ADT7467ARQZ-R7 datasheet(18/77 Pages) ONSEMI

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ADT7467

Rev. 3 | Page 18 of 77 | www.onsemi.com

The construction of a filter allows the ADT7467 and the remote

temperature sensor to operate in noisy environments. Figure 24

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

R = 100 Ω, C = 1 nF

This filtering reduces both common-mode noise and

differential noise.

D+

1nF

100Ω

REMOTE

TEMPERATURE

SENSOR

D–

100Ω

Figure 24. Filter Between Remote Sensor and ADT7467

Factors Affecting Diode Accuracy

Remote Sensing Diode

The ADT7467 is designed to work with either substrate transistors

built into processors or 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 the collector). If an NPN

transistor is used, the collector and base are connected to D+

and the emitter is connected to D−. If a PNP transistor is used,

the collector and base are connected to D− and the emitter is

connected 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

ADT7467 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 whose nf does

not equal 1.008. See the processor’s data sheet for the nf

values.

ΔT = (nf − 1.008)/1.008 × (273.15 K + T)

•

To correct for this error, the user can write the ΔT value to

the offset register, and the ADT7467 automatically adds it

to or subtracts it from the temperature measurement.

•

Some CPU manufacturers specify the high and low current

levels of the substrate transistors. The high current level of

the ADT7467, IHIGH, is 96 μA, and the low level current, ILOW,

is 6 μA. If the ADT7467 current levels do not match the

current levels specified by the CPU manufacturer, it may

be necessary to remove an offset. The CPU’s data sheet

should provide information relating to nf to compensate for

differences. An offset can 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 used with the ADT7467, the best

accuracy is obtained by choosing devices according to the

following criteria:

•

Base-emitter voltage is greater than 0.25 V at 6 μA with the

highest operating temperature.

•

Base-emitter voltage is less than 0.95 V at 100 μA with the

lowest operating temperature.

•

Base resistance is less than 100 Ω.

•

There is a small variation in hFE (for example, 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.

Table 7. Twos Complement Temperature Data Format

Temperature

Digital Output (10-Bit)1

−128°C

1000 0000 00

−125°C

1000 0011 00

−100°C

1001 1100 00

−75°C

1011 0101 00

−50°C

1100 1110 00

−25°C

1110 0111 00

−10°C

1111 0110 00

0°C

0000 0000 00

+10.25°C

0000 1010 01

+25.5°C

0001 1001 10

+50.75°C

0011 0010 11

+75°C

0100 1011 00

+100°C

0110 0100 00

+125°C

0111 1101 00

+127°C

0111 1111 00

1 Bold numbers denote 2 LSBs of measurement in Extended Resolution

Table 8. Offset 64 Temperature Data Format

Temperature

Digital Output (10-Bit)1

−64°C

0000 0000 00

−1°C

0011 1111 00

0°C

0100 0000 00

+1°C

0100 0001 00

+10°C

0100 1010 00

+25°C

0101 1001 00

+50°C

0111 0010 00

+75°C

1000 1001 00

+100°C

1010 0100 00

+125°C

1011 1101 00

+191°C

1111 1111 00

1 Bold numbers denote 2 LSBs of measurement in Extended Resolution

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