Analog Devices dBCool ADT7475 Manual page 16

ADT7475
The technique used in the ADT7475 is to measure the change
in V when the device is operated at two different currents.
BE
This is given by
ΔV = KT/q × 1n(N)
BE
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 ΔV , the sensor is switched between operating
BE
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 ΔV
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.
. This voltage is measured
BE
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 (Reg. 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
Figure 22. Measuring Temperature Using an NPN Transistor
Figure 23. Measuring Temperature Using a PNP Transistor
Rev. 0 | Page 16 of 64
ADT7475
D+
NPN
D–
ADT7475
D+
2N3906
D–
PNP