±1°C Temperature Monitor with
Series Resistance Cancellation
ADT7461
Rev. A
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Tel: 781.329.4700 www.analog.com
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FEATURES
On-chip and remote temperature sensor
0.25°C resolution/1°C accuracy on remote channel
1°C resolution/3°C accuracy on local channel
Automatically cancels up to 3 kΩ (typ) of resistance in series
with remote diode to allow noise filtering
Extended, switchable temperature measurement range 0°C
to +127°C (default) or –55°C to +150°C
Pin- and register-compatible with ADM1032
2-wire SMBus serial interface with SMBus alert support
Programmable over/under temperature limits
Offset registers for system calibration
Up to two overtemperature fail-safe THERM outputs
Small 8-lead SOIC or MSOP package
170 µA operating current, 5.5 µA standby current
APPLICATIONS
Desktop and notebook computers
Industrial controllers
Smart batteries
Automotive
Enbedded systems
Burn-in applications
Instrumentation
GENERAL DESCRIPTION
The ADT74611 is a dual-channel digital thermometer and
under/over temperature alarm, intended for use in PCs and
thermal management systems. It is pin- and register-compatible
with the ADM1032. The ADT7461 has three additional features:
series resistance cancellation (where up to 3 kΩ (typical) of
resistance in series with the temperature monitoring diode may
be automatically cancelled from the temperature result, allowing
noise filtering); configurable ALERT output; and an extended,
switchable temperature measurement range.
The ADT7461 can accurately measure the temperature of a
remote thermal diode to ±1°C and the ambient temperature to
±3°C. The temperature measurement range defaults to 0°C to
+127°C, compatible with the ADM1032, but can be switched to
a wider measurement range of−55°C to +150°C. The ADT7461
communicates over a 2-wire serial interface compatible with
system management bus (SMBus) standards. An ALERT output
signals when the on-chip or remote temperature is out of range.
The THERM output is a comparator output that allows on/off
control of a cooling fan. The ALERT output can be reconfigured
as a second THERM output, if required.
1 Protected by U.S. Patents 5,195,827; 5,867,012; 5,982,221; 6,097,239;
6,133,753; 6,169,442; other patents pending.
04110-0-012
DIGITAL MUX
DIGITAL MUX
SCLKSDATAGNDV
DD
6
4
ADDRESS POINTER
REGISTER
LOCAL TEMPERATURE
LOW LIMIT REGISTER
LOCAL TEMPERATURE
HIGH LIMIT REGISTER
REMOTE TEMPERATURE
LOW LIMIT REGISTER
REMOTE TEMPERATURE
HIGH LIMIT REGISTER
LIMIT
COMPARATOR
LOCAL TEMPERATURE
VALUE REGISTER
REMOTE TEMPERATURE
VALUE REGISTER
ADC
ANALOG
MUX
ON-CHIP
TEMPERATURE
SENSOR
RUN/STANDBYBUSY
REMOTE OFFSET
REGISTER
EXTERNAL DIODE OPEN-CIRCUIT
ADT7461
STATUS REGISTER
SMBus INTERFACE
LOCAL THERM LIMIT
REGISTER
EXTERNAL THERM LIMIT
REGISTER
CONFIGURATION
REGISTER
INTERRUPT
MASKING
751 8
THERM ALERT/
THERM2
D+
D– SRC
BLOCK
2
3
CONVERSION RATE
REGISTER
Figure 1. Functional Block Diagram
ADT7461
Rev. A | Page 2 of 24
TABLE OF CONTENTS
Specifications....................................................................................3
SMBus Timing Specifications ........................................................4
Absolute Maximum Ratings...........................................................5
ESD Caution.................................................................................5
Pin Configuration and Function Descriptions............................6
Typical Performance Characteristics ............................................7
Functional Description ...................................................................9
Series Resistance Cancellation...................................................9
Temperature Measurement Method .........................................9
Temperature Measurement Results.........................................10
Temperature Measurement Range ..........................................10
Temperature Data Format........................................................10
ADT7461 Registers ...................................................................11
Serial Bus Interface....................................................................14
Addressing the Device ..............................................................14
ALERT Output...........................................................................16
Low Power Standby Mode........................................................16
Sensor Fault Detection .............................................................16
The ADT7461 Interrupt System..............................................16
Application Information ..........................................................18
Factors Affecting Diode Accuracy ..........................................18
Thermal Inertia and Self-Heating...........................................18
Layout Considerations..............................................................19
Application Circuit....................................................................20
Outline Dimensions......................................................................21
Ordering Guide .........................................................................21
REVISION HISTORY
10/04—Changed from Rev. 0 to Rev. A
Change to SMBus specifications................................................4
Changes to Figure 6 and Figure 10............................................7
Added Figure 9 and Figure 13....................................................7
Changes to Temperature Measurement section ....................10
Changes to Figure 19 and Figure 25........................................16
Changes to Serial Bus Interface section..................................14
10/03—Revision 0: Initial Version
ADT7461
Rev. A | Page 3 of 24
SPECIFICATIONS
TA = −40°C to +120°C, VDD = 3 V to 5.5 V, unless otherwise noted.
Table 1.
Parameter Min Typ Max Unit Test Conditions
POWER SUPPLY
Supply Voltage, VDD 3.0 3.30 5.5 V
Average Operating Supply Current, IDD 170 215 µA 0.0625 conversions/sec rate1
5.5 10 µA Standby mode , –40°C ≤ TA ≤ +85°C
5.5 20 µA Standby mode, +85°C ≤ TA ≤ +120°C
Undervoltage Lockout Threshold 2.2 2.55 2.8 V VDD input, disables ADC, rising edge
Power-On-Reset Threshold 1 2.5 V
TEMPERATURE-TO-DIGITAL CONVERTER
Local Sensor Accuracy ±1 ±3 °C −40°C ≤ TA ≤ +100°C, 3 V ≤ VDD ≤ 3.6 V
Resolution 1 °C
Remote Diode Sensor Accuracy ±1 °C +60°C ≤ TA ≤ +100°C, −55°C ≤ TD 2 ≤ +150°C, 3 V ≤ VDD ≤ 3.6 V
±3 °C −40°C ≤ TA ≤ +120°C, −55°C ≤ TD 2 ≤ +150°C, 3 V ≤ VDD ≤ 5.5 V
Resolution 0.25 °C
Remote Sensor Source Current 96 µA High level3
36 µA Middle level3
6 µA Low level3
Conversion Time 32.13 114.6 ms From stop bit to conversion complete (both channels), one-
shot mode with averaging switched on
3.2 12.56 ms One-shot mode with averaging off (i.e., conversion rate = 16,
32, or 64 conversions per second)
Maximum Series Resistance Cancelled 3 kΩ Resistance split evenly on both the D+ and D– inputs
OPEN-DRAIN DIGITAL OUTPUTS
(THERM, ALERT/THERM2)
