Series Resistance Cancellation; Factors Affecting Diode Accuracy - Analog Devices dBCool ADT7467 Manual

The ADT7467 has a major advantage over other devices for
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 effect
of any filter resistance seen in series with the remote sensor is
automatically canceled from the temperature result.
The construction of a filter allows the ADT7467 and the remote
temperature sensor to operate in noisy environments. Figure 24
shows a low-pass R-C-R filter, with the following values:
R = 100 Ω, C = 1 nF
This filtering reduces both common-mode noise and
differential noise.
REMOTE
TEMPERATURE
SENSOR
Figure 24. Filter between Remote Sensor and ADT7467

SERIES RESISTANCE CANCELLATION

Parasitic resistance to the ADT7467 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 1 Ω of parasitic resistance in series with the remote
diode.
The ADT7467 automatically cancels out the effect of this series
resistance on the temperature reading, giving a more accurate
result without the need for user characterization of this
resistance. The ADT7467 is designed to automatically cancel,
typically, up to 3 kΩ of resistance. By using an advanced
temperature measurement method, this is transparent to the
user. 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 Noise Filtering section for details.

FACTORS AFFECTING DIODE ACCURACY

Remote Sensing Diode
The ADT7467 is designed to work with either substrate
transistors built into processors or discrete transistors. Substrate
transistors are generally PNP types with the collector connected
to the substrate. Discrete types can be either PNP or NPN
transistors connected as a diode (base-shorted to the collector).
If an NPN transistor is used, the collector and base are
connected to D+ and the emitter is connected to D−. If a PNP
transistor is used, the collector and base are connected to D−
and the emitter is connected to D+.
100Ω
D+
1nF
100Ω
D–
Rev. 0| Page 17 of 80
To reduce the error due to variations in both substrate and
discrete transistors, a number of factors should be taken into
consideration:
•
The ideality factor, n , of the transistor is a measure of the
f
deviation of the thermal diode from ideal behavior. The
ADT7467 is trimmed for an n
following equation to calculate the error introduced at a
temperature T (°C), when using a transistor whose n
not equal 1.008. See the processor data sheet for the n
values.
∆ T = (n
− 1.008)/1.008 × (273.15 K + T)
f
To factor this in, the user can write the ∆T value to the
offset register. The ADT7467 then automatically adds it to
or subtracts it from the temperature measurement.
•
Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7467, I , is 96 µA and the low level current, I
HIGH
is 6 µA. If the ADT7467 current levels do not match the
current levels specified by the CPU manufacturer, it might
be necessary to remove an offset. The CPU's data sheet
advises whether this offset needs to be removed and how to
calculate it. This offset can be programmed to the offset
register. It is important to note that, if more than one offset
must be considered, the algebraic sum of these offsets must
be programmed to the offset register.
If a discrete transistor is used with the ADT7467, the best
accuracy is obtained by choosing devices according to the
following criteria:
•
Base-emitter voltage greater than 0.25 V at 6 µA, at the
highest operating temperature.
•
Base-emitter voltage less than 0.95 V at 100 µA, at the
lowest operating temperature.
•
Base resistance less than 100 Ω.
•
Small variation in h (say 50 to 150) that indicates tight
FE
control of V characteristics.
BE
Transistors, such as 2N3904, 2N3906, or equivalents in SOT-23
packages, are suitable devices to use.
ADT7467
value of 1.008. Use the
f
does
f
f
LOW
,