ADT7481 datasheet(22/24 Pages) AD

22 / 24 page

ADT7481

Rev. 0 | Page 22 of 24

APPLICATIONS INFORMATION

NOISE FILTERING

For temperature sensors operating in noisy environments, previ-

ous practice was to place a capacitor across the D+ and D− pins

to help combat the effects of noise. However, large capacitances

affect the accuracy of the temperature measurement, leading to a

recommended maximum capacitor value of 1,000 pF.

FACTORS AFFECTING DIODE ACCURACY

Remote Sensing Diode

The ADT7481 is designed to work with substrate transistors

built into processors or with discrete transistors. Substrate

transistors will generally be PNP types with the collector

connected to the substrate. Discrete types can be either a PNP

or an NPN transistor connected as a diode (base shorted to

collector). If an NPN transistor is used, the collector and base

are connected to D+ and the emitter to D−. If a PNP transistor

is used, the collector and base are connected to D− and the

emitter to D+.

To reduce the error due to variations in both substrate and

discrete transistors, a number of factors should be taken into

consideration:

• The ideality factor, n

, of the transistor is a measure of the de-

viation of the thermal diode from ideal behavior. The ADT7481

is trimmed for an nf value of 1.008. Use the following equation

to calculate the error introduced at a temperature, T (°C), when

using a transistor where nf does not equal 1.008. Consult the

processor data sheet for the nf values.

(

)

(

)

Kelvin

273

008

–

To factor this in, the user can write the ΔT value to the offset

from, the temperature measurement by the ADT7481.

• Some CPU manufacturers specify the high and low current

levels of the substrate transistors. The high current level of

the ADT7481, IHIGH, is 233 μA. The low level current, ILOW, is

14 μA. If the ADT7481 current levels do not match the

current levels specified by the CPU manufacturer, it may

become necessary to remove an offset. The CPU data sheet

will advise whether this offset needs to be removed and how

to calculate it. This offset may be programmed to the 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 μA, at the

highest operating temperature.

• Base-emitter voltage less than 0.95 V at 100 μA, at the lowest

operating temperature.

• Base resistance less than 100 Ω.

• Small variation in h

(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, θJA, 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

ADT7481 Datasheet(PDF) 22 Page - Analog Devices

ADT7481 Datasheet(HTML) 22 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL