7411(2017) 데이터 시트보기 (PDF) - Analog Devices
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7411
(Rev.:2017)
(Rev.:2017)
Analog Devices
7411 Datasheet PDF : 36 Pages
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ADT7411
Temperature Measurement Method
Internal Temperature Measurement
The ADT7411 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. As both positive and negative temperatures can be
measured, the temperature data is stored in twos complement
format, as shown in Table 6. 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 ADT7411 can measure the temperature of one external
diode sensor or diode-connected transistor.
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, the absolute
value of VBE varies from device to device, and individual
calibration is required to null this out, so the technique is
unsuitable for mass production.
The technique used in the ADT7411 is to measure the change
in VBE when the device is operated at two different currents.
This is given by
ΔVBE = KT/q × In (N)
where:
K is Boltzmann’s constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvin.
N is the ratio of the two currents.
Figure 23 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
temperature 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 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.
Data Sheet
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 ∆VBE, the sensor is switched between operating
currents 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 rectification of the waveform to produce a dc voltage
proportional to ∆VBE. This voltage is measured by the ADC
to give a temperature 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, particularly
when measuring the very small voltages from a remote diode
sensor. The following precautions should be taken:
1. Place the ADT7411 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 (see Figure 28).
GND
D+
D–
GND
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
Figure 28. 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 as
1°C corresponds 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.
Rev. C | Page 18 of 36
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