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ADM1021 Datasheet(PDF) 11 Page - Analog Devices

Part No. ADM1021
Description  Low Cost Microprocessor System Temperature Monitor
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADM1021 Datasheet(HTML) 11 Page - Analog Devices

 
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ADM1021
–11–
REV. 0
APPLICATIONS INFORMATION
FACTORS AFFECTING ACCURACY
Remote Sensing Diode
The ADM1021 is designed to work with substrate transistors
built into processors, or with discrete transistors. Substrate
transistors will generally be PNP types with the collector con-
nected to the substrate. Discrete types can be either PNP or
NPN, connected as a diode (base shorted to collector). If an
NPN transistor is used then the collector and base are con-
nected to D+ and the emitter to D–. If a PNP transistor is used
then the collector and base are connected to D– and the emitter
to D+.
The user has no choice in the case of substrate transistors, but if
a discrete transistor is used the best accuracy will be obtained by
choosing devices according to the following criteria:
1. Base-emitter voltage greater than 0.25 V at 6
µA, at the high-
est operating temperature.
2. Base-emitter voltage less than 0.95 V at 100
µA, at the lowest
operating temperature.
3. Base resistance less than 100
Ω.
4. Small variation in hfe (say 50 to 150) which indicates tight
control of Vbe characteristics.
Transistors such as 2N3904, 2N3906 or equivalents in SOT-23
package 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 that being measured, and a number of factors
can affect this. Ideally, the sensor should be in good thermal
contact with the part of the system being measured, for example
the processor. If it is not, the thermal inertia caused by the mass
of the sensor will cause a lag in the response of the sensor to a
temperature change. In the case of the remote sensor this should
not be a problem, as it will be either a substrate transistor in the
processor or a small package device such as SOT-23 placed in
close proximity to it.
The on-chip sensor, however, will often be remote from the
processor and will only be monitoring the general ambient tem-
perature around the package. The thermal time constant of the
QSOP-16 package is about 10 seconds.
In practice, the package will have electrical, and hence thermal,
connection to the printed circuit board, so the temperature rise
due to self-heating will be negligible.
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments, and the
ADM1021 is measuring very small voltages from the remote
sensor, so care must be taken to minimize noise induced at the
sensor inputs. The following precautions should be taken:
1. Place the ADM1021 as close as possible to the remote sens-
ing diode. Provided that the worst noise sources such as
clock generators, data/address buses and CRTs are avoided,
this distance can be four to eight inches.
2. Route the D+ and D– tracks close together, in parallel, with
grounded guard tracks on each side. Provide a ground plane
under the tracks if possible.
3. Use wide tracks to minimize inductance and reduce noise
pickup. 10 mil track minimum width and spacing is recom-
mended.
GND
D+
D-
GND
10 mil.
10 mil.
10 mil.
10 mil.
10 mil.
10 mil.
10 mil.
Figure 17. Arrangement of Signal Tracks
4. Try to minimize the number of copper/solder joints, which
can cause thermocouple effects. Where copper/solder joints
are used, make sure that they are in both the D+ and D–
path and at the same temperature.
Thermocouple effects should not be a major problem as 1
°C
corresponds to about 240
µV, and thermocouple voltages are
about 3
µV/°C of temperature difference. Unless there are
two thermocouples with a big temperature differential be-
tween them, thermocouple voltages should be much less
than 240
µV.
5. Place a 0.1
µF bypass capacitor close to the V
DD pin and
2200 pF input filter capacitors across D+, D– close to the
ADM1021.
6. If the distance to the remote sensor is more than eight inches,
the use of twisted pair cable is recommended. This will work
up to about 6 to 12 feet.
7. For really long distances (up to 100 feet), use shielded twisted
pair such as Belden #8451 microphone cable. Connect the
twisted pair to D+ and D– and the shield to GND close to
the ADM1021. Leave the remote end of the shield uncon-
nected to avoid ground loops.
Because the measurement technique uses switched current
sources, excessive cable and/or filter capacitance can affect the
measurement. When using long cables, the filter capacitor may
be reduced or removed.
Cable resistance can also introduce errors. 1
Ω series resistance
introduces about 0.5
°C error.


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