ADT7482ARMZ datasheet(22/24 Pages) AD

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ADT7482

Rev. 0 | Page 22 of 24

APPLICATIONS INFORMATION

NOISE FILTERING

For temperature sensors operating in noisy environments, the

previous practice was to place a capacitor across the D+ pin and

the 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 1000 pF.

While this capacitor reduces the noise, it does not eliminate it,

making it difficult to use the sensor in a very noisy environment.

The ADT7482 has a major advantage over other devices for

eliminating the effects of noise on the external sensor. The

series resistance cancellation feature allows a filter to be

constructed between the external temperature sensor and the

part. The effect of any filter resistance seen in series with the remote

sensor is automatically cancelled from the temperature result.

The construction of a filter allows the ADT7482 and the remote

temperature sensor to operate in noisy environments. Figure 23

shows a low-pass R-C-R filter, with the following values:

R = 100 Ω and C = 1 nF

This filtering reduces both common-mode noise and

differential noise.

D+

1nF

100

REMOTE

TEMPERATURE

SENSOR

D–

100

Figure 23. Filter Between Remote Sensor and ADT7482

FACTORS AFFECTING DIODE ACCURACY

Remote Sensing Diode

The ADT7482 is designed to work with substrate transistors

built into processors or with discrete transistors. Substrate

transistors are generally PNP types with the collector connected

to the substrate. Discrete types can be either PNP or NPN

transistors 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, nf, of the transistor is a measure of the

deviation of the thermal diode from ideal behavior. The

ADT7482 is trimmed for an nf value of 1.008. The

following equation can be used to calculate the error

introduced at a temperature T (°C), when using a transistor

whose nf does not equal 1.008. Consult the processor data

sheet for the nf values.

(

)

(

)

Kelvin

273

008

–

To factor this in, write the ΔT value to the offset register. It

is then automatically added to or subtracted from the

temperature measurement by the ADT7482.

•

Some CPU manufacturers specify the high and low current

levels of the substrate transistors. The high current level of

the ADT7482, IHIGH, is 220 μA and the low level current,

ILOW, is 13.5 μA. If the ADT7482 current levels do not

match the current levels specified by the CPU manufacturer,

it may be necessary to remove an offset. The CPU data

sheet advises whether this offset needs to be removed and

how to calculate it. This offset can 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 ADT7482, 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 hFE (such as 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. 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 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. In practice, the ADT7482 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 ADT7482 or the remote sensor causes

the chip temperature of the device or remote sensor to rise

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