ADT7473 Analog Devices, Inc., ADT7473 Datasheet - Page 17

• Manufacturer Part Number: ADT7473
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

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
two ∆ V
of any series resistance on the temperature measurement.
The resulting ∆V
low-pass filter to remove noise and then to a chopper-stabili
amplifier. This amplifies and rectifies the waveform to produc
a dc voltage proportional to ∆V
voltage, and a temperature measurement is produced. To re
the effects of noise, digital filtering is performed by averaging
the results of 16 measurement cycles.
The results of remote temperature m
10-bit, twos complement format, a
resolution for the temperature measurements is held in the
Extended Resolution Register 2 (0x77). This gives temperature
readings with a resolution of 0.25°C.
Noise Filtering
For temperature sensors operating in noisy environments,
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 measureme
leading to a recommended maximum capacitor value of 100
This capacitor reduces the noise, but does not eliminate it,
making use of the sensor difficult in a very noisy environmen
The ADT7473 has a major advantage over other devices fo
eliminating the effects of noise on the external sensor. Using the
series resistance cancellation feature, a filter can be constructed
between the external temperature sensor and the part. The
of any filter resistance seen in series with the rem
automatically canceled from the temperatur
Figure 22. Measuring Temperature Using an NPN Transistor
BE
Figure 23. Measuring Temperature Using a PNP Transistor
measurements. This method can also cancel the effect
BE2
. The temperature can then be calculated using the
2N3904
2N3906
BE
NPN
PNP
, the operating current through the sensor is
BE
waveforms are passed through a 65 kHz
BE1
, a
nd then between I and N2 × I,
BE
. The ADC digitizes this
s listed in Table 8. The extra
D+
D–
D+
D–
ADT7473
ADT7473
easurements are stored in
e result.
ote sensor is
r
0 pF.
effect
duce
t.
zed
Rev. A | Page 17 of 76
nt,
e
The construction of a filter allows t
temperature sensor to operate in noisy
shows a low-pass R-C filter with the following values:
This filtering reduces both common-mod
differential noise.
SERIES RESISTANCE CANCELLATION
Parasitic resistance to the ADT7473 D+ and D− inputs (seen in
series with the remote diode) is caused by a variety of factors
including PCB track resistance and track length. This series
resistance appears as a temperature offset in the remote sensor’s
temperature measurement. This error typically causes a 0.5°C offset
per Ω of parasitic resistance in series with the remote diode.
The ADT7473 automatically cancels out the effect of this s
resistanc
result without the need for user characterization
tance. The ADT7473 is designed to automatically cancel up to
3 kΩ of resistance, typically. This is transparent to the user by
using an advanced temperature measurement method. This
feature allows resistances to be added to the sensor path to
produce a filter, allowing the part to be used in noisy
environments. See the Nois
FACTORS AFFECTING DIODE ACCURACY
Remote Sensing Diode
The ADT7473 is designed to work with either substrate tran
tors built into processors or discrete transistors. Substrate
transistors are generally PNP types with the collector connected
to the sub
transistors connected as a diode (base-shorted
If an NPN transistor is used, the collector and b
nected to D+ and the emitter is connected
transistor is used, the collector and base are connected to D−
and the emitter is connected to D+.
To reduce the error due t
discrete transistors, a number of factors should be taken into
consideration:
•
R = 100 Ω, C = 1 nF
The ideality factor, n
deviation of the thermal diode from ideal behavior. The
ADT7473 is trimmed for an n
following equation to calculate the error introduced
temp
not equal 1.0
to obtain the n
e on the temperature reading, giving a more accurate
TEMPERATURE
Figure 24. Filter Between Remote Sensor and ADT7473
∆ T = (n
strate. Discrete types can be either PNP or NPN
erature, T(°C), when usin
REMOTE
SENSOR
f
08. Refer to the da
− 1.00
f
valu
es.
8)/1.008 ×
o variations in both substrate and
f
, of the transistor is a measure of the
e Filtering section for details.
f
he ADT7473 and the remote
g a transistor whose n
(273.1
value of 1.008. Us
ta sheet
100Ω
100Ω
environments. Figure 24
e noise and
5 K + T)
to D−. If a PNP
1nF
for the related CPU
to the collector).
ase are con-
of this resis-
D+
D–
ADT7473
e the
at a
f
does
eries
sis-