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ADT7468 Datasheet PDF : 81 Pages
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ADT7468
produce a dc voltage proportional to ∆VBE. The ADC digitizes
this voltage, and a temperature measurement is produced. To
reduce the effects of noise, digital filtering is performed by
averaging the results of 16 measurement cycles.
The results of remote temperature measurements are stored in
10-bit, twos complement format, as illustrated in Table 7. The
extra resolution for the temperature measurements is held in
the Extended Resolution Register 2 (Reg. 0x77). This gives
temperature readings with a resolution of 0.25°C.
Noise Filtering
For temperature sensors operating in noisy environments,
previous practice was to place a capacitor across the D+ and D−
pins to decrease the effects of noise. However, large capacitances
affect the accuracy of the temperature measurement, leading to a
recommended maximum capacitor value of 1000 pF. A capacitor
of this value reduces the noise, but does not eliminate it, making
use of the sensor difficult in a very noisy environment.
The ADT7468 has a major advantage over other devices for
eliminating the effects of noise on the external sensor. Using the
series resistance cancellation feature, a filter can be constructed
between the external temperature sensor and the part. The effect
of any filter resistance seen in series with the remote sensor is
automatically canceled from the temperature result.
The construction of a filter allows the ADT7468 and the remote
temperature sensor to operate in noisy environments. Figure 24
shows a low-pass R-C-R filter, with the following values:
R = 100 Ω, C = 1 nF.
This filtering reduces both common-mode noise and
differential noise.
REMOTE
TEMPERATURE
SENSOR
100Ω
100Ω
D+
1nF
D–
Figure 24. Filter Between Remote Sensor and ADT7468
Series Resistance Cancellation
Parasitic resistance to the ADT7468 D+ and D− inputs (seen in
series with the remote diode) is caused by a variety of factors,
including PCB track resistance and track length. This series
resistance appears as a temperature offset in the remote sensor’s
temperature measurement. This error typically causes a
0.5°C offset per Ω of parasitic resistance in series with the
remote diode.
The ADT7468 automatically cancels out the effect of this series
resistance on the temperature reading, giving a more accurate
result, without the need for user characterization of this
resistance. The ADT7468 is designed to automatically cancel,
typically, up to 3 kΩ of resistance. By using an advanced
temperature measurement method, this is transparent to the
user. This feature allows resistances to be added to the sensor
path to produce a filter, allowing the part to be used in noisy
environments. See the Noise Filtering section for details.
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7468 is designed to work with either substrate
transistors built into processors or with discrete transistors.
Substrate transistors are generally PNP types with the collector
connected to the substrate. Discrete types can be either PNP or
NPN transistors connected as a diode (base-shorted to the
collector). If an NPN transistor is used, the collector and base
are connected to D+, and the emitter is connected to D−. If a
PNP transistor is used, the collector and base are connected to
D− and the emitter is connected to D+.
To reduce the error due to variations in both substrate and
discrete transistors, a number of factors should be taken into
consideration:
• The ideality factor, nf, of the transistor is a measure of the
deviation of the thermal diode from ideal behavior. The
ADT7468 is trimmed for an nf value of 1.008. Use the
following equation to calculate the error introduced at a
temperature T (°C), when using a transistor whose nf does
not equal 1.008. See the processor data sheet for the
nf values.
∆T = (nf − 1.008) × (273.15 K + T)
To correct for this error, the user can write the ∆T value to
the offset register. The ADT7468 then automatically adds
or subtracts it from the temperature measurement.
• Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7468, IHIGH, is 96 µA and the low level current,
ILOW, is 6 µA. If the ADT7468 current levels do not match
the current levels specified by the CPU manufacturer, it
might be necessary to remove an offset. The CPU’s data
sheet should advise whether this offset needs to be
removed and how to calculate it. This offset can be
programmed to the offset register. It is important to note
that, if more than one offset must be considered, the
algebraic sum of these offsets must be programmed to the
offset register.
If a discrete transistor is used with the ADT7468, the best
accuracy is obtained by choosing devices according to the
following criteria:
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