ADT7461
Rev. 3 | Page 19 of 23 | www.onsemi.com
APPLICATION INFORMATION
Noise Filtering
For temperature sensors operating in noisy environments, the
industry standard practice was to place a capacitor across the
D+ and D− pins 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 1,000 pF. While this capacitor reduces the noise, it does
not eliminate it, making it difficult to use the sensor in a very
noisy environment.
The ADT7461 has a major advantage over other devices for
eliminating the effects of noise on the external sensor. The
series resistance cancellation feature allows a filter to 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 cancelled from the temperature result.
The construction of a filter allows the ADT7461 and the remote
temperature sensor to operate in noisy environments. Figure 23
shows a low-pass R-C-R filter with the following values:
R = 100 Ω and C = 1 nF. This filtering reduces both common-
mode noise and differential noise.
04110-0-009
D+
1nF
100Ω
REMOTE
EMPERATURE
SENSOR
D–
100Ω
Figure 23. Filter Between Remote Sensor and ADT7461
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7461 is designed to work with substrate transistors
built into processors or with discrete transistors. Substrate
transistors are generally PNP types with the collector connected
to the substrate. Discrete types can be either PNP or NPN
transistor connected as a diode (base-shorted to collector). If an
NPN transistor is used, the collector and base are connected to
D+ and the emitter to D−. If a PNP transistor is used, the
collector and base are connected to D− and the emitter to D+.
To reduce the error due to variations in both substrate and
discrete transistors, several factors should be taken into
consideration:
• The ideality factor, n
F
, of the transistor is a measure of the
deviation of the thermal diode from ideal behavior. The
ADT7461 is trimmed for an n
F
value of 1.008. The
following equation may be used to calculate the error
introduced at a temperature T (°C), when using a transistor
whose n
f
does not equal 1.008. Consult the processor data
sheet for the n
F
values.
ΔT = (n
F
− 1.008)/1.008 × (273.15 Kelvin + T)
To factor this in, the user can write the ΔT value to the
offset register. It is then automatically added to or
subtracted from the temperature measurement by
the ADT7461.
• Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7461, I
HIGH
, is 96 μA, and the low level current,
I
LOW
, is 6 μA. If the ADT7461 current levels do not match
the current levels specified by the CPU manufacturer, it
may become necessary to remove an offset. The CPUs data
sheet advises whether this offset needs to be removed and
how to calculate it. This offset may 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 being used with the ADT7461, 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
FE
(50 to 150) that indicates tight
control of V
BE
characteristics.
Transistors, such as the 2N3904, 2N3906, or equivalents in
SOT-23 packages are suitable devices to use.
THERMAL INERTIA AND SELF-HEATING
Accuracy depends on the temperature of the remote sensing
diode and/or the internal temperature sensor being at the same
temperature as the environment being measured; many factors
can affect this. Ideally, the sensor should be in good thermal
contact with the part of the system being measured. If it is not,
the thermal inertia caused by the sensor’s mass causes a lag in
the response of the sensor to a temperature change. With a
remote sensor, this should not be a problem since it will be
either a substrate transistor in the processor or a small package
device, such as the SOT-23, placed in close proximity to it.
The on-chip sensor, however, is often remote from the
processor and only monitors the general ambient temperature
around the package. The thermal time constant of the SOIC-8
package in still air is about 140 seconds, and if the ambient air
temperature quickly changed by 100 degrees, it would take
about 12 minutes (5 time constants) for the junction tempera-
ture of the ADT7461 to settle within 1 degree of this. In
practice, the ADT7461 package is in electrical, and hence
