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ADT7490
Table 17. TWOS COMPLEMENT TEMPERATURE DATA
FORMAT
Temperature Digital Output (10-bit) (Note 1)
–128 ? C 1000 0000 00 (Diode Fault)
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+
–63 ? C
–50 ? C
–25 ? C
–10 ? C
0 ? C
10.25 ? C
25.5 ? C
50.75 ? C
75 ? C
100 ? C
1100 0001 00
1100 1110 00
1110 0111 00
1111 0110 00
0000 0000 00
0000 1010 01
0001 1001 10
0011 0010 11
0100 1011 00
0110 0100 00
D+ input. If an NPN transistor is used, the emitter is
connected to the D– input and the base to the D+ input.
Figure 25 and Figure 26 show how to connect the ADT7490
to an NPN or PNP transistor for temperature measurement.
ADT7490
2N3904
NPN
D ?
Figure 25. Measuring Temperature by Using
an NPN Transistor
125 ? C 0111 1101 00
127 ? C 0111 1111 00
1. Bold numbers denote 2 LSBs of measurement in the Extended
Resolution 2 register (Register 0x77) with 0.25 ? C resolution.
2N3906
PNP
ADT7490
D+
D ?
Table 18. OFFSET 6 4 DATA FORMAT
Figure 26. Measuring Temperature by Using
Temperature
–64 ? C
–63 ? C
–1 ? C
0 ? C
1 ? C
10 ? C
25 ? C
50 ? C
75 ? C
100 ? C
125 ? C
Digital Output (10-bit) (Note 1)
0000 0000 00 (Diode Fault)
0000 0001 00
0011 1111 00
0100 0000 00
0100 0001 00
0100 1010 00
0101 1001 00
0111 0010 00
1000 1001 00
1010 0100 00
1011 1101 00
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 1,000 pF).
However, a better option in noisy environments is to add a
filter, as described in the section.
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
191 ? C 1111 1111 00
1. Bold numbers denote 2 LSBs of measurement in the Extended
Resolution 2 register (Register 0x77) with 0.25 ? C resolution.
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 (V BE ) of a transistor operated at
constant current. Unfortunately, this technique requires
calibration to null out the effect of the absolute value of V BE ,
which varies from device to device.
The technique used in the ADT7490 is to measure the
change in V BE 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 resistances in series with the external
temperature sensor.
Figure 28 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
transistor operated at a constant current exhibits a negative
temperature coefficient of about ? 2 mV/ ? C. Unfortunately,
the absolute value of V BE 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
V BE when the device is operated at three different currents.
This is given by:
q
D V BE + kT In(N) (eq. 2)
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 D V BE , 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 27. The currents through the temperature diode are
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