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ADT7411ARQZ-ADT7411ARQZ-REEL7
SPI/I2C® Compatible, 10-Bit Digital Temperature Sensor and Eight Channel ADC
SPI®/I2C® Compatible, 10-Bit Digital
Temperature Sensor and 8-Channel ADC
Rev. A
FEATURES
10-bit temperature-to-digital converter
10-bit 8-channel ADC
DC input bandwidth
Input range: 0 V to 2.25 V, and 0 V to VDD
Temperature range: –40°C to +120°C
Temperature sensor accuracy of ±0.5°C
Supply range: 2.7 V to 5.5 V
Power-down current 1 µA
Internal 2.25 VREF option
Double-buffered input logic 2C, SPI, QSPI™, MICROWIRE™, and DSP compatible
4-wire serial interface
SMBus packet error checking (PEC) compatible
16-lead QSOP package
APPLICATIONS
Portable battery-powered instruments
Personal computers
Smart battery chargers
Telecommunications systems electronic test equipment
Domestic appliances
Process control
PIN CONFIGURATION 02882-A
AIN6AIN7
AIN5AIN8AIN4SCL/SCLK
GNDSDA/DIN
VDDDOUT/ADD
D+/AIN1INT/INT
D–/AIN2AIN3
NC = NO CONNECT
Figure 1.
GENERAL DESCRIPTION The ADT7411 combines a 10-bit temperature-to-digital con-
verter and a 10-bit 8-channel ADC in a 16-lead QSOP package.
This includes a band gap temperature sensor and a 10-bit ADC
to monitor and digitize the temperature reading to a resolution
of 0.25°C. The ADT7411 operates from a single 2.7 V to 5.5 V
supply. The input voltage on the ADC channels has a range of
0 V to 2.25 V and the input bandwidth is dc. The reference for
the ADC channels is derived internally. The ADT7411 provides
two serial interface options: a 4-wire serial interface compatible
with SPI, QSPI, MICROWIRE, and DSP interface standards,
and a 2-wire SMBus/I2C interface. It features a standby mode
that is controlled via the serial interface.
The ADT7411’s wide supply voltage range, low supply current,
and SPI/I2C compatible interface make it ideal for a variety of
applications, including personal computers, office equipment,
and domestic appliances.
*Protected by the following U.S. Patent Numbers: 6,169,442; 5,867,012; 5,764174. Other patents pending.
TABLE OF CONTENTS Specifications.....................................................................................3
Functional Block Diagram..............................................................6
Absolute Maximum Ratings............................................................7
ESD Caution..................................................................................7
Pin Configuration and Functional Description...........................8
Terminology......................................................................................9
Typical Performance Characteristics...........................................10
Theory of Operation......................................................................13
Power-Up Calibration................................................................13
Conversion Speed.......................................................................13
Functional Description..................................................................14
Analog Inputs..............................................................................14
Functional Description—Measurement..................................15
ADT7411 Registers....................................................................19
Serial Interface............................................................................27
Outline Dimensions.......................................................................34
Ordering Guide..........................................................................34
REVISION HISTORY
Revision A
3/04–Data Sheet Changed from Rev. 0 to Rev. A Format Updated Universal
Change to Equation.............................................................................17
8/03–Revision 0: Initial Version
SPECIFICATIONS
Table 1. VDD = 2.7 V to 5.5 V, GND = 0 V, unless otherwise noted. Temperature ranges are −40°C to +120°C. Round robin is the continuous sequential measurement of the following channels: VDD, internal temperature, external temperature (AIN1, AIN2), AIN3, AIN4, AIN5,
AIN6, AIN7, and AIN8. Guaranteed by design and characterization, not production tested.
4 The SDA and SCL timing is measured with the input filters turned on so as to meet the FAST-Mode I2C specification. Switching off the input filters improves the transfer
02882-A
SCL
SDADATA IN
Figure 2. I2C Bus Timing Diagram
02882-A
SCLK
DIN
DOUT
Figure 3. SPI Bus Timing Diagram
02882-A
-004
200µAIOH
1.6VOUTPUTPINCL
50pF
200µAIOLFigure 4. Load Circuit for Access Time and Bus Relinquish Time
7 IDD specification is valid for full-scale analog input voltages. Interface inactive. ADC active. Load currents excluded.
