ADT7461A

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17

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,

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:

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.

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.

Figure 22. Typical Arrangement of Signal Tracks

GND

D+

D–

GND

5 MIL

5 MIL

5 MIL

5 MIL

5 MIL

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5 MIL

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.

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.

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.

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.

Application Circuit

Figure 23 shows a typical application circuit for the

ADT7461A, using a discrete sensor transistor connected via

a shielded, twisted pair cable. The pullups on SCLK,

SDATA, and ALERT are required only if they are not

provided elsewhere in the system.

The SCLK pin and the SDATA pin of the ADT7461A can

be interfaced directly to the SMBus of an I/O controller, such