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ADT7468
TEMPERATURE MEASUREMENT
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, measuring the base-
emitter voltage (V BE ) of a transistor, operated at constant
current. Unfortunately, this technique requires calibration to
null the effect of the absolute value of V BE , which varies from
device to device.
The technique used in the ADT7468 is to measure the change in
V BE when the device is operated at three currents. Previous
devices have used only two operating currents, but the use of a
third current allows automatic cancellation of resistances in
series with the external temperature sensor.
Figure 24 shows the input signal conditioning used to measure
the output of an external temperature sensor. This figure shows
the external sensor as a substrate transistor, but it could equally
be a discrete transistor. If a discrete transistor is used, the
collector is not grounded and should be linked to the base. To
for mass production. The technique used in the ADT7468 is to
measure the change in V BE when the device is operated at three
currents.
This is given by
? V BE = KT / q × 1 n ( 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 23 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.
prevent ground noise from interfering with the measurement,
I
N2 × I N1 × I I BIAS
V DD
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. C1 can optionally be added as a noise filter (the
REMOTE
SENSING
TRANSISTOR
recommended maximum value is 1000 pF). However, a better
option in noisy environments is to add a filter, as described in
the Noise Filtering section.
D+
D–
LPF
f C = 65kHz
V OUT+
TO ADC
V OUT–
Local Temperature Measurement
The ADT7468 contains an on-chip band gap temperature
sensor whose output is digitized by the on-chip 10-bit ADC.
The 8-bit MSB temperature data is stored in the local tempera-
ture register (Address 0x26). Because both positive and negative
temperatures can be measured, the temperature data is stored in
Offset 64 format or twos complement format, as shown in
Table 7 and Table 8. Theoretically, the temperature sensor and
ADC can measure temperatures from ?128°C to +127°C (or
?61°C to +191°C in the extended temperature range) with a
resolution of 0.25°C. However, this exceeds the operating
temperature range of the device, so local temperature
measurements outside the ADT7468 operating temperature
range are not possible.
Remote Temperature Measurement
The ADT7468 can measure the temperature of two remote
diode sensors or diode-connected transistors connected to
Pins 17 and 18, or Pins 15 and 16.
The forward voltage of a diode or diode-connected transistor
operated at a constant current exhibits a negative temperature
coefficient of about –2 mV/°C. Unfortunately, the absolute
value of V BE varies from device to device and individual
calibration is required, and therefore the technique is unsuitable
Figure 23. 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 to the D+ input. Figure
25 and Figure 26 show how to connect the ADT7468 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 ?V BE , the operating current through the sensor is
switched among three related currents. Shown in Figure 23,
N1 × I and N2 × I are different multiples of the current I. The
currents through the temperature diode are switched between
I and N1 × I, giving ?V BE1 , and then between I and N2 × I,
giving ?V BE2 . The temperature can then be calculated using
the two ?V BE measurements. This method can also cancel the
effect of any series resistance on the temperature measurement.
The resulting ?V BE waveforms are passed through a 65 kHz
low-pass filter to remove noise and then sent to a chopper-
stabilized amplifier that amplifies and rectifies the waveform to
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