FUNCTIONAL BLOCK DIAGRAM 02882-A
-001
D+/AIN1D–/AIN2AIN3AIN4AIN5AIN6AIN7AIN8
SDA/DIN
GND
VDD
SCL/SCLK
DOUT/ADD
INT/INTFigure 5. Functional Block Diagram
ABSOLUTE MAXIMUM RATINGS
Table 2. Values relate to package being used on a 4-layer board
9 Junction-to-case resistance is applicable to components featuring a
preferential flow direction, e.g., components mounted on a heat sink.
Junction-to-ambient resistance is more useful for air-cooled PCB-mounted
components.
Table 3. I2C Address Selection 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
electro-static discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
PIN CONFIGURATION AND FUNCTIONAL DESCRIPTION 02882-A
AIN6AIN7
AIN5AIN8AIN4SCL/SCLK
GNDSDA/DIN
VDDDOUT/ADD
D+/AIN1INT/INT
D–/AIN2AIN3
NC = NO CONNECT
Figure 6. Pin Configuration
Table 4. Pin Function Description TERMINOLOGY
Relative Accuracy Relative accuracy or integral nonlinearity (INL) is a measure of
the maximum deviation, in LSBs, from a straight line passing
through the endpoints of the ADC transfer function. A typical
INL versus code plot can be seen in Figure 10.
Total Unadjusted Error (TUE) Total unadjusted error is a comprehensive specification that
includes the sum of the relative accuracy error, gain error, and
offset error under a specified set of conditions.
Offset Error This is a measure of the offset error of the ADC. It can be
negative or positive. It is expressed in mV.
Gain Error This is a measure of the span error of the ADC. It is the
deviation in slope of the actual ADC transfer characteristic
from the ideal expressed as a percentage of the full-scale range.
Offset Error Drift This is a measure of the change in offset error with changes in
temperature. It is expressed in (ppm of full-scale range)/°C.
Gain Error Drift This is a measure of the change in gain error with changes in
temperature. It is expressed in (ppm of full-scale range)/°C.
Long -term Temperature Drift This is a measure of the change in temperature error with the
passage of time. It is expressed in degrees Celsius. The concept
of long-term stability has been used for many years to describe
by what amount an IC’s parameter would shift during its
lifetime. This is a concept that has been typically applied to both
voltage references and monolithic temperature sensors.
Unfortunately, integrated circuits cannot be evaluated at room
temperature (25°C) for 10 years or so to determine this shift. As
a result, manufacturers typically perform accelerated lifetime
testing of integrated circuits by operating ICs at elevated
temperatures (between 125°C and 150°C) over a shorter period
of time (typically, between 500 and 1,000 hours). As a result of
this operation, the lifetime of an integrated circuit is
significantly accelerated due to the increase in rates of reaction
within the semiconductor material.
DC Power Supply Rejection Ratio (PSRR) The power supply rejection ratio (PSRR) is defined as the ratio
of the power in the ADC output at full-scale frequency f, to the
power of a 100 mV sine wave applied to the VDD supply of
frequency fs: )PfsPSRRlog10
Pf = power at frequency f in ADC output
Pfs = power at frequency fs coupled into the VDD supply
Round Robin This term is used to describe the ADT7411 cycling through
the available measurement channels in sequence, taking a
measurement on each channel.
TYPICAL PERFORMANCE CHARACTERISTICS02882-A
VCC (V)
ICC
(mA)
1.95
Figure 7. Supply Current vs. Supply Voltage at 25ºC
02882-A
AC P
RR (dB)
FREQUENCY (kHz)
Figure 8. PSRR vs. Supply Ripple Frequency
02882-A
VCC (V)
ICC
02882-A
ADC CODE
INL E
RROR (LS
0.8
Figure 10. ADC INL with Ref = VDD (3.3V)
02882-A
TEMPERATURE (°C)
RATURE
RROR (
°C)
1.0Figure 11. Temperature Error at 3.3 V and 5 V
02882-A
RROR (LS
TEMPERATURE (°C)406080100120
02882-A
ATURE
RROR (
PCB LEAKAGE RESISTANCE (MΩ)405060708090100Figure 13. External Temperature Error vs. PCB Track Resistance
02882-A
RATURE
RROR (
NOISE FREQUENCY (Hz)100200300400500600Figure 14. External Temperature Error vs.
