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ADT7473

dBCOOL® Remote Thermal Monitor and Fan Control

ON Semiconductor 

ADT7473

VDD

I N2 y I N1 y I IBIAS

REMOTE

SENSING

TRANSISTOR D+

D–

LPF

fC = 65kHz

VOUT+

VOUT–

TO ADC

Figure 24. Signal Conditioning for Remote Diode Temperature Sensors

If a discrete transistor is used, the collector is not grounded

and should be linked to the base. If a PNP transistor is used,

the base is connected to the D– input and the emitter is

connected to the D+ input. If an NPN transistor is used, the

emitter is connected to the D– input and the base is

connected to the D+ input. Figure 25 and Figure 26 show

how to connect the ADT7473/ADT7473−1 to an NPN or

PNP transistor for temperature measurement. To prevent

ground noise from interfering with the measurement, the

more negative terminal of the sensor is not referenced to

ground, but is biased above ground by an internal diode at

the D– input.

2N3904

NPN

ADT7473/

ADT7473−1

D+

D–

Figure 25. Measuring Temperature Using an NPN

Transistor

2N3906

PNP

ADT7473/

ADT7473−1

D+

D–

Figure 26. Measuring Temperature Using a PNP

Transistor

To measure DVBE, the operating current through the

sensor is switched among three related currents. N1 x I and

N2 x I are different multiples of the current I, as shown in

Figure 24. The currents through the temperature diode are

switched between I and N1 x I, giving DVBE1, and then

between I and N2 x I, giving DVBE2. The temperature can

then be calculated using the two DVBE measurements. This

method can also cancel the effect of any series resistance on

the temperature measurement.

The resulting DVBE 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 DVBE.

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

10. 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/ADT7473−1 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/

ADT7473−1 and the remote temperature sensor to operate

in noisy environments. Figure 27 shows a low−pass R−C

filter with the following values:

R + 100 W, C + 1 nF

(eq. 1)

This filtering reduces both common−mode noise and

differential noise.

REMOTE

TEMPERATURE

SENSOR

100Ω

100Ω

D+

1nF

D–

Figure 27. Filter Between Remote Sensor and

ADT7473/ADT7473−1

Series Resistance Cancellation

Parasitic resistance to the ADT7473/ADT7473−1 D+ and

D− inputs (seen in series with the remote diode) is caused by

a variety of factors including PCB track resistance and track

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