ADT7461

http://onsemi.com

17

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 the environment being measured;

many factors can affect this. Ideally, the sensor should be 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. With a remote sensor, this should not be

a problem since it will be 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. The thermal time constant

of the SOIC-8 package in still air is about 140 seconds, and

if the ambient air temperature quickly changed by 100

degrees, it would take about 12 minutes (5 time constants)

for the junction temperature of the ADT7461 to settle within

1 degree of this. In practice, the ADT7461 package is 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 also affects the accuracy.

Self-heating due to the power dissipated in the ADT7461 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. With the ADT7461, 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,

Layout Considerations

Digital boards can be electrically noisy environments, and

the ADT7461 is measuring very small voltages from the

remote sensor, so care must be taken to minimize noise

induced at the sensor inputs. The following precautions

should be taken:

1. Place the ADT7461 as close as possible to the

remote sensing diode. Provided the worst noise

sources, such as clock generators, data/address

buses, and CRTs, are avoided, this distance can be

4 inches to 8 inches.

2. 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 23. Typical Arrangement of Signal Tracks

GND

D+

D–

GND

5 MIL

5 MIL

5 MIL

5 MIL

5 MIL

5 MIL

5 MIL

3. 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.

4. Place a 0.1

pin. In extremely noisy environments, an input filter

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

the ADT7461. This capacitance can effect the

temperature measurement, so care must be taken to

ensure 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.

5. If the distance to the remote sensor is more than

8 inches, the use of twisted pair cable is

recommended. This works up to about 6 to 12 feet.

For extremely 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

ADT7461. 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 may be reduced or removed.

Application Circuit

Figure 24 shows a typical application circuit for the

ADT7461 using a discrete sensor transistor connected via a

shielded, twisted pair cable. The pull-ups on SCLK, SDATA,

and ALERT are required only if they are not already provided

elsewhere in the system.

The SCLK and SDATA pins of the ADT7461 can be

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