Output Low Voltage, VOL 0.4 V IOUT = −6.0 mA3
High Level Output Leakage Current, IOH 0.1 1 µA VOUT = VDD 3
ALERT Output Low Sink Current 1 mA ALERT forced to 0.4 V
SMBus INTERFACE3, 4
Logic Input High Voltage, VIH 2.1 V 3 V ≤ VDD ≤ 3.6 V
SCLK, SDATA
Logic Input Low Voltage, VIL 0.8 V 3 V ≤ VDD ≤ 3.6 V
SCLK, SDATA
Hysteresis 500 mV
SMBus Output Low Sink Current 6 mA SDATA forced to 0.6 V
Logic Input Current, IIH, IIL −1 +1 µA
SMBus Input Capacitance, SCLK, SDATA 5 pF
SMBus Clock Frequency 400 kHz
SMBus Timeout5 25 64 ms User programmable
SCLK Falling Edge to SDATA Valid Time 1 µs Master clocking in data
1 See for information on other conversion rates. Table 8
2 Guaranteed by characterization, but not production tested.
3 Guaranteed by design, but not production tested.
4 See section for more information. SMBUS Timing Specifications
5 Disabled by default. Details on how to enable it are in the SMBus section of this data sheet.
ADT7461
Rev. A | Page 4 of 24
SMBus TIMING SPECIFICATIONS
Table 2. SMBus Timing Specifications1
Parameter Limit at TMIN and TMAX Unit Description
fSCLK 400 kHz max
tLOW 1.3 µs min Clock low period, between 10% points
tHIGH 0.6 µs min Clock high period, between 90% points
tR 300 ns max Clock/data rise time
tF 300 ns max Clock/data fall time
tSU; STA 600 ns min Start condition setup time
tHD; STA2 600 ns min Start condition hold time
tSU; DAT3 100 ns min Data setup time
tHD; DAT 300 ns min Data hold time
tSU; STO4 600 ns min Stop condition setup time
tBUF 1.3 µs min Bus bree time between stop and start conditions
1 Guaranteed by design, but not production tested.
2 Time from 10% of SDATA to 90% of SCLK.
3 Time for 10% or 90% of SDATA to 10% of SCLK.
4 Time for 90% of SCLK to 10% of SDATA.
04110-0-001
SCLK
SDATA
t
R
t
F
t
LOW
t
HD;DAT
t
HD;STA
t
HIGH
t
SU;DAT
STOP START STOPSTART
t
SU;STA
t
SU;STO
t
HD;STA
t
BUF
Figure 2. Serial Bus Timing
ADT7461
Rev. A | Page 5 of 24
ABSOLUTE MAXIMUM RATINGS
Table 3.
Parameter Rating
Positive Supply Voltage (VDD) to GND −0.3 V, +5.5 V
D+ −0.3 V to VDD + 0.3 V
D− to GND −0.3 V to +0.6 V
SCLK, SDATA, ALERT −0.3 V to +5.5 V
THERM −0.3 V to VDD + 0.3 V
Input Current, SDATA, THERM −1 mA, +50 mA
Input Current, D− ±1 mA
ESD Rating, All Pins (Human Body Model) 2000 V
Maximum Junction Temperature (TJ Max) 150°C
Storage Temperature Range −65°C to +150°C
IR Reflow Peak Temperature 220°C
Pb-Free Parts Only 260°C (±0.5°C)
Lead Temperature (Soldering 10 sec) 300°C
Thermal Characteristics
8-Lead SOIC Package
θJA = 121°C/W
8-Lead MSOP Package
θJA = 142°C/W
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
ADT7461
Rev. A | Page 6 of 24
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
04110-0-013
SCLK
SDATA
ALERT/THERM2
GND
1
V
DD ADT7461
TOP VIEW
(Not to Scale)
8
2
D+
7
3
D–
6
4
THERM
5
Figure 3. Pin Configuration
Table 4. Pin Function Descriptions
Pin No. Mnemonic Description
1 VDD Positive Supply, 3 V to 5.5 V.
2 D+ Positive Connection to Remote Temperature Sensor.
3 D− Negative Connection to Remote Temperature Sensor.
4 THERM Open-drain output that can be used to turn a fan on/off or throttle a CPU clock in the event of an
overtemperature condition. Requires pull-up to VDD.
5 GND Supply Ground Connection.
6 ALERT/THERM2 Open-Drain Logic Output Used as Interrupt or SMBus Alert. This may also be configured as a second THERM
output. Requires pull-up resistor.
7 SDATA Logic Input/Output, SMBus Serial Data. Open-Drain Output. Requires pull-up resistor.
8 SCLK Logic Input, SMBus Serial Clock. Requires pull-up resistor.
ADT7461
Rev. A | Page 7 of 24
TYPICAL PERFORMANCE CHARACTERISTICS
04110-0-017
60
0 20406080
LEAKAGE RESISTANCE (MΩ)100
TEMPERATURE ERROR (°C)
–80
–60
–40
–20
0
20
40
D+ TO GND
D+ TO V
CC
Figure 4. Temperature Error vs. Leakage Resistance
04110-0-022
–3 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
TEMPERATURE ERROR (°C)
–0.8
–0.7
–0.6
–0.5
–0.4
–0.3
–0.2
–0.1
0
Figure 5. Temperature Error vs. Actual Temperature Using 2N3906
04110-0-027
4
0 100 200 300
FREQUENCY (MHz)
400 500 600
TEMPERATURE ERROR (°C)
–2
–1
0
1
2
3
40mV NO FILTER
60mV NO FILTER
40mV WITH FILTER
60mV WITH FILTER
Figure 6. Temperature Error vs. Differential Mode Noise Frequency
(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)
04110-0-015
20
020
FREQUENCY (MHz) 40
TEMPERATURE ERROR (°C)
–15
–10
–5
0
5
10
15
250mV EXTERNAL
250mV INTERNAL 100mV EXTERNAL
100mV INTERNAL
Figure 7. Temperature Error vs. Power Supply Noise Frequency
04110-0-018
0
0 5 10 15 20
CAPACITANCE (nF) 25
TEMPERATURE ERROR (°C)
–70
–60
–50
–40
–30
–20
–10
Figure 8. Temperature Error vs. Capacitance between D+ and D−
04110-0-024
180
160
140
120
100
0 100 200 300
FREQUENCY (MHz)
400 500 600
TEMPERATURE ERROR (°C)
80
60
40
20
0
–20
100mV NO FILTER
100mV WITH FILTER
Figure 9. Temperature Error vs. 100 mV Differential Mode Noise Frequency
(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)
ADT7461
Rev. A | Page 8 of 24
04110-0-025
5
0 100 200 300
FREQUENCY (MHz)
400 500 600
TEMPERATURE ERROR (°C)
–1
0
1
2
3
4
40mV NO FILTER
60mV NO FILTER
40mV WITH FILTER
60mV WITH FILTER
Figure 10. Temperature Error vs. Common-Mode Noise Frequency
(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)
04110-0-020
40
0 100 200 300
3V
5.5V
SCL CLOCK FREQUENCY (kHz) 40050 150 250 350
IDD (µA)
0
5
10
15
20
25
30
35
Figure 11. Standby Supply Current vs. Clock Frequency
04110-0-021
7
3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4
V
DD
(V)
I
DD
(µA)
0
1
2
3
4
5
6
Figure 12. Standby Current vs. Supply Voltage
04110-0-026
55
45
35
25
15
5
0 100 200 300
FREQUENCY (MHz)
400 500 600
TEMPERATURE ERROR (°C)
–5
100mV NO FILTER
100mV WITH FILTER
Figure 13. Temperature Error vs. 100 mV Common-Mode Noise Frequency
(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)
04110-0-019
800
0.01 0.1 1 10
3V
5.5V
CONVERSION RATE (Hz) 100
I
DD
(µA)
0
100
200
300
400
500
600
700
Figure 14. Operating Supply Current vs. Conversion Rate
04110-0-023
0 2 10 200 1k 2k 3k 4k
SERIES RESISTANCE (Ω)
TEMPERATURE ERROR (°C)
–5
0
5
10
15
20
25
30
35
40
45
50
3.3V T = –30
3.3V T = +25
3.3V T = +120
5.5V T = –30
5.5V T = +25
5.5V T = +120
Figure 15. Temperature Error vs. Series Resistance
ADT7461
Rev. A | Page 9 of 24
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 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 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 section on Noise Filtering for more details.
