ADT7485A

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11

noise pickup. A 5 mil track minimum width and

spacing is recommended.

Figure 17. Arrangements of Signal Tracks

5mil

5mil

5mil

5mil

5mil

5mil

5mil

GND

D1+

GND

D1–

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.

because 1

°C 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.

inches, the use of a twisted pair cable is recommended.

This works for distances of about 6 feet to 12 feet.

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

°C 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

±128°C with 0.25°C

resolution. The offset format is the same as the temperature

data format; 16−bit, twos complement notation, as shown in

Table 11. 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.

ORDERING INFORMATION

Device Order Number*

Package Type

Package Option

ADT7485AARMZ−R

10−Lead MSOP

RM−10

3000 Tape & Reel

ADT7485AARMZ−R7

1000 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.

*These are Pb−Free packages.