ADT7461
Rev. 3 | Page 9 of 23 | www.onsemi.com
FUNCTIONAL DESCRIPTION
The ADT7461 is a local and remote temperature sensor and
over/under temperature alarm, with the added ability to auto-
matically cancel the effect of 3 kΩ (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.
The limit registers can be programmed and the device con-
trolled 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.
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 resis-
tance. The ADT7461 is designed to automatically cancel typically
up to 3 kΩ 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 environ-
ments. See the Noise Filtering section for more details.
TEMPERATURE MEASUREMENT METHOD
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode by measuring the
base-emitter voltage (V
BE
) of a transistor operated at constant
current. However, this technique requires calibration to null out
the effect of the absolute value of V
BE
, which varies from device
to device.
The technique used in the ADT7461 is to measure the change
in V
BE
when the device is operated at three different 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 16 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 collec-
tor 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.
To meas ure ΔV
BE
, the operating current through the sensor is
switched among three related currents. Figure 16 shows
N1 × I and N2 × I as different multiples of the current, I. The
currents through the temperature diode are switched between
I and N1 × I, giving ΔV
BE1
, and then between I and N2 × I,
giving ΔV
BE2
. The temperature may then be calculated using the
two ΔV
BE
measurements. This method can also be shown to
cancel the effect of any series resistance on the temperature
measurement.
The resulting ΔV
BE
waveforms are passed 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 ΔV
BE
. The ADC digitizes this volt-
age 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 tempera-
ture sensor is performed in the same manner.
