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參數(shù)資料
| 型號： | ADT7462ACPZ-500RL7 |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | DIGITAL TEMP SENSOR-SERIAL, 8BIT(s), 4Cel, SQUARE, SURFACE MOUNT |
| 封裝： | 5 X 5 MM, LFCSP-32 |
| 文件頁數(shù)： | 17/88頁 |
| 文件大?。?/td> | 836K |
| 代理商： | ADT7462ACPZ-500RL7 |
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ADT7462
Rev. 2 | Page 24 of 88 | www.onsemi.com
Offset Registers
The ADT7462 has temperature offset registers at Register 0x56
to Register 0x59 for the local, Remote 1, Remote 2, and Remote 3
temperature channels. By doing a one-time calibration of the
system, the user can determine the offset caused by system
board noise and cancel it using the offset registers. The offset
registers automatically add a twos complement, 8-bit reading to
every temperature measurement. The LSBs add 0.5°C offset to
the temperature reading so the 8-bit register effectively allows
temperature offsets of up to ±64°C with a resolution of 0.5°C.
This ensures that the readings in the temperature measurement
registers are as accurate as possible.
Temperature Offset Registers
Register 0x56 Local Temperature Offset = 0x00 (0°C default)
Register 0x57 Remote 1 Temperature Offset = 0x00 (0°C default)
Register 0x58 Remote 2 Temperature Offset = 0x00 (0°C default)
Register 0x59 Remote 3 Temperature Offset = 0x00 (0°C default)
Layout Considerations
Digital boards can be electrically noisy environments. The
ADT7462 measures very small voltages from the remote sensor,
so care must be taken to minimize noise induced at the sensor
inputs. The following precautions should be taken:
Place the ADT7462 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 4 inches to 8 inches.
Route the D+ and D tracks close together, in parallel,
with grounded guard tracks on each side. To minimize
inductance and reduce noise pickup, a 5 mil track width
and spacing is recommended. If possible, provide a ground
plane under the tracks.
5MIL
GND
D+
GND
D–
0556
9-
025
Figure 35. Typical Arrangement of Signal Tracks
Minimize the number of copper/solder joints that can
cause thermocouple effects. Where copper/solder joints are
used, make sure that they are in both the D+ and D path
and at the same temperature.
Thermocouple effects should not be a major problem
because 1°C corresponds to about 200 mV, and thermocouple
voltages are about 3 mV/°C of temperature difference.
Unless there are two thermocouples with a large tempera-
ture differential between them, thermocouple voltages
should be much less than 200 mV.
Place a 0.1 μF bypass capacitor close to the VCC pin. In
extremely noisy environments, an input filter capacitor
can be placed across D+ and D close to the ADT7462.
This capacitance can affect the temperature measurement,
so care must be taken to ensure that any capacitance seen
at D+ and D is a maximum of 1000 pF. This maximum
value includes the filter capacitance, plus any cable or stray
capacitance between the pins and the sensor diode.
If the distance to the remote sensor is more than 8 inches,
the use of twisted pair cable is recommended. This works
from about 6 feet up to 12 feet.
For really long distances (up to 100 feet), use shielded
twisted pair, such as Belden No. 8451 microphone cable.
Connect the twisted pair to D+ and D and the shield to
GND close to the ADT7462. Leave the remote end of the
shield unconnected to avoid ground loops.
Because the measurement technique uses switched current
sources, excessive cable or filter capacitance can affect the
measurement. When using long cables, the filter capaci-
tance can be reduced or removed.
Noise Filtering
For temperature sensors operating in noisy environments, the
industry-standard practice is to place a capacitor across the D+
and D pins to help combat the effects of noise. However, large
capacitances affect the accuracy of the temperature measure-
ment, leading to a recommended maximum capacitor value of
1000 pF. While this capacitor does reduce noise, it does not
eliminate it, making it difficult to use the sensor in a very noisy
environment.
The ADT7462 has a major advantage over other devices in
eliminating the effects of noise on the external sensor. The
series resistance cancellation feature allows a filter to be
constructed between the external temperature sensor and the
device. The effect of any filter resistance seen in series with the
remote sensor is automatically canceled from the temperature
result.
The construction of a filter allows the ADT7462 and the remote
temperature sensor to operate in noisy environments. Figure 36
shows a low-pass RCR filter, with the following values:
R = 100 Ω
C = 1 nF
This filtering reduces both common-mode noise and
differential noise.
100
1nF
REMOTE
SENSOR
05
569-
02
6
D+
D–
Figure 36. Filter Between Remote Sensor and the ADT7462
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