Common-Mode Noise Frequency
02882-A
VDD (V)
RROR (LS
2.73.13.53.94.34.75.15.5
Figure 15. ADC Offset Error and Gain Error vs. VDD
02882-A
RATURE
RROR (
CAPACITANCE (nF)35404550Figure 16. External Temperature Error vs. Capacitance
between D+ and D−
02882-A
RATURE
RROR (
NOISE FREQUENCY (MHz)
300400500600Figure 17. External Temperature Error vs. Differential Mode
Noise Frequency
02882-A
NOISE FREQUENCY (Hz)100200300400500600
RATURE
RROR (
–0.6Figure 18. Internal Temperature Error vs. Power Supply Noise Frequency
02882-A
RATURE
°C)
TIME (s)4050060Figure 19. Temperature Sensor Response to Thermal Shock
THEORY OF OPERATION Directly after the power-up calibration routine, the ADT7411
goes into idle mode. In this mode, the device is not performing
any measurements and is fully powered up.
To begin monitoring, write to the Control Configuration 1
register (Address 18h) and set Bit C0 = 1. The ADT7411 goes
into its power-up default measurement mode, which is round
robin. The device proceeds to take measurements on the VDD
channel, internal temperature sensor channel, external temp-
erature sensor channel, or AIN1 and AIN2, AIN3, AIN4, AIN5,
AIN6, AIN7, and finally AIN8. Once it finishes taking measure-
ments on the AIN8 channel, the device immediately loops back
to start taking measurements on the VDD channel and repeats
the same cycle as before. This loop continues until the mon-
itoring is stopped by resetting Bit C0 of the Control Config-
uration 1 register to 0. It is also possible to continue monitoring
as well as switching to single-channel mode by writing to the
Control Configuration 2 register (Address 19h) and setting
Bit C4 = 1. Further explanations of the single-channel and
round robin measurement modes are given in later sections. All
measurement channels have averaging enabled on them at
power-up. Averaging forces the device to take an average of 16
readings before giving a final measured result. To disable aver-
aging and consequently decrease the conversion time by a factor
of 16, set C5 = 1 in the Control Configuration 2 register.
There are eight single-ended analog input channels on the
ADT7411: AIN1 to AIN8. AIN1 and AIN2 are multiplexed with
the external temperature sensors D+ and D− terminals. Bits C1
and C2 of the Control Configuration 1 register (Address 18h)
are used to select between AIN1/2 and the external temperature
sensor. The input range on the analog input channels is
dependent on whether the ADC reference used is the internal
VREF or VDD. To meet linearity specifications, it is recommended
that the maximum VDD value is 5 V. Bit C4 of the Control
Configuration 3 register is used to select between the internal
reference and VDD as the analog inputs’ ADC reference.
The dual serial interface defaults to the I2C protocol on power-
up. To select and lock in the SPI protocol, follow the selection
process as described in the Serial Interface Selection section.
The I2C protocol cannot be locked in, while the SPI protocol on
selection is automatically locked in. The interface can only be
switched back to I2C when the device is powered off and on.
When using I2C, the CS pin should be tied to either VDD or
GND.
There are a number of different operating modes on the
ADT7411 devices and all of them can be controlled by the
configuration registers. These features consist of enabling and
disabling interrupts, polarity of the INT/INT pin, enabling and
disabling the averaging on the measurement channels, SMBus
timeout, and software reset.
POWER-UP CALIBRATION It is recommended that no communication to the part is
initiated until approximately 5 ms after VDD has settled to within
10% of its final value. It is generally accepted that most systems
take a maximum of 50 ms to power up. Power-up time is
directly related to the amount of decoupling on the voltage
supply line.
During the 5 ms after VDD has settled the part is performing a
calibration routine; any communication to the device will
interrupt this routine and could cause erroneous temperature
measurements. If it is not possible to have VDD at its nominal
value by the time 50 ms has elapsed or that communication to
the device has started prior to VDD settling, then it is recom-
mended that a measurement be taken on the VDD channel
before a temperature measurement is taken. The VDD measure-
ment is used to calibrate out any temperature measurement
error due to different supply voltage values.
CONVERSION SPEED The internal oscillator circuit used by the ADC has the
capability to output two different clock frequencies. This means
that the ADC is capable of running at two different speeds
when doing a conversion on a measurement channel. Thus the
time taken to perform a conversion on a channel can be
reduced by setting C0 of the Control Configuration 3 register
(Address 1Ah). This increases the ADC clock speed from
1.4 kHz to 22 kHz. At the higher clock speed, the analog filters
on the D+ and D− input pins (external temperature sensor) are
switched off. This is why the power-up default setting is to have
the ADC working at the slow speed. The typical times for fast
and slow ADC speeds are given in the specification pages.
The ADT7411 powers up with averaging on. This means every
channel is measured 16 times and internally averaged to reduce
noise. The conversion time can also be reduced by turning the
averaging off. This is done by setting Bit C5 of the Control
Configuration 2 register (Address 19h) to a 1.
