ADT7461
Functional Description
Temperature Measurement Method
The ADT7461 is a local and remote temperature sensor
and over/under temperature alarm, with the added ability to
automatically cancel the effect of 3 kW (typical) of
resistance in series with the temperature monitoring diode.
When the ADT7461 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 the
remote temperature sensor. The ADC digitizes these signals
and the results are stored in the local and remote temperature
value registers.
The local and remote measurement results are compared
with the corresponding high, low, and THERM temperature
limits, stored in eight 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 an external diode fault causes 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.
A simple method of measuring temperature is to exploit
the negative temperature coefficient of a diode by measuring
the base-emitter voltage (V ) of a transistor operated at
BE
constant current. However, this technique requires
calibration to null out the effect of the absolute value of V
which varies from device to device.
,
BE
The technique used in the ADT7461 is to measure the
change in V when the device is operated at three different
BE
currents. Previous devices have used only two operating
currents, but it is the use of a third current that allows
automatic cancellation of resistances in series with the
external temperature sensor.
Figure 15 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 will not be grounded and
should be 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 be added as a noise filter (a recommended maximum
value of 1,000 pF). However, a better option in noisy
environments is to add a filter, as described in the Noise
Filtering section. See the Layout Considerations section for
more information on C1.
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 6 between
ALERT and THERM2, and selecting the conversion rate.
To measure DV , the operating current through the
BE
sensor is switched among three related currents. Figure 15
shows N1 x I and N2 x I as different multiples of the current,
I. The currents through the temperature diode are switched
Series Resistance Cancellation
Parasitic resistance to the D+ and D− inputs to the
ADT7461, seen in series with the remote diode, is caused by
a variety of factors, including PCB track resistance and track
length. This series resistance appears as a temperature offset
in the remote sensor’s temperature measurement. This error
typically causes a 0.5°C offset per ohm of parasitic resistance
in series with the remote diode.
The ADT7461 automatically cancels out the effect of this
series resistance on the temperature reading, giving a more
accurate result, without the need for user characterization of
this resistance. The ADT7461 is designed to automatically
cancel typically up to 3 kW of resistance. By using an
advanced temperature measurement method, this is
transparent to the user. This feature allows resistances to be
added to the sensor path to produce a filter, allowing the part
to be used in noisy environments. See the Noise Filtering
section for more details.
between I and N1 x I, giving DV , and then between I and
BE1
N2 x I, giving DV . The temperature may then be
BE2
calculated using the two DV measurements. This method
BE
can also be shown to cancel the effect of any series resistance
on the temperature measurement.
The resulting DV
waveforms are passed through a
BE
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 DV . The
BE
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
for low conversion rates. At rates of 16, 32, and 64
conversions per second, no digital averaging takes place.
Signal conditioning and measurement of the internal
temperature sensor is performed in the same manner.
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