ADM1021AARQ View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
ADM1021AARQ Datasheet PDF : 16 Pages
| |||
ADM1021A
APPLICATIONS INFORMATION
FACTORS AFFECTING ACCURACY
Remote Sensing Diode
The ADM1021A is designed to work with substrate transistors
built into processors, or with discrete transistors. Substrate
transistors will generally be PNP types with the collector connected
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, 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+.
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 con-
tact 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 tem-
perature change. In the case of the remote sensor this should not
be a problem, as it will be either a substrate transistor in the pro-
cessor 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 because
the ADM1021A is measuring very small voltages from the
remote sensor, care must be taken to minimize noise induced at
the sensor inputs. The following precautions should be taken:
1. Place the ADM1021A as close as possible to the remote
sensing 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 recommended.
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– paths
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 between
them, thermocouple voltages should be much less than 240 µV.
5. Place a 0.1 µF bypass capacitor close to the VDD pin, and
2200 pF input filter capacitors across D+, D– close to the
ADM1021A.
GND
D+
D–
GND
10 mil
10 mil
10 mil
10 mil
10 mil
10 mil
10 mil
Figure 7. Arrangement of Signal Tracks
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.
REV. D
–11–
Share Link: 