Theory of Operation
The ADT7481 is a local and dual remote temperature
sensor and over/under temperature alarm. When the
ADT7481 is operating normally, the on−board ADC
operates in a free−running mode. The analog input
multiplexer alternately selects either the on−chip
temperature sensor to measure its local temperature, or
either of the remote temperature sensors. The ADC digitizes
these signals and the results are stored in the local, Remote 1,
and Remote 2 temperature value registers.
The local and remote measurement results are compared
with the corresponding high, low, and THERM temperature
limits, stored in on−chip registers. Out−of−limit comparisons
generate flags that are stored in the status register. A result that
exceeds the high temperature limit, the low temperature limit,
or remote diode open circuit will cause the ALERT output to
assert low. Exceeding THERM temperature limits causes the
THERM output to assert low. The ALERT output can be
reprogrammed as a second THERM output.
The limit registers can be programmed, and the device
controlled and configured via the serial SMBus. The
contents of any register can also be read back via the SMBus.
Control and configuration functions consist of switching
the device between normal operation and standby mode,
selecting the temperature measurement scale, masking or
enabling the ALERT output, switching Pin 8 between
ALERT and THERM2, and selecting the conversion rate.
Temperature Measurement Method
A simple method of measuring temperature is to exploit
the negative temperature coefficient of a diode, measuring
the base−emitter voltage (V
) of a transistor operated at
This technique requires calibration to null the effect of the
absolute value of V
, which varies from device to device.
The technique used in the ADT7481 measures the change
when the device is operated at two different currents.
Figure 14 shows the input signal conditioning used to
measure the output of a remote temperature sensor. This
figure shows the remote 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 is linked
to the base. To prevent ground noise 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. C1 may optionally be added
as a noise filter with a recommended maximum value of
, the operating current through the
sensor is switched among two related currents. The currents
through the temperature diode are switched between I, and
N x I, giving
. The temperature can then be calculated
waveforms pass 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
The ADC digitizes this voltage producing a temperature
measurement. To reduce the effects of noise, digital filtering
is performed by averaging the results of 16 measurement
cycles for low conversion rates. At rates of 16, 32, and 64
conversions/second, no digital averaging takes place.
Signal conditioning and measurement of the local
temperature sensor is performed in the same manner.
CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000pF MAX.
Figure 14. Input Signal Conditioning
Temperature Measurement Results
The results of the local and remote temperature
measurements are stored in the local and remote temperature
value registers and are compared with limits programmed
into the local and remote high and low limit registers.
The local temperature measurement is an 8−bit
measurement with 1°C resolution. The remote temperature
measurements are 10−bit measurements, with the 8 MSBs
stored in one register and the 2 LSBs stored in another
register. Table 1 is a list of the temperature measurement
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