ADT7461ARMZ-002 ON Semiconductor, ADT7461ARMZ-002 Datasheet - Page 7

• Manufacturer Part Number: ADT7461ARMZ-002
• Description: IC TEMP MONITOR +/-1C DUAL 8MSOP
• Manufacturer: ON Semiconductor

Specifications of ADT7461ARMZ-002

• Function: Temp Monitoring System (Sensor)
• Topology: ADC, Comparator, Multiplexer, Register Bank
• Sensor Type: External & Internal
• Sensing Temperature: 0°C ~ 127°C, External Sensor
• Output Type: SMBus™
• Output Alarm: Yes
• Output Fan: Yes
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 120°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Full Temp Accuracy: +/- 1 C, +/- 3 C
• Digital Output - Bus Interface: Serial (2-Wire)
• Maximum Operating Temperature: + 127 C
• Minimum Operating Temperature: 0 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Functional Description
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
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.
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.
controlled and configured via the serial SMBus. The
contents of any register can also be read back via the SMBus.
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
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.
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.
The ADT7461 is a local and remote temperature sensor
The local and remote measurement results are compared
The limit registers can be programmed and the device
Control and configuration functions consist of switching
Parasitic resistance to the D+ and D− inputs to the
The ADT7461 automatically cancels out the effect of this
alternately
selects
either
the
on-chip
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7
Temperature Measurement Method
the negative temperature coefficient of a diode by measuring
the base-emitter voltage (V
constant current. However, this technique requires
calibration to null out the effect of the absolute value of V
which varies from device to device.
change in V
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.
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.
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
between I and N1 x I, giving DV
N2 x I, giving DV
calculated using the two DV
can also be shown to cancel the effect of any series resistance
on the temperature measurement.
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
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.
temperature sensor is performed in the same manner.
A simple method of measuring temperature is to exploit
The technique used in the ADT7461 is to measure the
Figure 15 shows the input signal conditioning used to
To measure DV
The resulting DV
Signal conditioning and measurement of the internal
BE
when the device is operated at three different
BE
BE
BE2
, the operating current through the
waveforms are passed through a
. The temperature may then be
BE
BE
) of a transistor operated at
measurements. This method
BE1
, and then between I and
BE
. The
BE
,