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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 (Address 0x26). Because both positive and negative
temperatures can be measured, the temperature data is stored in
Offset 64 format or twos complement format, as shown in
Table 6 and Table 7. 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 operating temperature
range are not possible.
Remote Temperature Measurement
The ADT7473 can measure the temperature of two remote
diode sensors or diode-connected transistors connected to
Pins 10 and 11, or Pins 12 and 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
( )
Δ
=
/ ×
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.
when the device is
BE
×
I
N2
I
N1
REMOTE
SENSING
TRANSISTOR
D+
D–
Figure 21. Signal Conditioning for Remote Diode Temperature Sensors
Figure 21 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.
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 23 and
Figure 24 show how to connect the ADT7473 to an NPN or
PNP transistor for temperature measurement. 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.
To measure ∆ V
switched among three related currents. N1 × I and N2 × I are
different multiples of the current I, as shown in Figure 21. The
currents through the temperature diode are switched between
I and N1 × I, giving ∆ V
giving ∆ V
BE2
two ∆ V
measurements. This method can also cancel the effect
BE
of any series resistance on the temperature measurement.
The resulting ΔV
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 ΔV
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 6. The extra
resolution for the temperature measurements is held in the
extended resolution register 2 (Reg. 0x77). This gives
temperature readings with a resolution of 0.25°C.
V
DD
×
I
I
BIAS
LPF
f
= 65kHz
C
Rev. 0 | Page 15 of 76
, the operating current through the sensor is
BE
, and then between I and N2 × I,
BE1
. The temperature can then be calculated using the
waveforms are passed through a 65 kHz
BE
. The ADC digitizes this
BE
V
OUT+
TO ADC
V
OUT–
ADT7473
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