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ADT7473
D V BE + kT q
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 ADT7473/ADT7473 ? 1
measures 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 24 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
ln(N) (eq. 1)
where:
k is Boltzmann’s constant.
T is the absolute temperature in Kelvin.
q is the charge on the carrier.
N is the ratio of the two currents.
Figure 24 shows the input signal conditioning used to
measure the output of a remote temperature sensor. This
figure shows the external sensor as a substrate transistor,
provided for temperature monitoring on some
microprocessors. It could also be a discrete transistor such
as a 2N3904/2N3906.
Table 14. TWOS COMPLEMENT TEMPERATURE DATA
FORMAT
it could equally be a discrete transistor. If a discrete
transistor is used, the collector is not grounded and should
be linked to the base. 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 1,000 pF). However, a better option in
noisy environments is to add a filter, as described in the
Noise Filtering section.
Local Temperature Measurement
The ADT7473/ADT7473 ? 1 contains an on-chip band gap
temperature sensor whose output is digitized by the on-chip
10-bit ADC. The 8-bit MSB temperature data is stored in the
local temperature register (0x26). Because both positive and
Temperature
–128 ? C
–63 ? C
–50 ? C
–25 ? C
–10 ? C
0 ? C
10.25 ? C
25.5 ? C
50.75 ? C
75 ? C
100 ? C
125 ? C
127 ? C
Digital Output (10-bit) (Note 1)
1000 0000 00 (Diode Fault)
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
0111 1101 00
0111 1111 00
negative temperatures can be measured, the temperature
data is stored in Offset 64 format or twos complement
format, as shown in Table 14 and Table 15. Theoretically,
the temperature sensor and ADC can measure temperatures
from ? 63 ? C to +127 ? C (or ? 63 ? C to +191 ? C in the extended
temperature range) with a resolution of +0.25 ? C. However,
this exceeds the operating temperature range of the device,
1. Bold numbers denote 2 LSBs of measurement in the Extended
Resolution Register 2 (Register 0x77) with 0.25 ? C resolution.
Table 15. EXTENDED RANGE, TEMPERATURE DATA
FORMAT
Temperature Digital Output (10-bit) (Note 1)
–64 ? C 0000 0000 00 (Diode Fault)
so local temperature measurements outside the
ADT7473/ADT7473 ? 1 operating temperature range are not
possible.
Remote Temperature Measurement
The ADT7473/ADT7473 ? 1 can measure the temperature
–63 ? C
–1 ? C
0 ? C
1 ? C
10 ? C
0000 0001 00
0011 1111 00
0100 0000 00
0100 0001 00
0100 1010 00
of two remote diode sensors or diode-connected transistors
connected to Pin 10 and Pin 11 or to 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 V BE 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 ADT7473/ADT7473 ? 1 is to measure the change
25 ? C 0101 1001 00
50 ? C 0111 0010 00
75 ? C 1000 1001 00
100 ? C 1010 0100 00
125 ? C 1011 1101 00
191 ? C 1111 1111 00
1. Bold numbers denote 2 LSBs of measurement in the Extended
Resolution Register 2 (Register 0x77) with 0.25 ? C resolution.
in V BE when the device is operated at three different
currents. This is given by:
http://onsemi.com
15
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