TEMPERATURE MEASUREMENT METHOD
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, measuring the
base-emitter voltage (VBE) of a transistor operated at constant
current. However, this technique requires calibration to null out
the effect of the absolute value of VBE, which varies from device
to device.
The technique used in the ADT7461 is to measure the change
in VBE 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 measure ∆VBE, the operating current through the sensor is
switched among three related currents. Shown in Figure 16,
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 ∆VBE1, and then between I and N2 × I, giving
∆VBE2. The temperature may then be calculated using the two
∆VBE measurements. This method can also be shown to cancel
the effect of any series resistance on the temperature measurement.
The resulting ∆VBE 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 ∆VBE. The ADC digitizes this vol-
tage 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/second, no digital averaging
takes place.
Signal conditioning and measurement of the internal tempera-
ture sensor is performed in the same manner.
ADT7461
Rev. A | Page 10 of 24
04110-0-002
IN1
×
IN2
×
II
BIAS
V
DD
V
OUT+
TO ADC
V
OUT–
REMOTE
SENSING
TRANSISTOR
*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
D+
D–
C1*
BIAS
DIODE LOW-PASS FILTER
f
C
= 65kHz
Figure 16. 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 value is in Register 0x00 and has a reso-
lution of 1°C. The external temperature value is stored in two
registers, with the upper byte in Register 0x01 and the lower
byte in Register 0x10. Only the two MSBs in the external temp-
erature low byte are used. This gives the external temperature
measurement a resolution of 0.25°C. Table 5 shows the data
format for the external temperature low byte.
Table 5. Extended Temperature Resolution (Remote
Temperature Low Byte)
Extended Resolution Remote Temperature Low Byte
0.00°C 0 000 0000
0.25°C 0 100 0000
0.50°C 1 000 0000
0.75°C 1 100 0000
When reading the full external temperature value, both the high
and low byte, the two registers should be read in succession.
Reading one register does not lock the other, so both should be
read before the next conversion finishes. In practice, there is
more than enough time to read both registers, as transactions
over the SMBus are significantly faster than a conversion time.
TEMPERATURE MEASUREMENT RANGE
The temperature measurement range for both internal and
external measurements is, by default, 0°C to +127°C. However,
the ADT7461 can be operated using an extended temperature
range. It can measure the full temperature range of an external
diode, from −55°C to +150°C. The user can switch between
these two temperature ranges by setting or clearing Bit 2 in the
configuration register. A valid result is available in the next
measurement cycle after changing the temperature range.
In extended temperature mode, the upper and lower temp-
erature that can be measured by the ADT7461 is limited by the
remote diode selection. The temperature registers themselves
can have values from −64°C to +191°C. However, most temp-
erature sensing diodes have a maximum temperature range of
−55°C to +150°C. Above 150°C, they may lose their semicon-
ductor characteristics and approximate conductors instead. This
results in a diode short. In this case, a read of the temperature
result register will give the last good temperature measurement.
The user should be aware that the temperature measurement on
the external channel may not be accurate for temperatures that
are outside the operating range of the remote sensor.
It should be noted that while both local and remote temperature
measurements can be made while the part is in extended temp-
erature mode, the ADT7461 itself should not be exposed to temp-
eratures greater than those specified in the absolute maximum
ratings section. Further, the device is only guaranteed to operate as
specified at ambient temperatures from −40°C to +120°C.
TEMPERATURE DATA FORMAT
The ADT7461 has two temperature data formats. When the
temperature measurement range is from 0°C to +127°C
(default), the temperature data format for both internal and
external temperature results is binary. When the measurement
range is in extended mode, an offset binary data format is used
for both internal and external results. Temperature values in the
offset binary data format are offset by 64°C. Examples of temp-
eratures in both data formats are shown in Table 6.
Table 6. Temperature Data Format (Local and Remote
Temperature High Byte)
Temperature Binary Offset Binary1
–55°C 0 000 00002 0 000 1001
0°C 0 000 0000 0 100 0000
+1°C 0 000 0001 0 100 0001
+10°C 0 000 1010 0 100 1010
+25°C 0 001 1001 0 101 1001
+50°C 0 011 0010 0 111 0010
+75°C 0 100 1011 1 000 1011
+100°C 0 110 0100 1 010 0100
+125°C 0 111 1101 1 011 1101
+127°C 0 111 1111 1 011 1111
+150°C 0 111 11113 1 101 0110
1 Offset binary scale temperature values are offset by 64°C.
2 Binary scale temp. measurement returns 0°C for all temperatures < 0°C.
3 Binary scale temp. measurement returns 127°C for all temperature > 127°C.
ADT7461
Rev. A | Page 11 of 24
The user may switch between measurement ranges at any time.
Switching the range will also switch the data format. The next
temperature result following the switching will be reported back
to the register in the new format. However, the contents of the
limit registers will not change. It is up to the user to ensure that
when the data format changes, the limit registers are repro-
grammed as necessary. More information on this can be found
in the Limit Registers section.
ADT7461 REGISTERS
The ADT7461 contains 22 8-bit registers in total. These regis-
ters are used to store the results of remote and local temperature
measurements and high and low temperature limits and to confi-
gure and control the device. A description of these registers fol-
lows. Additional details are given in Table 7 through Table 11.
Address Pointer Register
The address pointer register itself does not have or require an
address, as the first byte of every write operation is automa-
tically written to this register. The data in this first byte always
contains the address of another register on the ADT7461, which
is stored in the address pointer register. It is to this register
address that the second byte of a write operation is written to or
to which a subsequent read operation is performed.
The power-on default value of the address pointer register is
0x00, so if a read operation is performed immediately after
power-on, without first writing to the address pointer, the value
of the local temperature will be returned, since its register
address is 0x00.
Temperature Value Registers
The ADT7461 has three registers to store the results of local and
remote temperature measurements. These registers can only be
written to by the ADC and can be read by the user over the
SMBus. The local temperature value register is at Address 0x00.
The external temperature value high byte register is at
Address 0x01, with the low byte register at Address 0x10.
The power-on default for all three registers is 0x00.
Configuration Register
The configuration register is Address 0x03 at read and Address
0x09 at write. Its power-on default is 0x00. Only four bits of the
configuration register are used. Bits 0, 1, 3, and 4 are reserved
and should not be written to by the user.