FUNCTIONAL DESCRIPTION
ANALOG INPUTS
Single-Ended Inputs The ADT7411 offers eight single-ended analog input channels.
The analog input range is from 0 V to 2.25 V or 0 V to VDD. To
maintain the linearity specification it is recommended that the
maximum VDD value be set at 5 V. Selection between the two
input ranges is done by Bit C4 of the Control Configuration
3 register (Address 1Ah). Setting this bit to 0 sets up the analog
input ADC reference to be sourced from the internal voltage
reference of 2.25 V. Setting the bit to 1 sets up the ADC
reference to be sourced from VDD.
The ADC resolution is 10 bits and is mostly suitable for dc input
signals or very slowly varying ac signals. Bits C1:2 of the
Control Configuration 1 register (Address 18h) are used to set
up Pins 7 and 8 as AIN1 and AIN2. Figure 20 shows the overall
view of the 8-channel analog input path.
02882-A
SAMPLING
COMPARATOR
INT VREFSW1
REF/2
Figure 20. Octal Analog Input Path
Converter Operation The analog input channels use a successive approximation ADC
based around a capacitor DAC. Figure 21 and Figure 22 show
simplified schematics of the ADC. Figure 21 shows the ADC
during acquisition phase. SW2 is closed and SW1 is in position
A. The comparator is held in a balanced condition and the
sampling capacitor acquires the signal on AIN.
02882-A
SAMPLING
COMPARATOR
INT VREFSW1
REF/2
Figure 21. ADC Acquisition Phase
When the ADC eventually goes into conversion phase (see
Figure 22) SW2 opens and SW1 moves to position B, causing
the comparator to become unbalanced. The control logic and
the DAC are used to add and subtract fixed amounts of charge
from the sampling capacitor to bring the comparator back into
a balanced condition. When the comparator is rebalanced, the
conversion is complete. The control logic generates the ADC
output code. Figure 24 shows the ADC transfer function for
single-ended analog inputs.
02882-A
SAMPLING
COMPARATOR
INT VREF
VDD
AINSW1
REF/2
Figure 22. ADC Conversion Phase
02882-A
VDD
VOUT+
VOUT–
REMOTESENSINGTRANSISTOR(2N3906)
OPTIONAL CAPACITOR, UP TO3nF MAX. CAN BE ADDED TOIMPROVE HIGH FREQUENCYNOISE REJECTION IN NOISYENVIRONMENTS
Figure 23. Signal Conditioning for External Diode Temperature Sensor
ADC Transfer Function The output coding of the ADT7411 analog inputs is straight
binary. The designed code transitions occur midway between
successive integer LSB values (i.e., 1/2 LSB, 3/2 LSB). The LSB is
VDD/1024 or Int VREF/1024, Int VREF = 2.25 V. The ideal transfer
characteristic is shown in Figure 24.
02882-A
+VREF– 1LSB0V 1/2 LSB
ANALOG INPUT
ADC CODE
000...000
Figure 24. Transfer Function
To work out the voltage on any analog input channel, the
following method can be used: 1VferenceReLSB=
Convert the value read back from the AIN value register into
decimal. sizeLSBdAINValueAINVoltage×=
where d = decimal
Example:
Internal reference used. Therefore, VREF = 2.25 V.
AINVoltagesizeLSBvalueAIN
512×===
Analog Input ESD Protection Figure 26 shows the input structure that provides ESD protec-
tion on any of the analog input pins. The diode provides the
main ESD protection for the analog inputs. Care must be taken
that the analog input signal never drops below the GND rail by
more than 200 mV. If this happens, the diode will become
forward biased and start conducting current into the substrate.
The 4 pF capacitor is the typical pin capacitance and the resistor
is a lumped component made up of the on resistance of the
multiplexer switch.
02882-A
-024
DIODEINTERNALSENSETRANSISTOR
VDD
VOUT+
VOUT–Figure 25. Top Level Structure of Internal Temperature Sensor
02882-A
-025
100ΩFigure 26. Equivalent Analog Input ESD Circuit
AIN Interrupts The measured results from the AIN inputs are compared with
the AIN VHIGH (greater than comparison) and VLOW (less than or
equal to comparison) limits. An interrupt occurs if the AIN
inputs exceed or equal the limit registers. These voltage limits
are stored in on-chip registers. Note that the limit registers are
eight bits long while the AIN conversion result is 10 bits long.
