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| 型號： | ADT7485ARMZ-REEL7 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | DIGITAL TEMP SENSOR-SERIAL, 16BIT(s), 1Cel, SQUARE, SURFACE MOUNT |
| 封裝： | LEAD FREE, PLASTIC, MO-187-BA, MSOP-10 |
| 文件頁數(shù)： | 6/16頁 |
| 文件大?。?/td> | 235K |
| 代理商： | ADT7485ARMZ-REEL7 |
ADT7485
Preliminary Technical Data
Rev. PrA | Page 14 of 16
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 15 shows how to connect the ADT7485 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.
D1+
D1–
ADT7485
2N3904
NPN
D1+
D1–
ADT7485
2N3906
PNP
05
19
7
-00
5
Figure 16. Connections for NPN and PNP Transistors
The ADT7485 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 ADT7485 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
GND
D1+
GND
D1–
0
51
97
-0
0
6
Figure 17. 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 ADT7485.
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 six 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 ADT7485. 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.
TEMPERATURE OFFSET
As CPUs run faster, it is more difficult to avoid high frequency
clocks when routing the D1+ and D1 tracks around a system
board. Even when the recommended layout guidelines are
followed, there may still be temperature errors, attributed to
noise being coupled on to the D1+ and D1 lines. High
frequency noise generally has the effect of producing
temperature measurements that are consistently too high by a
specific amount. The ADT7485 has a temperature offset
command code of 0xe0 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 ADT7485. The offset is automatically added
to every temperature measurement. The maximum offset is
±128°C with 0.25°C resolution. The offset format is the same as
the temperature data format—16-bit, twos compliment
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.
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