieve the mechanical stress of the PCB. These conditions reflect real world use case scenario and common manufacturing techniques.

During the long-term stability testing, precautions are taken to make sure that only the long-term stability drift is being measured. The boards are maintained at 35°C ± 0.02°C in an oil bath. The oil bath makes sure that the temperature is constant across the device over time. The measurements are captured every 30 minutes with a calibrated 8.5 digit multimeter.

Typical long-term stability characteristic are expressed as a deviation over time. Figure 7-2 shows the typical drift value for the REF54 V_OUT is 25 ppm from 0 to 1000 hours. The REF54 experiences the highest drift in the initial 1000 hr, subsequent deviation is typically lower than previous 1000 hours.

Figure 7-2 Long Term Stability SOIC -1000 hours (V_OUT)