Bit 7 of the configuration register is used to mask the ALERT
output. If Bit 7 is 0, the ALERT output is enabled. This is the
power-on default. If Bit 7 is set to 1, the ALERT output is
disabled. This only applies if Pin 6 is configured as ALERT. If
Pin 6 is configured as THERM2, then the value of Bit 7 has no
effect.
If Bit 6 is set to 0, which is power-on default, the device is in
operating mode with the ADC converting. If Bit 6 is set to 1, the
device is in standby mode and the ADC does not convert. The
SMBus does, however, remain active in standby mode, so values
can be read from or written to the ADT7461 via the SMBus in
this mode. The ALERT and THERM outputs are also active in
standby mode. Changes made to the registers in standby mode
that affect the THERM or ALERT outputs will cause these
signals to be updated.
Bit 5 determines the configuration of Pin 6 on the ADT7461. If
Bit 5 is 0, (default) then Pin 6 is configured as an ALERT output.
If Bit 5 is 1, then Pin 6 is configured as a THERM2 output. Bit 7,
the ALERT mask bit, is only active when Pin 6 is configured as
an ALERT output. If Pin 6 is setup as a THERM2 output, then
Bit 7 has no effect.
Bit 2 sets the temperature measurement range. If Bit 2 is 0
(default value), the temperature measurement range is set
between 0°C to +127°C. Setting Bit 2 to 1 means that the
measurement range is set to the extended temperature range.
Table 7. Configuration Register Bit Assignments
Bit Name Function
Power-On
Default
7 MASK1 0 = ALERT Enabled
1 = ALERT Masked 0
6 RUN/STOP 0 = Run
1 = Standby 0
5 ALERT/THERM2 0 = ALERT
1 = THERM2 0
4–3 Reserved 0
2 Temperature
Range Select
0 = 0°C to 127°C
1 = Extended Range 0
1–0 Reserved 0
Conversion Rate Register
The conversion rate register is Address 0x04 at read and
Address 0x0A at write. The lowest four bits of this register are
used to program the conversion rate by dividing the internal
oscillator clock by 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, or 1024 to
give conversion times from 15.5 ms (Code 0x0A) to 16 seconds
(Code 0x00). For example, a conversion rate of 8 conversions
per second means that beginning at 125 ms intervals the device
performs a conversion on the internal and the external
temperature channels.
This register can be written to and read back over the SMBus.
The higher four bits of this register are unused and must be set
to 0. The default value of this register is 0x08, giving a rate of
16 conversions per second. Use of slower conversion times
greatly reduces the device power consumption, as shown
in Table 8.
ADT7461
Rev. A | Page 12 of 24
Table 8. Conversion Rate Register Codes
Code Conversion/Second
Average Supply
Current µA Typ
at VDD = 5.5 V
0x00 0.0625 121.33
0x01 0.125 128.54
0x02 0.25 131.59
0x03 0.5 146.15
0x04 1 169.14
0x05 2 233.12
0x06 4 347.42
0x07 8 638.07
0x08 16 252.44
0x09 32 417.58
0x0A 64 816.87
0x0B to 0xFF Reserved
Limit Registers
The ADT7461 has eight limit registers: high, low, and THERM
temperature limits for both local and remote temperature
measurements. The remote temperature high and low limits
span two registers each, to contain an upper and lower byte for
each limit. There is also a THERM hysteresis register. All limit
registers can be written to and read back over the SMBus. See
Table 12 for details of the limit registers’ addresses and their
power-on default values.
When Pin 6 is configured as an ALERT output, the high limit
registers perform a > comparison while the low limit registers
perform a ≤ comparison. For example, if the high limit register
is programmed with 80°C, then measuring 81°C will result in an
out-of-limit condition, setting a flag in the status register. If the
low limit register is programmed with 0°C, measuring 0°C or
lower will result in an out-of-limit condition.
Exceeding either the local or remote THERM limit asserts
THERM low. When Pin 6 is configured as THERM2, exceeding
either the local or remote high limit asserts THERM2 low. A
default hysteresis value of 10°C is provided that applies to both
THERM channels. This hysteresis value may be reprogrammed
to any value after power-up (Register Address 0x21).
It is important to remember that the temperature limits data
format is the same as the temperature measurement data
format. So if the temperature measurement uses default binary,
then the temperature limits also use the binary scale. If the
temperature measurement scale is switched, however, the
temperature limits do not switch automatically. The user must
reprogram the limit registers to the desired value in the correct
data format. For example, if the remote low limit is set at 10°C
and the default binary scale is being used, the limit register
value should be 0000 1010b. If the scale is switched to offset
binary, the value in the low temperature limit register should be
reprogrammed to be 0100 1010b.
Status Register
The status register is a read-only register, at Address 0x02. It
contains status information for the ADT7461.
Bit 7 of the status register indicates that the ADC is busy con-
verting when it is high. The other bits in this register flag the
out-of-limit temperature measurements (Bits 6 to 3 and Bits 1
to 0) and the remote sensor open circuit (Bit 2).
If Pin 6 is configured as an ALERT output, the following applies. If
the local temperature measurement exceeds its limits, Bit 6 (high
limit) or Bit 5 (low limit) of the status register asserts to flag this
condition. If the remote temperature measurement exceeds its
limits, then Bit 4 (high limit) or Bit 3 (low limit) asserts. Bit 2 asserts
to flag an open-circuit condition on the remote sensor. These five
flags are NOR’d together, so if any of them is high, the ALERT
interrupt latch will be set and the ALERT output will go low.
Reading the status register clears the five flags, Bits 6 to 2, pro-
vided the error conditions causing the flags to be set have gone
away. A flag bit can be reset only if the corresponding value reg-
ister contains an in-limit measurement or if the sensor is good.
The ALERT interrupt latch is not reset by reading the status
register. It resets when the ALERT output has been serviced by
the master reading the device address, provided the error condi-
tion has gone away and the status register flag bits are reset.
When Flag 1 and/or Flag 0 are set, the THERM output goes low
to indicate that the temperature measurements are outside the
programmed limits. The THERM output does not need to be
reset, unlike the ALERT output. Once the measurements are
within the limits, the corresponding status register bits are reset
automatically and the THERM output goes high. The user may
add hysteresis by programming Register 0x21. The THERM
output will be reset only when the temperature falls to limit
value–hysteresis value.
When Pin 6 is configured as THERM2, only the high temp-
erature limits are relevant. If Flag 6 and/or Flag 4 are set, the
THERM2 output goes low to indicate that the temperature
measurements are outside the programmed limits. Flag 5 and
Flag 3 have no effect on THERM2. The behavior of THERM2 is
otherwise the same as THERM.
Table 9. Status Register Bit Assignments
Bit Name Function
7 BUSY 1 when ADC converting
6 LHIGH* 1 when local high temperature limit tripped
5 LLOW* 1 when local low temperature limit tripped
4 RHIGH* 1 when remote high temperature limit tripped
3 RLOW* 1 when remote low temperature limit tripped
2 OPEN* 1 when remote sensor open circuit
1 RTHRM 1 when remote THERM limit tripped
0 LTHRM 1 when local THERM limit tripped
*These flags stay high until the status register is read or they are reset by POR.
