ADT7476
Rev. B | Page 21 of 72
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
ADT7476 is to measure the change in V
BE
when the device is
operated at two different currents.
This is given by
Δ
V
BE
=
KT
/
q
×
ln
(
N
)
where:
K
is Boltzmann’s constant.
T
is the absolute temperature in Kelvin.
q
is the charge on the carrier.
ln
(
N
) is the log of the ratio of the two currents (
N
).
Figure 27 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 can 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 25 and Figure 26
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 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
ΔV
BE
, the sensor is switched between operating currents of I
and N ×
I. The resulting waveform is passed through a 65 kHz
low-pass filter to remove noise and to a chopper-stabilized
amplifier that performs the functions of amplification and
rectification of the waveform to produce a dc voltage
proportional to ΔV
BE
. This voltage is measured by the ADC to
give a temperature output in 10-bit, twos complement format.
To further reduce the effects of noise, digital filtering is
performed by averaging the results of 16 measurement cycles.
A remote temperature measurement takes nominally 38 ms.
The results of remote temperature measurements are stored in
10-bit, twos complement format, as illustrated in Table 10. The
extra resolution for the temperature measurements is held in
the Extended Resolution Register 2 (0x77). This gives temper-
ature readings with a resolution of 0.25°C.
Noise Filtering
For temperature sensors operating in noisy environments, the
previous practice was to place a capacitor across the D+ pin and
the D− pin to help combat the effects of noise. However, large
capacitances affect the accuracy of the temperature measurement,
leading to a recommended maximum capacitor value of 1000 pF.
This capacitor reduces the noise but does not eliminate it, which
makes it difficult to use of the sensor in a very noisy environment.
In most cases, a capacitor is not required because differential
inputs by their very nature have a high immunity to noise.
2N3904
NPN
ADT7476
D+
D–
05382-027
Figure 25. Measuring Temperature Using an NPN Transistor
2N3906
PNP
ADT7476
D+
D–
05382-028
Figure 26. Measuring Temperature Using a PNP Transistor
05
38
2-
0
26
D+
BIAS
DIODE
V
DD
TO ADC
V
OUT+
V
OUT–
REMOTE
SENSING
TRANSISTOR
D–
THERMDA
THERMDC
I
N × I
I
BIAS
LOW-PASS FILTER
f
C
= 65kHz
CPU
Figure 27. Signal Conditioning for Remote Diode Temperature Sensors
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