ADT7461AARMZ datasheet(20/24 Pages) AD

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ADT7461A

Rev. A | Page 20 of 24

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. Many factors can affect

this. Ideally, place the sensor in good thermal contact with the

part of the system being measured. If it is not, 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 is either a substrate

transistor in the processor or a small package device, such as the

SOT-23, placed in close proximity to it.

The on-chip sensor, however, is often remote from the processor

and only monitors the general ambient temperature around the

package. How accurately the temperature of the board and/or

the forced airflow reflects the temperature to be measured

dictates the accuracy of the measurement. Self-heating due to

the power dissipated in the ADT7461A 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. In the

case of the ADT7461A, the worst-case condition occurs when

the device 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, θJA, of the

8-lead MSOP is approximately 142°C/W.

LAYOUT CONSIDERATIONS

Digital boards can be electrically noisy environments, and the

ADT7461A is measuring 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 ADT7461A as close as possible to the remote

sensing diode. Provided that the worst noise sources, that

is, clock generators, data/address buses, and CRTs are

avoided, this distance can be 4 inches 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 pickup, a 5 mil track width

and spacing is recommended. Provide a ground plane

under the tracks, if possible.

GND

D+

D–

GND

5MIL

Figure 23. Typical Arrangement of Signal Tracks

•

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

1°C corresponds to about 200 mV, and thermocouple

voltages are about 3 mV/°C of temperature difference.

Unless there are two thermocouples with a big temperature

differential between them, thermocouple voltages should

be much less than 200 mV.

•

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

extremely noisy environments, place an input filter

capacitor across D+ and D− close to the ADT7461A. This

capacitance can effect the temperature measurement, so

ensure that any capacitance seen at D+ and D− is, at

maximum, 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 is needed.

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

twisted pair, such as the Belden No. 8451 microphone

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

shield to GND close to the ADT7461A. 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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