ADT7461
Rev. A | Page 13 of 24
Offset Register
Offset errors may be introduced into the remote temperature
measurement by clock noise or by the thermal diode being
located away from the hot spot. To achieve the specified
accuracy on this channel, these offsets must be removed.
The offset value is stored as a 10-bit, twos complement value in
Registers 0x11 (high byte) and 0x12 (low byte, left justified).
Only the upper 2 bits of Register 0x12 are used. The MSB of
Register 0x11 is the sign bit. The minimum offset that can be
programmed is −128°C, and the maximum is +127.75°C. The
value in the offset register is added or subtracted to the
measured value of the remote temperature.
The offset register powers up with a default value of 0°C and
will have no effect unless the user writes a different value to it.
Table 10. Sample Offset Register Codes
Offset Value 0x11 0x12
−128°C 1000 0000 00 00 0000
−4°C 1111 1100 00 00 0000
−1°C 1111 1111 00 000000
−0.25°C 1111 1111 10 00 0000
0°C 0000 0000 00 00 0000
+0.25°C 0000 0000 01 00 0000
+1°C 0000 0001 00 00 0000
+4°C 0000 0100 00 00 0000
+127.75°C 0111 1111 11 00 0000
One-Shot Register
The one-shot register is used to initiate a conversion and
comparison cycle when the ADT7461 is in standby mode, after
which the device returns to standby. Writing to the one-shot
register address (0x0F) causes the ADT7461 to perform a
conversion and comparison on both the internal and the
external temperature channels. This is not a data register as
such, and it is the write operation to Address 0x0F that causes
the one-shot conversion. The data written to this address is
irrelevant and is not stored.
Consecutive ALERT Register
The value written to this register determines how many out-of-
limit measurements must occur before an ALERT is generated.
The default value is that one out-of-limit measurement gen-
erates an ALERT. The maximum value that can be chosen is 4.
The purpose of this register is to allow the user to perform
some filtering of the output. This is particularly useful at the
fastest three conversion rates, where no averaging takes place.
This register is at Address 0x22.
Table 11. Consecutive ALERT Register Bit
Register Value
Number of Out-of-Limit
Measurements Required
yxxx 000x 1
yxxx 001x 2
yxxx 011x 3
yxxx 111x 4
x = Don’t care bit.
y = SMBus timeout bit. Default = 0. See Serial Bus Interface section.
Table 12. List of Registers
Read Address (Hex) Write Address (Hex) Name Power-On Default
Not Applicable Not Applicable Address Pointer Undefined
00 Not Applicable Local Temperature Value 0000 0000 (0x00)
01 Not Applicable External Temperature Value High Byte 0000 0000 (0x00)
02 Not Applicable Status Undefined
03 09 Configuration 0000 0000 (0x00)
04 0A Conversion Rate 0000 1000 (0x08)
05 0B Local Temperature High Limit 0101 0101 (0x55) (85°C)
06 0C Local Temperature Low Limit 0000 0000 (0x00) (0°C)
07 0D External Temperature High Limit High Byte 0101 0101 (0x55) (85°C)
08 0E External Temperature Low Limit High Byte 0000 0000 (0x00) (0°C)
Not Applicable 0F One-Shot
10 Not Applicable External Temperature Value Low Byte 0000 0000
11 11 External Temperature Offset High Byte 0000 0000
12 12 External Temperature Offset Low Byte 0000 0000
13 13 External Temperature High Limit Low Byte 0000 0000
14 14 External Temperature Low Limit Low Byte 0000 0000
19 19 External THERM Limit 0110 1100 (0x55) (85°C)
20 20 Local THERM Limit 0101 0101 (0x55) (85°C)
21 21 THERM Hysteresis 0000 1010 (0x0A) (10°C)
22 22 Consecutive ALERT 0000 0001 (0x01)
FE Not Applicable Manufacturer ID 0100 0001 (0x41)
FF Not Applicable Die Revision Code 0101 0001 (0x51)
*Writing to Address 0F causes the ADT7461 to perform a single measurement. It is not a data register as such and it does not matter what data is written to it.
ADT7461
Rev. A | Page 14 of 24
SERIAL BUS INTERFACE
Control of the ADT7461 is carried out via the serial bus. The
ADT7461 is connected to this bus as a slave device, under the
control of a master device.
After a conversion sequence completes, there should be no
SMBus transactions to the ADT7361 for at least one conversion
time, to allow the next conversion to complete. The conversion
time depends on the value programmed in the conversion rate
register.
The ADT7461 has an SMBus timeout feature. When this is
enabled, the SMBus will timeout after typically 25 ms of no
activity. However, this feature is not enabled by default. Bit 7 of
the consecutive alert register (Address = 0x22) should be set to
enable it.
Consult the SMBus 1.1 specification for more information
(www.smbus.org).
ADDRESSING THE DEVICE
In general, every SMBus device has a 7-bit device address,
except for some devices that have extended 10-bit addresses.
When the master device sends a device address over the bus, the
slave device with that address will respond. The ADT7461 is
available with one device address, 0x4C (1001 100b).
The serial bus protocol operates as follows:
1. The master initiates data transfer by establishing a start
condition, defined as a high-to-low transition on the serial
data line SDATA, while the serial clock line SCLK remains
high. This indicates that an address/data stream will follow.
All slave peripherals connected to the serial bus respond to
the start condition and shift in the next eight bits, consis-
ting of a 7-bit address (MSB first) plus an R/W bit, which
determines the direction of the data transfer, i.e., whether
data will be written to or read from the slave device. The
peripheral whose address corresponds to the transmitted
address responds by pulling the data line low during the
low period before the ninth clock pulse, known as the
acknowledge bit. All other devices on the bus now remain
idle while the selected device waits for data to be read from
or written to it. If the R/W bit is a 0, the master will write to
the slave device. If the R/W bit is a 1, the master will read
from the slave device.
2. Data is sent over the serial bus in a sequence of nine clock
pulses, eight bits of data followed by an acknowledge bit
from the slave device. Transitions on the data line must
occur during the low period of the clock signal and remain
stable during the high period, since a low-to-high transition
when the clock is high may be interpreted as a stop signal.
The number of data bytes that can be transmitted over the
serial bus in a single read or write operation is limited only
by what the master and slave devices can handle.
3. When all data bytes have been read or written, stop con-
ditions are established. In write mode, the master will pull
the data line high during the tenth clock pulse to assert a
stop condition. In read mode, the master device will
override the acknowledge bit by pulling the data line high
during the low period before the ninth clock pulse. This is
known as no acknowledge. The master will then take the
data line low during the low period before the tenth clock
pulse, then high during the tenth clock pulse to assert a
stop condition.
Any number of bytes of data may be transferred over the serial
bus in one operation, but it is not possible to mix read and write
in one operation because the type of operation is determined at
the beginning and cannot subsequently be changed without
starting a new operation. In the case of the ADT7461, write
operations contain either one or two bytes, while read opera-
tions contain one byte.
