LM90

SNIS126A – MAY 2004 – REVISED MARCH 2013

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The non-ideality factor,

η, is the only other parameter not accounted for and depends on the diode that is used

for measurement. Since

ΔVBE is proportional to both η and T, the variations in η cannot be distinguished from

variations in temperature. Since the non-ideality factor is not controlled by the temperature sensor, it will directly

add to the inaccuracy of the sensor. For the Pentium III Intel specifies a ±1% variation in

η from part to part. As

an example, assume a temperature sensor has an accuracy specification of ±3°C at room temperature of 25 °C

and the process used to manufacture the diode has a non-ideality variation of ±1%. The resulting accuracy of the

temperature sensor at room temperature will be:

TACC = ± 3°C + (±1% of 298 °K) = ±6 °C

(5)

The additional inaccuracy in the temperature measurement caused by

η, can be eliminated if each temperature

sensor is calibrated with the remote diode that it will be paired with. The following table shows the variations in

non-ideality for a variety of processors.

Processor Family

η, non-ideality

min

typ

max

Pentium II

1

1.0065

1.0173

Pentium III CPUID 67h

1

1.0065

1.0125

Pentium III CPUID

1.0057

1.008

1.0125

68h/PGA370Socket/Celeron

Pentium 4, 423 pin

0.9933

1.0045

1.0368

Pentium 4, 478 pin

0.9933

1.0045

1.0368

MMBT3904

1.003

AMD Athlon MP model 6

1.002

1.008

1.016

Compensating for Diode Non-Ideality

In order to compensate for the errors introduced by non-ideality, the temperature sensor is calibrated for a

particular processor. These temperature sensors are always calibrated to the typical non-ideality of a given

processor type. The LM90 is calibrated for the non-ideality of a mobile Pentium III processor, 1.008. When a

temperature sensor calibrated for a particular processor type is used with a different processor type or a given

processor type has a non-ideality that strays from the typical, errors are introduced. Figure 16 shows the

minimum and maximum errors introduced to a temperature sensor calibrated specifically to the typical value of

the processor type it is connected to. The errors in this figure are attributed only to the variation in non-ideality

from the typical value. In Figure 17 is a plot of the errors that result from using a temperature sensor calibrated

for a Pentium II, the LM84, with a typical Pentium 4 or AMD Athlon MP Model 6.

Figure 16. Error Caused by Non-Ideality Factor

Figure 17. Errors Induced when Temperature

Sensor is Not Calibrated to Typical Non-Ideality

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