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| 型號(hào)： | ADT7482ARMZ-U2 |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | DIGITAL TEMP SENSOR-SERIAL, 11BIT(s), 2.5Cel, SQUARE, SURFACE MOUNT |
| 封裝： | LEAD FREE, MO-187-BA, MSOP-10 |
| 文件頁(yè)數(shù)： | 15/24頁(yè) |
| 文件大?。?/td> | 335K |
| 代理商： | ADT7482ARMZ-U2 |
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ADT7482
Rev. 1 | Page 22 of 24 | www.onsemi.com
APPLICATIONS INFORMATION
NOISE FILTERING
For temperature sensors operating in noisy environments, the
previous practice was to place a capacitor across the D+ pin and
the D pins to help combat the effects of noise. However, large
capacitances affect the accuracy of the temperature measurement,
leading to a recommended maximum capacitor value of 1000 pF.
While this capacitor reduces the noise, it does not eliminate it,
making it difficult to use the sensor in a very noisy environment.
The ADT7482 has a major advantage over other devices for
eliminating the effects of noise on the external sensor. The
series resistance cancellation feature allows a filter to 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 cancelled from the temperature result.
The construction of a filter allows the ADT7482 and the remote
temperature sensor to operate in noisy environments. Figure 23
shows a low-pass R-C-R filter, with the following values:
R = 100 Ω and C = 1 nF
This filtering reduces both common-mode noise and
differential noise.
04110-
0-
009
D+
1nF
100Ω
REMOTE
TEMPERATURE
SENSOR
D–
100Ω
Figure 23. Filter Between Remote Sensor and ADT7482
FACTORS AFFECTING DIODE ACCURACY
Remote Sensing Diode
The ADT7482 is designed to work with 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 collector). If
an NPN transistor is used, the collector and base are connected
to D+ and the emitter to D. If a PNP transistor is used, the
collector and base are connected to D and the emitter 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
ADT7482 is trimmed for an
nf value of 1.008. The
following equation can be used to calculate the error
introduced at a temperature T (°C), when using a transistor
whose
nf does not equal 1.008. Consult the processor data
sheet for the
nf values.
(
)
(
)
T
Kelvin
n
T
f
+
×
=
Δ
15
.
273
008
.
1
/
008
.
1
–
To factor this in, write the ΔT value to the offset register. It
is then automatically added to or subtracted from the
temperature measurement by the ADT7482.
Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7482, IHIGH, is 220 μA and the low level current,
ILOW, is 13.5 μA. If the ADT7482 current levels do not
match the current levels specified by the CPU manufacturer,
it may be necessary to remove an offset. The CPU data
sheet advises 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 being used with the ADT7482, the best
accuracy is obtained by choosing devices according to the
following criteria:
Base-emitter voltage greater than 0.25 V at 6 μA, at the
highest operating temperature.
Base-emitter voltage less than 0.95 V at 100 μA, at the
lowest operating temperature.
Base resistance less than 100 Ω.
Small variation in hFE (such as 50 to 150) that indicates
tight control of VBE characteristics.
Transistors, such as 2N3904, 2N3906, or equivalents in SOT-23
packages, are suitable devices to use.
THERMAL INERTIA AND SELF-HEATING
Accuracy depends on the temperature of the remote sensing
diode and/or the local temperature sensor being at the same
temperature as that being measured. A number of factors can
affect this. Ideally, the sensor should be in good thermal contact
with the part of the system being measured. If it is not, the
thermal inertia caused by the sensor’s mass causes a lag in the
response of the sensor to a temperature change. In the case of
the remote sensor, this should not be a problem, since it is
either a substrate transistor in the processor or a small package
device, such as SOT-23, placed in close proximity to it.
The on-chip sensor, however, is often remote from the
processor and only monitors the general ambient temperature
around the package. In practice, the ADT7482 package is in
electrical, and hence thermal, contact with a PCB and may also
be in a forced airflow. How accurately the temperature of the
board and/or the forced airflow reflects the temperature to be
measured also affects the accuracy. Self-heating due to the
power dissipated in the ADT7482 or the remote sensor causes
the chip temperature of the device or remote sensor to rise
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