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ADT7476
Table 14. TWOS COMPLEMENT TEMPERATURE DATA
FORMAT
Temperature Digital Output (10-bit) (Note 1)
Remote Temperature Measurement
The ADT7476 can measure the temperature of two remote
diode sensors or diode-connected transistors connected to
Pin 17 and Pin 18, or Pin 15 and Pin 16.
–128 ° C
–50 ° C
–25 ° C
–10 ° C
0 ° C
+10.25 ° C
+25.5 ° C
+50.75 ° C
+75 ° C
+100 ° C
1000 0000 00 (Diode Fault)
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
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. As a result,
this technique is unsuitable for mass production. The
technique used in the ADT7476 is to measure the change in
V BE when the device is operated at two different currents.
This is given by:
q
D V BE + kT In(N) (eq. 1)
+125 ° C 0111 1101 00
+127 ° C 0111 1111 00
1. Bold numbers denote 2 LSB of measurement in the Extended
Resolution Register 2 (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)
where:
k is the 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 25 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,
which is provided on some microprocessors for temperature
–1 ° C
0 ° C
1 ° C
10 ° C
25 ° C
50 ° C
75 ° C
100 ° C
125 ° C
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
monitoring. It could also be a discrete transistor such as a
2N3904/2N3906.
If a discrete transistor is used, the collector is not grounded
and is 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 ADT7476 to an NPN
or PNP transistor for temperature measurement. To prevent
ground noise from interfering with the measurement, the
191 ° C 1111 1111 00
1. Bold numbers denote 2 LSB of measurement in the Extended
Resolution Register 2 (0x77) with 0.25 ° C resolution.
more negative terminal of the sensor is not referenced to
ground, but is biased above ground by an internal diode at
the D– input.
CPU
THERMDA
D+
I
N × I
I BIAS
V DD
V OUT+
REMOTE
SENSING
TRANSISTOR
THERMDC
D ?
BIAS
To ADC
V OUT ?
DIODE
LOW-PASS FILTER
f C = 65 kHz
Figure 25. Signal Conditioning for Remote Diode Temperature Sensors
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