Analog Devices dBCool ADT7473 Manual page 16

ADT7473
Local Temperature Measurement
The ADT7473 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 tempera-
ture register (0x26). Because both positive and negative
temperatures can e measured, the temperature data is stored b
Offset 64 format or twos complement format, as shown in
Table 8 and Table 9. 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, so local temperature
measurements outside the ADT7473 o
range are not possible.
Table 8. Twos Complement Temperature Data Format
Temperature Digital Output (10-Bit)
–128°C 1000 0000 00 (diode fault)
–63°C
1100 0001 00
–50°C
1100 1110 00
–25°C 1110 0111 00
–10°C 1111 0110 00
0°C 0000 0000 00
10.25°C
0000 1010 01
25.5°C
0001 1001 10
50.75°C 0011 0010 11
75°C 0100 1011 00
100°C 0110 0100 00
125°C
0111 1101 00
127°C 0111 1111 00
1
Bold numbers denote 2 LSBs of measurement in Extended Resolution
Register 2 (0x77) with 0.25°C resolution.
Table 9. Extended Range, Temperature Data Format
Temperature Digital Output (10-Bit)
–64°C 0000 0000 00 (diode fault)
–63°C 0000 0001 00
–1°C 0011 1111 00
0°C
0100 0000
1°C 0100 0001 00
10°C 0100 1010 00
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 Extended Resolution
Register 2 (0x77) with 0.25°C resolution.
in
perating temperature
1
1
00
Rev. A | Page 16 of 76
Remote Temperature Measurement
The ADT7473 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 V varies from device to device and individual
BE
calibration is required to null this out, so the technique is
unsuitable for mass production. The technique used in the
ADT7473 is to measure the change in V
operated at three different currents. This is given by
( )
∆ = / ×
V KT q 1 n N
BE
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 21 shows the input signal conditioning used to measure
the output of a remote temperature s
the external sensor as a substrate transistor, provided for
temperature monitoring on some microprocessors. It could als
be a discrete transistor such as a 2N3904/2N3906.
I
× ×
I N2 I N1 I BIAS
RE MOTE
SE NSING
TRANS ISTOR
D+
D–
Figure 21. S ignal Conditioning for Remote Diode Temperatur
If a discrete transistor is used, the collector is not grounded and
shou ld be linked to the base. If a PNP transistor is used, the
ba se is connected to the D– input and the emitter is conn
to t e D+ input. If an NPN transistor is used, the emitter is h
conn ected to the D– input and the base is connected to the D+
inpu t. Figure 22 and Figure 23 show how to connect the
ADT 7473 to an NPN or PNP transistor for temperature
mea surement. 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
inte rnal diode at the D– input.
when the devic
BE
ensor. This figure shows
V
DD
LPF
V
OUT+
V
f = 65kHz
OUT–
C
e Sensors
e is
o
TO ADC
ected