8.3 Long-Term Stability

One of the key performance parameters of the REF35 references is long-term stability also known as long-term

drift. 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 and the board does not have special cuts or grooves

around the devices to relieve the mechanical stress of the PCB. The devices and boards in this test do not

undergo high temperature burn in post-soldering prior to testing. These conditions reflect a real world use case

scenario and common manufacturing techniques.

During the long-term stability testing, precautions are taken to ensure that only the long-term stability drift is

being measured. The boards are maintained at 35°C in an oil bath. The oil bath ensures that the temperature

is constant across the device over time compared to an air oven. The measurements are captured every 30

minutes with a calibrated 8.5 digit multimeter.

Typical long-term stability characteristic is expressed as deviation of reference voltage output over time.

Figure 8-5 shows the typical drift value for the REF35 in SOT23-6 package is TBD ppm from 0 to 1000 hours.

It is important to understand that long-term stability is not ensured by design and that the value is typical. The

REF35 will experience the highest drift in the initial 1000 hr. Subsequent deviation is typically lower than first

1000 hr.

8.4 Thermal Hysteresis

Thermal hysteresis is measured with the REF35 soldered to a PCB, similar to a real-world application. Thermal

hysteresis for the device is defined as the change in output voltage after operating the device at 25°C, cycling

the device through the specified temperature range, and returning to 25°C. The PCB was baked at 150°C for 30

minutes before thermal hysteresis was measured. Hysteresis can be expressed by Equation 2:

6

PRE

POST

HYST

NOM

| V

V

|

V

10

ppm

V

§

·

u

¨

¸

©

¹

(2)

where

range of –40°C to +125°C and returns to 25°C.

Typical thermal hysteresis distribution is as shown in Figure 8-6.

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REF35

SNAS809 – DECEMBER 2021

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