ADT7485AARMZ-R【PDF 11/12 页】IC TEMP/VOLT DGL SENS SST 10MSOP

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ADT7485A

http://onsemi.com

Figure 16. Connections for NPN and PNP Transistors

ADT7485A

D1+

2N3904

NPN

ADT7485A

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

GND

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 1癈 corresponds to about

240 mV, and thermocouple voltages are about

3 mV/癈 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 0.5癈 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 ?28癈 with 0.25癈

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

ADT7485AARMZR

10-lead MSOP

RM10

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.

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