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ADT7488A データシート(PDF) 14 Page - Analog Devices

部品番号 ADT7488A
部品情報  SST Digital Temperature Sensor and Voltage Monitor
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メーカー  AD [Analog Devices]
ホームページ  http://www.analog.com
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ADT7488A データシート(HTML) 14 Page - Analog Devices

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ADT7488A
Rev. 0 | Page 14 of 16
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 15. SST Temperature Data Format
Twos Complement
Temperature (°C)
MSB
LSB
−125
1110 0000
1100 0000
−80
1110 1100
0000 0000
−40
1111 0110
0000 0000
−20
1111 1011
0011 1110
−5
1111 1110
1100 0000
−1
1111 1111
1100 0000
0
0000 0000
0000 0000
+1
0000 0000
0100 0000
+5
0000 0001
0100 0000
+20
0000 0100
1100 0010
+40
0000 1010
0000 0000
+80
0001 0100
0000 0000
+125
0001 1111
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 D− input and the emitter is 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 17 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.
D1+
D1–
ADT7488A
2N3904
NPN
D1+
D1–
ADT7488A
2N3906
PNP
Figure 17. Connections for NPN and PNP Transistors
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:
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.
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.
5mil
5mil
5mil
5mil
5mil
5mil
5mil
GND
D1+
GND
D1–
Figure 18. 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 μV, and thermocouple voltages
are about 3 μV/°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 μF bypass capacitor close to the ADT7488A.
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.
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 Ω series resistance introduces about 0.5°C error.


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