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| 型號： | ADT7488AARMZ-RL |
| 廠商： | ON Semiconductor |
| 文件頁數(shù)： | 11/13頁 |
| 文件大?。?/td> | 356K |
| 描述： | IC TEMP/VOLT DGTL W/SST 10MSOP |
| 產(chǎn)品變化通告： | MFG CHG Notification ADI to ON Semi |
| 標準包裝： | 3,000 |
| 功能： | 溫度監(jiān)控系統(tǒng)（傳感器） |
| 傳感器類型： | 內(nèi)部和外部 |
| 感應(yīng)溫度： | -40°C ~ 125°C，外部傳感器 |
| 精確度： | ±1.75°C（最小值） |
| 拓撲： | ADC，多路復用器，寄存器庫 |
| 輸出類型： | Simple Serial Transport?（SST） |
| 輸出警報： | 無 |
| 輸出風扇： | 無 |
| 電源電壓： | 3 V ~ 3.6 V |
| 工作溫度： | -40°C ~ 125°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 10-TFSOP，10-MSOP（0.118"，3.00mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 10-MSOP |
| 包裝： | 帶卷 (TR) |
ADT7488A
http://onsemi.com
11
connected to the D+ input. If an NPN transistor is used, the
emitter is connected to the D input and the base is
connected to the D+ input.
Figure 16 shows how to connect the ADT7488A to an
NPN or PNP transistor for temperature measurement. To
prevent ground noise from interfering with the
measurement, the more negative terminal of the sensor is not
referenced to ground, but is biased above ground by an
internal diode at the D input.
Figure 16. Connections for NPN and PNP Transistors
ADT7488A
D1+
D1
2N3904
NPN
ADT7488A
D1+
D1
2N3906
PNP
The ADT7488A shows an external temperature value of
0x8000 if the external diode is an open or short circuit.
Layout Considerations
Digital boards can be electrically noisy environments.
Take the following precautions to protect the analog inputs
from noise, particularly when measuring the very small
voltages from a remote diode sensor:
1. Place the ADT7488A as close as possible to the
remote sensing diode. Provided that the worst
noise sources, such as clock generators,
data/address buses, and CRTs, are avoided, this
distance can be four to eight inches.
2. Route the D1+ and D1 tracks close together in
parallel with grounded guard tracks on each side.
Provide a ground plane under the tracks if
possible.
3. Use wide tracks to minimize inductance and
reduce noise pickup. A 5 mil track minimum width
and spacing is recommended.
Figure 17. Arrangement of Signal Tracks
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
GND
D1
D1+
GND
4. Try to minimize the number of copper/solder
joints, which can cause thermocouple effects.
Where copper/solder joints are used, make sure
that they are in both the D1+ and D1 paths and
are at the same temperature.
5. Thermocouple effects should not be a major
problem because 1癈 corresponds to about
240 mV, and thermocouple voltages are about
3 mV/癈 of the temperature difference. Unless
there are two thermocouples with a big
temperature differential between them,
thermocouple voltages should be much less than
200 mV.
6. Place a 0.1 mF bypass capacitor close to the
ADT7488A.
7. If the distance to the remote sensor is more than
eight inches, the use of a twisted pair cable is
recommended. This works for distances of about
6 feet to 12 feet.
8. For very long distances (up to 100 feet), use
shielded twisted pair cables, such as Belden #8451
microphone cables. Connect the twisted pair cable
to D+ and D and the shield to GND, close to the
ADT7488A. Leave the remote end of the shield
unconnected to avoid ground loops.
Because the measurement technique uses switched
current sources, excessive cable and/or filter capacitance
can affect the measurement. When using long cables, the
filter capacitor can be reduced or removed. Cable resistance
can also introduce errors. A 1 W series resistance introduces
about 0.5癈 error.
Temperature Offset
As CPUs run faster, it is more difficult to avoid high
frequency clocks when routing the D+ and D tracks around
a system board. Even when the recommended layout
guidelines are followed, there may still be temperature
errors, attributed to noise being coupled onto the D+ and D
lines. High frequency noise generally has the effect of
producing temperature measurements that are consistently
too high by a specific amount. The ADT7488A has
temperature offset command codes of 0xe0 and 0xe1
through which a desired offset can be set. By doing a
one-time calibration of the system, the offset caused by
system board noise can be calculated and nulled by
specifying it in the ADT7488A. The offset is automatically
added to every temperature measurement. The maximum
offset is ?28癈 with 0.25癈 resolution. The offset format
is the same as the temperature data format; 16-bit, twos
complement notation, as shown in Table 15. The offset
should be programmed in little endian format, that is, LSB
before MSB. The offset value is also returned in little endian
format when read.
Application Schematic
A typical application circuit for the ADT7488A is shown
in Figure 18. The ADT7488A can be used in conjunction
with the ADP3192 controller to monitor the CPU power.
The result is a complete thermal and power monitor for the
CPU.
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