ADT7485A
http://onsemi.com
11
Figure 16. Connections for NPN and PNP Transistors
ADT7485A
D1+
D1−
2N3904
NPN
ADT7485A
D1+
D1−
2N3906
PNP
The ADT7485A shows an external temperature value of
0x8000 if the external diode is an open or short circuit.
Layout Considerations
Digital boards can be electrically noisy environments.
Take the following precautions to protect the analog inputs
from noise, particularly when measuring the very small
voltages from a remote diode sensor:
1. Place the ADT7485A 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 D1+ and D1− 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. A 5 mil track minimum width
and spacing is recommended.
Figure 17. Arrangement of Signal Tracks
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
GND
D1−
D1+
GND
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 D1+ and D1− paths and
are at the same temperature.
5. Thermocouple effects should not be a major
problem because 1C corresponds to about
240 mV, and thermocouple voltages are about
3 mV/C of the temperature difference. Unless
there are two thermocouples with a big
temperature differential between them,
thermocouple voltages should be much less than
200 mV.
6. Place a 0.1 mF bypass capacitor close to the
ADT7485A.
7. If the distance to the remote sensor is more than
eight inches, the use of a twisted pair cable is
recommended. This works for distances of about
6 feet to 12 feet.
8. For very long distances (up to 100 feet), use
shielded twisted pair cables, such as Belden #8451
microphone cables. Connect the twisted pair cable
to D+ and D− and the shield to GND, close to the
ADT7485A. Leave the remote end of the shield
unconnected 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 can be reduced or removed. Cable resistance
can also introduce errors. A 1 W series resistance introduces
about 0.5C error.
Temperature Offset
As CPUs run faster, it is more difficult to avoid high
frequency clocks when routing the D+ and D− tracks around
a system board. Even when the recommended layout
guidelines are followed, there may still be temperature
errors, attributed to noise being coupled onto the D+ and D−
lines. High frequency noise generally has the effect of
producing temperature measurements that are consistently
too high by a specific amount. The ADT7485A has
temperature offset command code of 0xe0 through which a
desired offset can be set. By doing a one-time calibration of
the system, the offset caused by system board noise can be
calculated and nulled by specifying it in the ADT7485A.
The offset is automatically added to every temperature
measurement. The maximum offset is 128C with 0.25C
resolution. The offset format is the same as the temperature
data format; 16-bit, twos complement notation, as shown in
Table 15. The offset should be programmed in little endian
format, that is, LSB before MSB. The offset value is also
returned in little endian format when read.
Table 16. ORDERING INFORMATION
Device Order Number* Package Type Package Option Shipping
†
ADT7485AARMZ−R 10-lead MSOP RM−10 3,000 Tape & Reel
†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging
Specifications Brochure, BRD8011/D.
*This is Pb-Free package.