To write data to one of the device data registers or to read data
from it, the address pointer register must be set so that the cor-
rect data register is addressed. The first byte of a write operation
always contains a valid address that is stored in the address
pointer register. If data is to be written to the device, the write
operation contains a second data byte that is written to the
register selected by the address pointer register.
This is illustrated in Figure 17. The device address is sent over
the bus followed by R/W set to 0. This is followed by two data
bytes. The first data byte is the address of the internal data
register to be written to, which is stored in the address pointer
register. The second data byte is the data to be written to the
internal data register.
ADT7461
Rev. A | Page 15 of 24
04110-0-003
A6
191
A5 A4 A3 A2 A1 A0 R/W D7 D6 D5 D4 D3 D2 D1 D0
SCLK
SDAT
9
A
START BY
MASTER ACK. BY
ADT7461
FRAME 1
SERIAL BUS ADDRESS BYTE FRAME 2
ADDRESS POINTER REGISTER BYTE
FRAME 3
DATA BYTE
SCLK (CONTINUED)
SDATA (CONTINUED)
ACK. BY
ADT7461
ACK. BY
ADT7461 STOP BY
MASTER
91
D7 D6 D5 D4 D3 D2 D1 D0
Figure 17. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
04110-0-004
SCLK
S
DATA
START BY
MASTER ACK. BY
ADT7461 ACK. BY
ADT7461 STOP BY
MASTER
FRAME 1
SERIAL BUS ADDRESS BYTE FRAME 2
ADDRESS POINTER REGISTER BYTE
191
A6 A5 A4 A3 A2 A1 A0 R/W D7 D6 D5 D4 D3 D2 D1 D0
9
Figure 18. Writing to the Address Pointer Register Only
04110-0-005
SCLK
S
DAT
A
START BY
MASTER ACK. BY
ADT7461
FRAME 1
SERIAL BUS ADDRESS BYTE FRAME 2
DATA BYTE FROM ADT7461
NACK. BY
MASTER STOP BY
MASTER
191 9
A6 A5 A4 A3 A2 A1 A0 R/W D7 D6 D5 D4 D3 D2 D1 D0
Figure 19. Reading from a Previously Selected Register
When reading data from a register there are two possibilities.
1. If the ADT7461’s address pointer register value is unknown
or not the desired value, it is necessary to set it to the cor-
rect value before data can be read from the desired data
register. This is done by writing to the ADT7461 as before,
but only the data byte containing the register read address
is sent, since data is not to be written to the register. This is
shown in Figure 18.
A read operation is then performed consisting of the serial
bus address, R/W bit set to 1, followed by the data byte read
from the data register. This is shown in Figure 19.
2. If the address pointer register is known to be at the desired
address, data can be read from the corresponding data reg-
ister without first writing to the address pointer register
and the bus transaction shown in Figure 18 can be omitted.
Note that although it is possible to read a data byte from a data
register without first writing to the address pointer register, if
the address pointer register is already at the correct value, it is
not possible to write data to a register without writing to the
address pointer register because the first data byte of a write is
always written to the address pointer register.
Also note that some of the registers have different addresses for
read and write operations. The write address of a register must
be written to the address pointer if data is to be written to that
register, but it may not be possible to read data from that
address. The read address of a register must be written to the
address pointer before data can be read from that register.
ADT7461
Rev. A | Page 16 of 24
ALERT OUTPUT
This is applicable when Pin 6 is configured as an ALERT output.
The ALERT output goes low whenever an out-of-limit measure-
ment is detected, or if the remote temperature sensor is open
circuit. It is an open-drain output and requires a pull-up to VDD.
Several ALERT outputs can be wire-ORed together, so that the
common line will go low if one or more of the ALERT outputs
goes low.
The ALERT output can be used as an interrupt signal to a pro-
cessor, or it may be used as an SMBALERT. Slave devices on the
SMBus cannot normally signal to the bus master that they want
to talk, but the SMBALERT function allows them to do so.
One or more ALERT outputs can be connected to a common
SMBALERT line that is connected to the master. When the
SMBALERT line is pulled low by one of the devices, the
following procedure occurs (see Figure 20.):
04110-0-006
ALERT RESPONSE
ADDRESS
MASTER SENDS
ARA AND READ
COMMAND DEVICE SENDS
ITS ADDRESS
RDSTART ACK DEVICE
ADDRESS NO
ACK STOP
MASTER
RECEIVES
SMBALERT
Figure 20. Use of SMBALERT
1. SMBALERT is pulled low.
2. Master initiates a read operation and sends the alert
response address (ARA = 0001 100). This is a general call
address that must not be used as a specific device address.
3. The device whose ALERT output is low responds to the
alert response address and the master reads its device
address. As the device address is seven bits, an LSB of 1 is
added. The address of the device is now known and it can
be interrogated in the usual way.
4. If more than one device’s ALERT output is low, the one
with the lowest device address will have priority, in accor-
dance with normal SMBus arbitration.
5. Once the ADT7461 has responded to the alert response
address, it will reset its ALERT output, provided that the
error condition that caused the ALERT no longer exists. If
the SMBALERT line remains low, the master will send the
ARA again, and so on until all devices whose ALERT out-
puts were low have responded.
LOW POWER STANDBY MODE
The ADT7461 can be put into low power standby mode by set-
ting Bit 6 of the configuration register. When Bit 6 is low, the
ADT7461 operates normally. When Bit 6 is high, the ADC is
inhibited, and any conversion in progress is terminated without
writing the result to the corresponding value register.
The SMBus is still enabled. Power consumption in the standby
mode is reduced to less than 10 µA if there is no SMBus activity
or 100 µA if there are clock and data signals on the bus.
When the device is in standby mode, it is still possible to initiate
a one-shot conversion of both channels by writing to the one-
shot register (Address 0x0F), after which the device will return
to standby. It does not matter what is written to the one-shot
register, all data written to it is ignored. It is also possible to
write new values to the limit register while in standby mode. If
the values stored in the temperature value registers are now
outside the new limits, an ALERT is generated, even though the
ADT7461 is still in standby.
SENSOR FAULT DETECTION
At its D+ input, the ADT7461 contains internal sensor fault
detection circuitry. This circuit can detect situations where an
external remote diode is either not connected or incorrectly
connected to the ADT7461. A simple voltage comparator trips if
the voltage at D+ exceeds VDD −1 V (typical), signifying an open
circuit between D+ and D−. The output of this comparator is
checked when a conversion is initiated. Bit 2 of the status
register (open flag) is set if a fault is detected. If the ALERT pin
is enabled, setting this flag will cause ALERT to assert low.
If the user does not wish to use an external sensor with the
ADT7461, then to prevent continuous setting of the OPEN flag,
the user should tie the D+ and D− inputs together.
THE ADT7461 INTERRUPT SYSTEM
The ADT7461 has two interrupt outputs, ALERT and THERM.
Both have different functions and behavior. ALERT is maskable
and responds to violations of software-programmed tempera-
ture limits or an open-circuit fault on the external diode.
THERM is intended as a fail-safe interrupt output that cannot
be masked.
If the external or local temperature exceeds the programmed
high temperature limits or equals or exceeds the low tempera-
ture limits, the ALERT output is asserted low. An open-circuit
fault on the external diode also causes ALERT to assert. ALERT
is reset when serviced by a master reading its device address,
provided the error condition has gone away and the status
register has been reset.
