9.3.2 Calculating Total System Accuracy

The voltage seen by the LM95214 also includes the I_FR_S voltage drop of the series resistance. The non-ideality factor, η, is the only other parameter not accounted for and depends on the diode that is used for measurement. Because ΔV_BE is proportional to both η and T, the variations in η cannot be distinguished from variations in temperature. Because the non-ideality factor is not controlled by the temperature sensor, it will directly add to the inaccuracy of the sensor. For the for Intel processor on 65 nm process, Intel specifies a +4.06%/−0.897% variation in η from part to part when the processor diode is measured by a circuit that assumes diode equation, Equation 4, as true. As an example, assume a temperature sensor has an accuracy specification of ±1.0°C at a temperature of 80°C (353 Kelvin) and the processor diode has a non-ideality variation of +1.19%/−0.27%. The resulting system accuracy of the processor temperature being sensed will be:

and

The next error term to be discussed is that due to the series resistance of the thermal diode and printed-circuit board traces. The thermal diode series resistance is specified on most processor data sheets. For the MMBT3904 transistor, this is specified at 0 Ω typical. The LM95214 accommodates the typical series resistance of a circuit with the offset register compensation. The error that is not accounted for is the spread of the thermal diodes series resistance. If a circuit has a series resistance spread that is 2.79 Ω to 6.24 Ω or 4.515 Ω ±1.73 Ω, the 4.515 Ω can be cancelled out with the offset register setting. The ±1.73 Ω spread cannot be cancelled out. The equation to calculate the temperature error due to series resistance (T_ER) for the LM95214 is simply:

Equation 6.

Solving Equation 6 for R_PCB equal to ±1.73 Ω results in the additional error due to the spread in the series resistance of ±1.07°C. The bulk of the error caused by the 4.515 Ω will cause a positive offset in the temperature reading of 2.79°C, which can be cancelled out by setting the offset register to –2.75°C. The spread in error cannot be canceled out, as it would require measuring each individual thermal diode device. This is quite difficult and impractical in a large volume production environment.

Equation 6 can also be used to calculate the additional error caused by series resistance on the printed circuit board. Because the variation of the PCB series resistance is minimal, the bulk of the error term is always positive and can simply be cancelled out by subtracting it from the output readings of the LM95214.