Series Resistance Cancellation; Factors Affecting Diode Accuracy - Analog Devices dBCool ADT7473 Manual

2N3904
NPN
Figure 22. Measuring Temperature Using an NPN Transistor
2N3906
PNP
Figure 23. Measuring Temperature Using a PNP Transistor
To measure ∆ V
, the operating current through the sensor is
BE
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
, a nd then between I and N2 × I,
BE1
giving ∆ V
. The temperature can then be calculated using the
BE2
two ∆ V
measurements. This method can also cancel the effect
BE
of any series resistance on the temperature measurement.
The resulting ∆V waveforms are passed through a 65 kHz
BE
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
ADT7473
D+
D–
ADT7473
D+
D–
. The ADC digitizes this
BE
duce
easurements are stored in
s listed in Table 8. The extra
0 pF.
t.
r
effect
ote sensor is
e result.
The construction of a filter allows t
temperature sensor to operate in noisy
shows a low-pass R-C filter with the following values:
R = 100 Ω, C = 1 nF
This filtering reduces both common-mod
differential noise.
TEMPERATURE
Figure 24. Filter Between Remote Sensor and ADT7473

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 e on the temperature reading, giving a more accurate
result without the need for user characterization
tance. The ADT7473 is designed to automatically cancel up to
zed
3 kΩ of resistance, typically. This is transparent to the user by
e
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 strate. Discrete types can be either PNP or NPN
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−
nt, and the emitter is connected to D+.
To reduce the error due t
discrete transistors, a number of factors should be taken into
consideration:
•
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 erature, T(°C), when usin
not equal 1.0
to obtain the n
∆ T = (n
Rev. A | Page 17 of 76
he ADT7473 and the remote
environments. Figure 24
e noise and
100Ω
REMOTE
SENSOR
100Ω
e Filtering section for details.
o variations in both substrate and
, of the transistor is a measure of the
f
value of 1.008. Us
f
g a transistor whose n
08. Refer to the da ta sheet
valu es.
f
− 1.00 8)/1.008 × (273.1 5 K + T)
f
ADT7473
D+
1nF
D–
eries
of this resis-
sis-
to the collector).
ase are con-
to D−. If a PNP
e the
at a
does
f
for the related CPU