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參數(shù)資料
| 型號： | ADT7518ARQZ-REEL |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | SPI/I2C Compatible, Temperature Sensor, 4-Channel ADC and Quad Voltage Output DAC |
| 中文描述： | DIGITAL TEMP SENSOR-SERIAL, 10BIT(s), 5Cel, RECTANGULAR, SURFACE MOUNT |
| 封裝： | LEAD FREE, MO-137-AB, QSOP-16 |
| 文件頁數(shù)： | 24/40頁 |
| 文件大?。?/td> | 1217K |
| 代理商： | ADT7518ARQZ-REEL |
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ADT7518
2 register and set C4 to 1 (if not done so already), then write all
0s to Bits C0:C2. All subsequent conversions will be done on the
V
DD
channel only. To change the channel selection to the inter-
nal temperature channel, write to the Control Configuration 2
register and set C0 = 1. When measuring in single-channel
mode, conversions on the channel selected occur directly after
each other. Any communication to the ADT7518 stops the
conversions, but they are restarted once the read or write
operation is completed.
Internal Temperature Measurement
The ADT7518 contains an on-chip band gap temperature
sensor whose output is digitized by the on-chip ADC. The
temperature data is stored in the Internal Temperature Value
register. Because both positive and negative temperatures can be
measured, the temperature data is stored in twos complement
format, as shown in Table 9. The thermal characteristics of the
measurement sensor could change and, therefore, an offset is
added to the measured value to enable the transfer function to
match the thermal characteristics. This offset is added before
the temperature data is stored. The offset value used is stored in
the internal temperature offset register.
External Temperature Measurement
The ADT7518 can measure the temperature of one external
diode sensor or diode-connected transistor.
Rev. A | Page 24 of 40
The forward voltage of a diode or diode-connected transistor,
operated at a constant current, exhibits a negative temperature
coefficient of about –2 mV/°C. Unfortunately, because the
absolute value of V
BE
varies from device to device, and indivi-
dual calibration is required to null this out, the technique is
unsuitable for mass production.
The technique used in the ADT7518 is to measure the change in
V
BE
when the device is operated at two different currents. This is
given by
( )
n
q
KT
V
BE
1
/
×
=
where:
K
is Boltzmann’s constant.
q
is the charge on the carrier.
T
is the absolute temperature in kelvins.
N
is the ratio of the two currents.
Figure 41 shows the input signal conditioning used to measure
the output of an external temperature sensor. This figure shows
the external sensor as a substrate transistor, provided for temp-
erature monitoring on some microprocessors, but it could
equally well be a discrete transistor.
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 to the D+ input. If
an NPN transistor is used, the emitter is connected to the D–
input and the base to the D+ input. A 2N3906 is recommended
as the external transistor.
To prevent ground noise 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. As the sensor is operating in a noisy environment, C1
is provided as a noise filter. See the Layout Considerations
section for more information on C1.
To measure V
BE
, the sensor is switched between operating cur-
rents of I and N × I. The resulting waveform is passed through a
low-pass filter to remove noise, then to a chopper-stabilized
amplifier that performs the functions of amplification and recti-
fication of the waveform to produce a dc voltage proportional
to V
BE
. This voltage is measured by the ADC to give a temper-
ature output in 10-bit twos complement format. To further
reduce the effects of noise, digital filtering is performed by
averaging the results of 16 measurement cycles.
Layout Considerations
Digital boards can be electrically noisy environments, and care
must be taken to protect the analog inputs from noise, particu-
larly when measuring the very small voltages from a remote
diode sensor. The following precautions should be taken:
1.
Place the ADT7518 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.
2.
Route the D+ and D– tracks close together, in parallel, with
grounded guard tracks on each side. Provide a ground
plane under the tracks, if possible.
3.
Use wide tracks to minimize inductance and reduce noise
pickup. A 10 mil track minimum width and spacing is
recommended.
GND
D+
D–
GND
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
0
Figure 50. Arrangement of Signal Tracks
4.
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 D+ and
D– path and at the same temperature. Thermocouple
effects should not be a major problem because 1°C corres-
ponds to about 240 μV, and thermocouple voltages are
about 3 μV/°C of temperature difference. Unless there are
two thermocouples with a big temperature differential
between them, thermocouple voltages should be much less
than 200 mV.
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