ADT7488A Просмотр технического описания (PDF) - ON Semiconductor
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ADT7488A Datasheet PDF : 12 Pages
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ADT7488A
rates. Signal conditioning and measurement of the internal
temperature sensor is performed in the same manner.
N1 × I N2 × I
I
VDD
IBIAS
REMOTE
SENSING
TRANSISTOR D1+
C1*
D1–
BIAS
DIODE
*CAPACITOR C1 IS OPTIONAL.
IT SHOULD ONLY BE USED
IN NOISY ENVIRONMENTS.
LOW−PASS FILTER
fC = 65kHz
VOUT+
TO
ADC
VOUT–
Figure 15. Signal Conditioning for Remote Diode
Temperature Sensors
Reading Temperature Measurements
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 0x06. The command codes and returned data are
described in Table 10.
Table 10. 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 ±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 11. SST Temperature Data Format
Temperature (5C)
−125
−80
−40
−20
−5
−1
0
+1
+5
+20
+40
+80
+125
Twos Complement
LSB
MSB
1110 0000
1100 0000
1110 1100
0000 0000
1111 0110
0000 0000
1111 1011
0011 1110
1111 1110
1100 0000
1111 1111
1100 0000
0000 0000
0000 0000
0000 0000
0100 0000
0000 0001
0100 0000
0000 0100
1100 0010
0000 1010
0000 0000
0001 0100
0000 0000
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 16 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.
2N3904
NPN
ADT7488A
D1+
D1–
2N3906
PNP
ADT7488A
D1+
D1–
Figure 16. 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.
GND
D1+
D1–
GND
5mil
5mil
5mil
5mil
5mil
5mil
5mil
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 mV, and
thermocouple voltages are about 3 mV/°C of the
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