ADT7473ARQZ データシートの表示（PDF） - Analog Devices

部品番号

コンポーネント説明

メーカー

ADT7473ARQZ

dBCool® Remote Thermal Monitor and Fan Controller

Analog Devices 

ADT7473

2N3904

NPN

ADT7473

D+

D–

Figure 22. Measuring Temperature Using an NPN Transistor

ADT7473

D+

2N3906

PNP

D–

Figure 23. Measuring Temperature Using a PNP Transistor

To measure ∆VBE, the operating current through the sensor is

switched among three related currents. N1 × I and N2 × I are

different multiples of the current I, as shown in Figure 21. The

currents through the temperature diode are switched between

I and N1 × I, giving ∆VBE1, and then between I and N2 × I,

giving ∆VBE2. The temperature can then be calculated using the

two ∆VBE measurements. This method can also cancel the effect

of any series resistance on the temperature measurement.

The resulting ∆VBE waveforms are passed through a 65 kHz

low-pass filter to remove noise and then to a chopper-stabilized

amplifier. This amplifies and rectifies the waveform to produce

a dc voltage proportional to ∆VBE. The ADC digitizes this

voltage, and a temperature measurement is produced. To reduce

the effects of noise, digital filtering is performed by averaging

the results of 16 measurement cycles.

The results of remote temperature measurements are stored in

10-bit, twos complement format, as listed in Table 8. The extra

resolution for the temperature measurements is held in the

Extended Resolution Register 2 (0x77). This gives temperature

readings with a resolution of 0.25°C.

Noise Filtering

For temperature sensors operating in noisy environments,

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

the D− pin 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.

This capacitor reduces the noise, but does not eliminate it,

making use of the sensor difficult in a very noisy environment.

The ADT7473 has a major advantage over other devices for

eliminating the effects of noise on the external sensor. Using the

series resistance cancellation feature, a filter can 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 canceled from the temperature result.

The construction of a filter allows the ADT7473 and the remote

temperature sensor to operate in noisy environments. Figure 24

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

R = 100 Ω, C = 1 nF

This filtering reduces both common-mode noise and

differential noise.

REMOTE

TEMPERATURE

SENSOR

100Ω

100Ω

D+

1nF

D–

Figure 24. Filter Between Remote Sensor and ADT7473

SERIES RESISTANCE CANCELLATION

Parasitic resistance to the ADT7473 D+ and D− inputs (seen in

series with the remote diode) is caused by a variety of factors

including PCB track resistance and track length. This series

resistance appears as a temperature offset in the remote sensor’s

temperature measurement. This error typically causes a 0.5°C offset

per Ω of parasitic resistance in series with the remote diode.

The ADT7473 automatically cancels out the effect of this series

resistance on the temperature reading, giving a more accurate

result without the need for user characterization of this resis-

tance. The ADT7473 is designed to automatically cancel up to

3 kΩ of resistance, typically. This is transparent to the user by

using an advanced temperature measurement method. This

feature allows resistances to be added to the sensor path to

produce a filter, allowing the part to be used in noisy

environments. See the Noise Filtering section for details.

FACTORS AFFECTING DIODE ACCURACY

Remote Sensing Diode

The ADT7473 is designed to work with either substrate transis-

tors 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 con-

nected 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

ADT7473 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. Refer to the data sheet for the related CPU

to obtain the nf values.

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

Rev. A | Page 17 of 76

Share Link: