ADT7475 datasheet(16/68 Pages) AD

16 / 68 page

ADT7475

Rev. A | Page 16 of 68

The technique used in the ADT7475 is to measure the change

in VBE when the device is operated at two different currents.

This is given by

ΔVBE = KT/q × 1n(N)

where:

K is Boltzmann’s constant.

q is the charge on the carrier.

T is the absolute temperature in Kelvin.

N is the ratio of the two currents.

Figure 21 shows the input signal conditioning used to measure

the output of a remote temperature sensor. This figure shows the

external sensor as a substrate transistor, provided for temperature

monitoring on some microprocessors. It could also be a discrete

transistor such as a 2N3904/2N3906.

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 to the

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

to the D− input and the base to the D+ input. Figure 22 and

Figure 23 show how to connect the ADT7475 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.

To measure ΔVBE, the sensor is switched between operating

currents of I and N × I. The resulting waveform is passed

through a 65 kHz low-pass filter to remove noise, and to a

chopper-stabilized amplifier that performs the functions of

amplification and rectification of the waveform to produce

a dc voltage proportional to ΔVBE. This voltage is measured

by the ADC to give a temperature output in 10-bit, twos

complement format. To further reduce the effects of noise,

digital filtering is performed by averaging the results of

16 measurement cycles.

A remote temperature measurement takes nominally 38 ms. The

results of remote temperature measurements are stored in 10-bit,

twos complement format, as shown in Table 6. The extra resolu-

tion 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

D− pin to help combat the effects of noise. However, large capaci-

tances 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.

Sometimes, this sensor noise is a problem in a very noisy

environment. In most cases, a capacitor is not required as

differential inputs, by their very nature, have a high immunity

to noise.

2N3904

NPN

ADT7475

D+

D–

Figure 22. Measuring Temperature Using an NPN Transistor

2N3906

PNP

ADT7475

D+

D–

Figure 23. Measuring Temperature Using a PNP Transistor

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