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LM86CIMM Datasheet(PDF) 23 Page - Texas Instruments |
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LM86CIMM Datasheet(HTML) 23 Page - Texas Instruments |
23 / 33 page Vbe = K ln (N) k T q IF = IS e Vbe KVt LM86 www.ti.com SNIS114E – DECEMBER 2001 – REVISED MARCH 2013 (3) In the above equation, η and IS are dependant upon the process that was used in the fabrication of the particular diode. By forcing two currents with a very controlled ration (N) and measuring the resulting voltage difference, it is possible to eliminate the IS term. Solving for the forward voltage difference yields the relationship: (4) The nonideality 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 nonideality 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 ±1°C at room temperature of 25 °C and the process used to manufacture the diode has a nonideality variation of ±1%. The resulting accuracy of the temperature sensor at room temperature will be: TACC = ± 1°C + (±1% of 298 °K) = ±4 °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 nonideality for a variety of processors. Processor Family η, nonideality min typ max Pentium II 1 1.0065 1.0173 Pentium III CPUID 67h 1 1.0065 1.0125 Pentium III CPUID 68h/PGA370Socket/Celeron 1.0057 1.008 1.0125 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 Nonideality In order to compensate for the errors introduced by nonideality, the temperature sensor is calibrated for a particular processor. National Semiconductor temperature sensors are always calibrated to the typical nonideality of a given processor type. The LM86 is calibrated for the nonideality 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 nonideality 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 nonideality 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. Copyright © 2001–2013, Texas Instruments Incorporated Submit Documentation Feedback 23 Product Folder Links: LM86 |
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