ADT7461
Rev. A | Page 17 of 24
The THERM output asserts low if the external or local temper-
ature exceeds the programmed THERM limits. THERM temp-
erature limits should normally be equal to or greater than the
high temperature limits. THERM is reset automatically when
the temperature falls back within the THERM limit. The exter-
nal limit is set by default to 85°C, as is the local THERM limit. A
hysteresis value can be programmed; in which case, THERM
resets when the temperature falls to the limit value minus the
hysteresis value. This applies to both local and remote measure-
ment channels. The power-on hysteresis default value is 10°C,
but this may be reprogrammed to any value after power-up.
The hysteresis loop on the THERM outputs is useful when
THERM is used for on/off control of a fan. The user’s system
can be set up so that when THERM asserts a fan can be
switched on to cool the system. When THERM goes high again,
the fan can be switched off. Programming a hysteresis value
protects from fan jitter, where the temperature hovers around
the THERM limit, and the fan is constantly being switched.
Table 13. THERM Hysteresis
THERM Hysteresis Binary Representation
0°C 0 000 0000
1°C 0 000 0001
10°C 0 000 1010
Figure 21 shows how the THERM and ALERT outputs operate.
A user may wish to use the ALERT output as a SMBALERT to
signal to the host via the SMBus that the temperature has risen.
The user could use the THERM output to turn on a fan to cool
the system, if the temperature continues to increase. This
method would ensure that there is a fail-safe mechanism to cool
the system, without the need for host intervention.
04110-0-007
1
32
4
THERM LIMIT
HIGH TEMP LIMIT
THERM LIMIT-HYSTERESIS
RESET BY MASTER
ALERT
THERM
100°C
TEMPERATURE
90°C
80°C
70°C
60°C
50°C
40°C
Figure 21. Operation of the ALERT and THERM Interrupts
1. If the measured temperature exceeds the high temperature
limit, the ALERT output will assert low.
2. If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low. This can be
used to throttle the CPU clock or switch on a fan.
3. The THERM output deasserts (goes high) when the
temperature falls to THERM limit minus hysteresis. In
Figure 21, the default hysteresis value of 10°C is shown.
4. The ALERT output deasserts only when the temperature
has fallen below the high temperature limit, and the master
has read the device address and cleared the status register.
Pin 6 on the ADT7461 can be configured as either an ALERT
output or as an additional THERM output. THERM2 will assert
low when the temperature exceeds the programmed local
and/or remote high temperature limits. It is reset in the same
manner as THERM, and it is not maskable. The programmed
hysteresis value applies to THERM2 also.
Figure 22 shows how THERM and THERM2 might operate
together to implement two methods of cooling the system. In
this example, the THERM2 limits are set lower than the
THERM limits. The THERM2 output could be used to turn on
a fan. If the temperature continues to rise and exceeds the
THERM limits, the THERM output could provide additional
cooling by throttling the CPU.
THERM2 LIMIT
THERM LIMIT
04110-0-008
THERM2 1
3
2
4
TEMPERATURE
THERM
90°C
80°C
70°C
60°C
50°C
40°C
30°C
Figure 22. Operation of the THERM and THERM2 Interrupts
1. When the THERM2 limit is exceeded, the THERM2 signal
asserts low.
2. If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low.
3. The THERM output deasserts (goes high) when the
temperature falls to THERM limit minus hysteresis. In
Figure 22, there is no hysteresis value shown.
4. As the system cools further, and the temperature falls
below the THERM2 limit, the THERM2 signal resets.
Again, no hysteresis value is shown for THERM2.
Both the external and internal temperature measurements will
cause THERM and THERM2 to operate as described.
ADT7461
Rev. A | Page 18 of 24
APPLICATION INFORMATION
Noise Filtering
For temperature sensors operating in noisy environments, the
industry standard practice was to place a capacitor across the D+
and D− pins to help combat the effects of noise. However, large
capacitances affect the accuracy of the temperature measurement,
leading to a recommended maximum capacitor value of 1,000 pF.
While this capacitor will reduce the noise, it will not eliminate it,
making it difficult to use the sensor in a very noisy environment.
The ADT7461 has a major advantage over other devices when it
comes to eliminating the effects of noise on the external sensor.
The series resistance cancellation feature allows a filter to be
constructed between the external temperature sensor and the
part. The effect of any filter resistance seen in series with the remote
sensor is automatically cancelled from the temperature result.
The construction of a filter allows the ADT7461 and the remote
temperature sensor to operate in noisy environments. Figure 23
shows a low-pass R-C-R filter, with the following values:
R = 100 Ω and C = 1 nF. This filtering reduces both common-
mode noise and differential noise.
04110-0-009
D+
1nF
100Ω
REMOTE
T
EMPERATURE
SENSOR D–
100Ω
Figure 23. Filter Between Remote Sensor and ADT7461
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7461 is designed to work with substrate transistors
built into processors or with discrete transistors. Substrate tran-
sistors will generally be PNP types with the collector connected
to the substrate. Discrete types can be either PNP or NPN tran-
sistor connected as a diode (base shorted to collector). If an
NPN transistor is used, the collector and base are connected to
D+ and the emitter to D−. If a PNP transistor is used, the col-
lector and base are connected to D− and the emitter to D+.
To reduce the error due to variations in both substrate and
discrete transistors, a several factors should be taken into
consideration:
• The ideality factor, nF, of the transistor is a measure of the
deviation of the thermal diode from ideal behavior. The
ADT7461 is trimmed for an nF value of 1.008. The follow-
ing equation may be used to calculate the error introduced
at a temperature T (°C), when using a transistor whose nf
does not equal 1.008. Consult the processor data sheet for
the nF values.
∆T = (nF − 1.008)/1.008 × (273.15 Kelvin + T)
To factor this in, the user can write the ∆T value to the
offset register. It will then be automatically added to or
subtracted from the temperature measurement by
the ADT7461.
• Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7461, IHIGH, is 96 µA and the low level current, ILOW,
is 6 µA. If the ADT7461 current levels do not match the
current levels specified by the CPU manufacturer, it may
become necessary to remove an offset. The CPUs data
sheet will advise whether this offset needs to be removed
and how to calculate it. This offset may be programmed to
the offset register. It is important to note that if more than
one offset must be considered, the algebraic sum of these
offsets must be programmed to the offset register.
If a discrete transistor is being used with the ADT7461, the best
accuracy will be obtained by choosing devices according to the
following criteria:
• Base-emitter voltage greater than 0.25 V at 6 µA, at the
highest operating temperature.
• Base-emitter voltage less than 0.95 V at 100 µA, at the
lowest operating temperature.
• Base resistance less than 100 Ω.
• Small variation in hFE (50 to 150) that indicates tight
control of VBE characteristics.
Transistors, such as the 2N3904, 2N3906, or equivalents in
SOT-23 packages are suitable devices to use.
