ADT7484A データシートの表示（PDF） - ON Semiconductor
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ADT7484A Datasheet PDF : 14 Pages
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ADT7484A/ADT7486A
SST Temperature Sensor Data Format
The data for temperature is structured to allow values in
the range of ±512°C 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
0.016°C resolution.
Table 8. SST Temperature Data Format
Temperature (5C)
Twos Complement
−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/
D+ ADT7486A
D–
2N3906
PNP
ADT7484A/
D+ ADT7486A
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:
• 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.
• 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.
• Use wide tracks to minimize inductance and reduce
noise pickup. A 5 mil track minimum width and
spacing is recommended.
GND
D+
D–
GND
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
Figure 16. Arrangements of Signal Tracks
• 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.
• Thermocouple effects should not be a major problem
because 1°C corresponds to about 240 mV, and
thermocouple voltages are about 3 mV/°C 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.
• Place a 0.1 mF bypass capacitor close to the device.
• 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 to 12 feet.
• 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 D1+ and D1−
and the shield to GND, close to the device. 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°C error.
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