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SNIS128C – AUGUST 2002 – REVISED MARCH 2013
The voltage seen by the LM89 also includes the I F R S voltage drop of the series resistance. The nonideality
factor, η , is the only other parameter not accounted for and depends on the diode that is used for measurement.
Since Δ V BE 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 4 and Mobile Pentium Processor-M Intel specifies a ±0.1% 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 ±0.1%. The
resulting accuracy of the temperature sensor at room temperature will be:
T ACC = ± 1°C + (±0.1% of 298 °K) = ±1.4 °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.
Processor Family
η , nonideality
MIN
TYP
MAX
Pentium III CPUID 67h
Pentium III CPUID 68h/PGA370Socket/Celeron
Pentium 4, 423 pin
Pentium 4, 478 pin
0.13 micron, Pentium 4
MMBT3904
AMD Athlon MP model 6
1
1.0057
0.9933
0.9933
1.0011
1.002
1.0065
1.008
1.0045
1.0045
1.0021
1.003
1.008
1.0125
1.0125
1.0368
1.0368
1.0030
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 LM89 is calibrated for the nonideality of a 0.13 micron, Mobile Pentium 4, 1.0021.
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.
Temperature errors associated with nonideality may be reduced in a specific temperature range of concern
through use of the offset registers (11h and 12h).
Please visit www.ti.com requesting further information on our recommended setting of the offset register for
different processor types.
PCB LAYOUT for MINIMIZING NOISE
Figure 15. Ideal Diode Trace Layout
In a noisy environment, such as a processor mother board, layout considerations are very critical. Noise induced
on traces running between the remote temperature diode sensor and the LM89 can cause temperature
conversion errors. Keep in mind that the signal level the LM89 is trying to measure is in microvolts. The following
guidelines should be followed:
1. V DD should be bypassed with a 0.1μF capacitor in parallel with 100pF. The 100pF capacitor should be placed
as close as possible to the power supply pin. A bulk capacitance of approximately 10μF needs to be in the
near vicinity of the LM89.
2. A 2.2nF diode bypass capacitor is required to filter high frequency noise. Place the 2.2nF capacitor as close
Copyright ? 2002–2013, Texas Instruments Incorporated
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