The REF70 is designed and tested for a minimal output voltage temperature drift, which is defined as the change in output voltage over temperature. Every unit shipped is tested at multiple temperatures to ensure that the product meets data sheet specifications. The temperature coefficient is calculated using the box method in which a box is formed by the min/max limits for the nominal output voltage over the operating temperature range. REF70 has a low maximum temperature coefficient of 2 ppm/°C from –40°C to +125°C. This method corresponds more accurately to the method of test and provides a closer estimate of actual error than the other methods. The box method specifies limits for the temperature error but does not specify the exact shape and slope of the device under test. Due to temperature curvature correction to achieve low-temperature drift, the temperature drift is expected to be non-linear. See SLYT183 for more information on the box method. The box method equation is shown in Equation 2:

Equation 2.

Figure 8-14 Output Voltage Vs Free-Air Temperature