THERMAL INERTIA AND SELF-HEATING
Accuracy depends on the temperature of the remote sensing
diode and/or the internal temperature sensor being at the same
temperature as that being measured. Many factors can affect
this. Ideally, the sensor should be in good thermal contact with
the part of the system being measured. If it is not, the thermal
inertia caused by the sensor’s mass causes a lag in the response
of the sensor to a temperature change. In the case of the remote
sensor, this should not be a problem since it will be either a
substrate transistor in the processor or a small package device,
such as the SOT-23, placed in close proximity to it.
The on-chip sensor, however, is often remote from the processor
and only monitors the general ambient temperature around the
package. The thermal time constant of the SOIC-8 package in
still air is about 140 seconds, and if the ambient air temperature
quickly changed by 100 degrees, it would take about 12 minutes
(5 time constants) for the junction temperature of the ADT7461
to settle within 1 degree of this. In practice, the ADT7461 pack-
age is in electrical, and hence thermal, contact with a PCB and
may also be in a forced airflow. How accurately the temperature
of the board and/or the forced airflow reflects the temperature
to be measured also affects the accuracy. Self-heating due to the
power dissipated in the ADT7461 or the remote sensor causes
the chip temperature of the device or remote sensor to rise
above ambient. However, the current forced through the remote
ADT7461
Rev. A | Page 19 of 24
sensor is so small that self-heating is negligible. In the case of
the ADT7461, the worst-case condition occurs when the device
is converting at 64 conversions per second while sinking the
maximum current of 1 mA at the ALERT and THERM output.
In this case, the total power dissipation in the device is about
4.5 mW. The thermal resistance, θJA, of the SOIC-8 package is
about 121°C/W.
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments, and the
ADT7461 is measuring very small voltages from the remote
sensor, so care must be taken to minimize noise induced at the
sensor inputs. The following precautions should be taken:
1. Place the ADT7461 as close as possible to the remote
sensing diode. Provided that the worst noise sources, i.e.,
clock generators, data/address buses, and CRTs are avoided,
this distance can be 4 inches to 8 inches.
2. Route the D+ and D– tracks close together, in parallel, with
grounded guard tracks on each side. To minimize
inductance and reduce noise pick-up, a 5 mil track width
and spacing is recommended. Provide a ground plane
under the tracks if possible.
04110-0-010
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
GND
D+
D–
GND
Figure 24. Typical Arrangement of Signal Tracks
3. Try to minimize the number of copper/solder joints
that can cause thermocouple effects. Where copper/solder
joints are used, make sure that they are in both the D+ and
D− path and at the same temperature.
Thermocouple effects should not be a major problem as
1°C corresponds to about 200 mV, and thermocouple
voltages are about 3 mV/°C of temperature difference.
Unless there are two thermocouples with a big temperature
differential between them, thermocouple voltages should
be much less than 200 mV.
4. Place a 0.1 µF bypass capacitor close to the VDD pin. In
extremely noisy environments, an input filter capacitor
may be placed across D+ and D− close to the ADT7461.
This capacitance can effect the temperature measurement,
so care must be taken to ensure that any capacitance seen
at D+ and D− is a maximum of 1,000 pF. This maximum
value includes the filter capacitance, plus any cable or stray
capacitance between the pins and the sensor diode.
5. If the distance to the remote sensor is more than 8 inches,
the use of twisted pair cable is recommended. This will
work up to about 6 feet to 12 feet.
For really long distances (up to 100 feet), use a shielded
twisted pair, such as the Belden No. 8451 microphone
cable. Connect the twisted pair to D+ and D− and the
shield to GND close to the ADT7461. Leave the remote end
of the shield unconnected to avoid ground loops.
Because the measurement technique uses switched current
sources, excessive cable or filter capacitance can affect the
measurement. When using long cables, the filter capacitance
may be reduced or removed.
ADT7461
Rev. A | Page 20 of 24
APPLICATION CIRCUIT
Figure 25 shows a typical application circuit for the ADT7461
using a discrete sensor transistor connected via a shielded,
twisted pair cable. The pull-ups on SCLK, SDATA, and ALERT
are required only if they are not already provided elsewhere in
the system.
The SCLK and SDATA pins of the ADT7461 can be interfaced
directly to the SMBus of an I/O controller, such as the Intel 820
chipset.
04110-0-011
SHIELD2N3906
OR
CPU THERMAL
DIODE ALERT/
THERM2
GND
THERM
D+
D–
ADT7461
SCLK
SDATA
V
DD
V
DD
3V TO 3.6V
TYP 10kΩ
TYP 10kΩ
0.1µF
FAN
CONTROL
CIRCUIT
FAN
ENABLE
SMBUS
CONTROLLER
5V OR 12V
Figure 25. Typical Application Circuit
ADT7461
Rev. A | Page 21 of 24
OUTLINE DIMENSIONS
0.25 (0.0098)
0.17 (0.0067)
1.27 (0.0500)
0.40 (0.0157)
0.50 (0.0196)
0.25 (0.0099)× 45°
8°
0°
1.75 (0.0688)
1.35 (0.0532)
SEATING
PLANE
0.25 (0.0098)
0.10 (0.0040)
41
85
5.00 (0.1968)
4.80 (0.1890)
4.00 (0.1574)
3.80 (0.1497)
1.27 (0.0500)
BSC
6.20 (0.2440)
5.80 (0.2284)
0.51 (0.0201)
0.31 (0.0122)
COPLANARITY
0.10
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
COMPLIANT TO JEDEC STANDARDS MS-012AA
Figure 26. 8-Lead Standard Small Outline Package [SOIC]
(R-8)
Dimensions Shown in Millimeters and (Inches)
0.80
0.60
0.40
8°
0°
4
85
4.90
BSC
PIN 1 0.65 BSC
3.00
BSC
SEATING
PLANE
0.15
0.00
0.38
0.22
1.10 MAX
3.00
BSC
COPLANARITY
0.10
0.23
0.08
COMPLIANT TO JEDEC STANDARDS MO-187AA
Figure 27. 8-Lead Micro Small Outline Package [MSOP]
(RM-8)
Dimensions Shown in Millimeters
ORDERING GUIDE
Model Temperature Range Package Description Package Option Branding SMBus Address
ADT7461AR −40°C to +125°C 8-Lead SOIC R-8 4C
ADT7461AR-REEL −40°C to +125°C 8-Lead SOIC R-8 4C
ADT7461AR-REEL7 −40°C to +125°C 8-Lead SOIC R-8 4C
ADT7461ARM −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C
ADT7461ARM-REEL −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C
ADT7461ARM-REEL7 −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C
ADT7461ARZ1−40°C to +125°C 8-Lead SOIC R-8 4C
ADT7461ARZ-REEL1 −40°C to +125°C 8-Lead SOIC R-8 4C
ADT7461ARZ-REEL71 −40°C to +125°C 8-Lead SOIC R-8 4C
ADT7461ARMZ 1 −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C
ADT7461ARMZ-REEL1 −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C
ADT7461ARMZ-REEL7 1 −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C
EVAL-ADT7461EB Evaluation Board
1 Z = Pb-free part.
ADT7461
Rev. A | Page 22 of 24
NOTES
ADT7461
Rev. A | Page 23 of 24
NOTES
ADT7461
Rev. A | Page 24 of 24
NOTES
© 2004 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
C04110-0-10/04(A)
