ADT7484A(2012) 查看數據表（PDF） - ON Semiconductor
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ADT7484A Datasheet PDF : 14 Pages
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ADT7484A/ADT7486A
Reading Temperature Measurements
The temperature measurement command codes are
detailed in Table 12. The temperature data returned is two
bytes in little endian format, that is, LSB before MSB. All
temperatures can be read together by using Command Code
0x00 with a read length of 0x04. The command codes and
returned data are described in Table 12.
Table 12. TEMPERATURE CHANNEL COMMAND
CODES
Temp
Channel
Internal
External 1
External 2
All Temps
Command
Code
0x00
0x01
0x02
0x00
Returned Data
LSB, MSB
LSB, MSB
LSB, MSB
Internal LSB, Internal MSB;
External 1 LSB, External 1 MSB;
External 2 LSB, External 2 MSB
SST Temperature Sensor Data Format
The data for temperature is structured to allow values in
the range of 512C to be reported. Thus, the temperature
sensor format uses a twos complement, 16-bit binary value
to represent values in this range. This format allows
temperatures to be represented with approximately a
0.016C resolution.
Table 13. SST TEMPERATURE DATA FORMAT
Twos Complement
Temperature (5C)
−125
−80
−40
−20
−5
−1
0
+1
+5
+20
+40
+80
+125
MSB
1110 0000
1110 1100
1111 0110
1111 1011
1111 1110
1111 1111
0000 0000
0000 0000
0000 0001
0000 0100
0000 1010
0001 0100
0001 1111
LSB
1100 0000
0000 0000
0000 0000
0011 1110
1100 0000
1100 0000
0000 0000
0100 0000
0100 0000
1100 0010
0000 0000
0000 0000
0100 0000
Using Discrete Transistors
If a discrete transistor is used, the collector is not grounded
and should be linked to the base. If a PNP transistor is used,
the base is connected to the D1− input and the emitter is
connected to the D1+ input. If an NPN transistor is used, the
emitter is connected to the D1− input and the base is
connected to the D1+ input. Figure 17 shows how to connect
the ADT7484A/ADT7486A 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 D1− input.
2N3904
NPN
ADT7484A/
ADT7486A
D+
D−
2N3906
PNP
ADT7484A/
ADT8486A
D+
D−
Figure 15. Connections for NPN and PNP Transistors
The ADT7484A/ADT7486A show 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 device 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.
GND
D+
D−
GND
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
Figure 16. Arrangement of Signal Tracks
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 1C corresponds to about
240 mV, and thermocouple voltages are about
3 mV/C of the temperature difference. Unless
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