ADT7481ARMZ-R7 datasheet(18/20 Pages) ONSEMI

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ADT7481

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calculate it. This offset may be programmed to the

offset register. It is important to note that if more than

one offset must be considered, the algebraic sum of

these offsets must be programmed to the offset register.

If a discrete transistor is being used with the ADT7481, the

best accuracy is obtained by choosing devices according to

the following criteria:

• Base−emitter voltage greater than 0.25 V at 6 mA, at the

highest operating temperature.

• Base−emitter voltage less than 0.95 V at 100 mA, at the

lowest operating temperature.

• Base resistance less than 100 W.

• Small variation in hFE (say 50 to 150) that indicates

tight control of VBE characteristics.

Transistors, such as 2N3904, 2N3906, or equivalents in

SOT−23 packages, are suitable devices to use.

Thermal Inertia and Self−Heating

Accuracy depends on the temperature of the remote

sensing diode and/or the local temperature sensor being at

the same temperature as that being measured. A number of

factors can affect this. Ideally, the sensor should be in good

thermal contact with the part of the system being measured;

otherwise, the thermal inertia caused by the sensor’s mass

causes 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, since it will either be a substrate transistor in the

processor or a small package device, such as an SOT−23,

placed in close proximity to it.

The on−chip sensor, however, will often be remote from

the processor and only monitors the general ambient

temperature around the package. In practice, the ADT7481

package will be in electrical, and hence, thermal contact with

a PCB and may also be in a forced airflow. How accurately

the temperature of the board and/or the forced airflow

reflects the temperature to be measured will also affect the

accuracy of the measurement. Self−heating, due to the

power dissipated in the ADT7481 or the remote sensor,

causes the chip temperature of the device (or remote sensor)

to rise above ambient. However, the current forced through

the remote sensor is so small that self−heating is negligible.

The worst−case condition occurs when the ADT7481 is

converting at 64 conversions per second while sinking the

maximum current of 1 mA at the ALERT and THERM

output. In this case, the total power dissipation in the device

is about 4.5 mW. The thermal resistance,

qJA, of the

MSOP−10 package is about 142

°C/W.

Layout Considerations

Digital boards can be electrically noisy environments, and

the ADT7481 measures very small voltages from the remote

sensor, so care must be taken to minimize noise induced at

the sensor inputs. Take the following precautions:

• Place the ADT7481 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 range from 4 to 8 inches.

• Route the D+ and D− tracks close together, in parallel,

with grounded guard tracks on each side. To minimize

inductance and reduce noise pick up, a 5 mil track

width and spacing is recommended. Provide a ground

plane under the tracks if possible.

Figure 21. Typical Arrangement of Signal Tracks

GND

D+

D–

GND

5MIL

• Try to minimize the number of copper/solder joints that

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

°C corresponds to about 200 mV, and thermocouple

voltages are about 3 mV/

°C of temperature difference.

• Unless there are two thermocouples with a large

temperature differential between them, thermocouple

voltages should be much less than 200 mV.

• Place a 0.1 mF bypass capacitor close to the VDD pin. In

extremely noisy environments, an input filter capacitor

may be placed across D+ and D− close to the

ADT7481. This capacitance can affect the temperature

measurement, so care must be taken to ensure that any

capacitance seen at D+ and D− is a maximum of 1,000

pF. This maximum value includes the filter capacitance,

plus any cable or stray capacitance between the pins

and the sensor diode.

• If the distance to the remote sensor is more than 8

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

total of 6 feet to 12 feet of cable is needed.

• For really long distances (up to 100 feet), use shielded

twisted pair, such as Belden No. 8451 microphone

cable. Connect the twisted pair to D+ and D− and the

shield to GND close to the ADT7481. Leave the remote

end of the shield unconnected to avoid ground loops.

Because the measurement technique uses switched

current sources, excessive cable or filter capacitance can

affect the measurement. When using long cables, the filter

capacitance can be reduced or removed.

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