Due to aging and environmental effects, all semiconductor devices experience physical changes of the semiconductor die and the packaging material over time. These changes and the associated package stress on the die cause the output voltage in precision voltage references to deviate over time. The value of such change is specified in the data sheet by a parameter called the long-term stability (also known as the long-term drift (LTD)). Equation 4 shows how LTD is calculated. Note that the LTD value is positive if the output voltage drifts higher over time and negative if the voltage drifts lower over time. Figure 7-22 through Figure 7-29 show the drift of the output voltage for REF50xx over the first 4000 operating hours.