EVAL-ADT7490EBZ データシートの表示（PDF） - Analog Devices
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EVAL-ADT7490EBZ Datasheet PDF : 76 Pages
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THERMAL DIODE TEMPERATURE MEASUREMENT
METHOD
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, measuring the base-
emitter voltage (VBE) of a transistor operated at constant
current. Unfortunately, this technique requires calibration to
null out the effect of the absolute value of VBE, which varies
from device to device.
The technique used in the ADT7490 is to measure the change
in VBE when the device is operated at three different currents.
Previous devices have used only two operating currents, but the
use of a third current allows automatic cancellation of resis-
tances in series with the external temperature sensor.
Figure 29 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, but it could equally
be a discrete transistor, such as a 2N3904/2N3906.
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. Figure 26 and Figure 27
show how to connect the ADT7490 to an NPN or PNP transis-
tor for temperature measurement.
ADT7490
2N3904
NPN
D+
D–
Figure 26. Measuring Temperature Using an NPN Transistor
ADT7490
D+
2N3906
PNP
D–
Figure 27. Measuring Temperature Using a PNP 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. C1 can optionally be added as a noise
filter (recommended maximum value of 1000 pF). However, a
better option in noisy environments is to add a filter, as
described in the Series Resistance Cancellation section.
ADT7490
Remote Temperature Measurement
The ADT7490 can measure the temperature of two remote
diode sensors or diode-connected transistors connected to
Pin 10 and Pin 11, or Pin 12 and Pin 13.
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. Therefore, the technique
is unsuitable for mass production. The technique used in the
ADT7490 is to measure the change in VBE when the device is
operated at three different currents. This is given by
ΔVBE
=
KT
q
× ln(N)
where:
K is the Boltzmann constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvin.
N is the ratio of the two currents.
To measure ΔVBE, the operating current through the sensor is
switched among three related currents. N1 × I and N2 × I are
different multiples of the current I, as shown in Figure 28. The
currents through the temperature diode are switched between
I and N1 × I, giving ΔVBE1, and then between I and N2 × I,
giving ΔVBE2. The temperature can then be calculated using the
two ΔVBE measurements. This method can also cancel the effect
of any series resistance on the temperature measurement.
The resulting ΔVBE waveforms are passed through a 65 kHz
low-pass filter to remove noise and then to a chopper-stabilized
amplifier. This amplifies and rectifies the waveform to produce
a dc voltage proportional to ΔVBE. The ADC digitizes this
voltage, and a temperature measurement is produced. To reduce
the effects of noise, digital filtering is performed by averaging
the results of 16 measurement cycles.
The results of remote temperature measurements are stored in
10-bit, twos complement format, as listed in Table 15. The extra
resolution for the temperature measurements is held in the
Extended Resolution Register 2 (0x77). This gives temperature
readings with a resolution of 0.25°C.
Rev. 0 | Page 21 of 76
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