If the voltage limits are not masked out, any out-of-limit
comparisons generate flags that are stored in the Interrupt
Status 1 register (Address 00h) and one or more out-of-limit
results will cause the INT/INT output to pull either high or low,
depending on the output polarity setting. It is good design prac-
tice to mask out interrupts for channels that are of no concern
to the application. Figure 27 shows the interrupt structure for
the ADT7411. It shows a block diagram representation of how
the various measurement channels affect the INT/INT pin.
FUNCTIONAL DESCRIPTION—MEASUREMENT
Temperature Sensor The ADT7411 contains an A/D converter with special input
signal conditioning to enable operation with external and on-
chip diode temperature sensors. When the ADT7411 is oper-
ating in single-channel mode, the A/D converter continually
processes the measurement taken on one channel only. This
channel is preselected by bits C0:C3 in the Control Config-
uration 2 register (Address 19h). When in round robin mode
the analog input multiplexer sequentially selects the VDD input
channel, on-chip temperature sensor to measure its internal
temperature, the external temperature sensor, or an AIN
channel, and then the rest of the AIN channels. These signals
are digitized by the ADC and the results stored in the various
value registers.
The measured results from the temperature sensors are com-
pared with the internal and external, THIGH, TLOW, limits. These
temperature limits are stored in on-chip registers. If the temp-
erature limits are not masked out, any out-of-limit comparisons
generate flags that are stored in Interrupt Status 1 register. One
or more out-of-limit results will cause the INT/INT output to
pull either high or low, depending on the output polarity setting.
Theoretically, the temperature measuring circuit can measure
temperatures from –128°C to +127°C with a resolution of
0.25°C. However, temperatures outside TA are outside the guar-
anteed operating temperature range of the device. Temperature
measurement from –128°C to +127°C is possible using an
external sensor.
Temperature measurement is initiated by three methods. The
first method is applicable when the part is in single-channel
measurement mode. The temperature is measured 16 times and
internally averaged to reduce noise. In single-channel mode, the
part is continuously monitoring the selected channel, i.e., as
soon as one measurement is taken, another one is started on the
same channel. The total time to measure a temperature channel
with the ADC operating at slow speed is typically 11.4 ms
(712 µs × 16) for the internal temperature sensor and 24.22 ms
(1.51 ms × 16) for the external temperature sensor. The new
temperature value is stored in two 8-bit registers and ready for
reading by the I2C or SPI interface. The user has the option of
disabling the averaging by setting Bit 5 in the Control Config-
uration 2 register (Address 19h). The ADT7411 defaults on
power-up with the averaging enabled.
The second method is applicable when the part is in round
robin measurement mode. The part measures both the internal
and external temperature sensors as it cycles through all
possible measurement channels. The two temperature channels
are measured each time the part runs a round robin sequence.
In round robin mode, the part is continuously measuring all
channels.
Temperature measurement is also initiated after every read or
write to the part when the part is in either single-channel
measurement mode or round robin measurement mode. Once
serial communication has started, any conversion in progress is
stopped and the ADC is reset. Conversion will start again
immediately after the serial communication has finished. The
temperature measurement proceeds normally as described
previously.
02882-A
INT/INT
(LATCHED OUTPUT)
TEMP
Figure 27. ADT7411 Interrupt Structure
VDD Monitoring The ADT7411 also has the capability of monitoring its own
power supply. The part measures the voltage on its VDD pin to a
resolution of 10 bits. The resulting value is stored in two 8-bit
registers, with the 2 LSBs stored in register Address 03h and the
8 MSBs stored in register Address 06h. This allows the user to
have the option of just doing a 1-byte read if 10-bit resolution is
not important. The measured result is compared with the VHIGH
and VLOW limits. If the VDD interrupt is not masked out then any
out-of-limit comparison generates a flag in the Interrupt Status
2 register, and one or more out-of-limit results will cause the
INT/INT output to pull either high or low, depending on the
output polarity setting.
Measuring the voltage on the VDD pin is regarded as monitoring
a channel along with the internal, external, and AIN channels.
The user can select the VDD channel for single-channel measure-
ment by setting Bit C4 = 1 and setting Bits C0 to C2 to all 0s in
the Control Configuration 2 register.
When measuring the VDD value, the reference for the ADC is
sourced from the internal reference. Table 5 shows the data
format. As the max VDD voltage measurable is 7 V, internal
scaling is performed on the VDD voltage to match the 2.25 V
internal reference value. The following is an example of how the
transfer function works: ferenceeRADC25.2= ferenceeRADCLSB197.2102425.22110=== .325.27e==ferenceRADCVFullscaleFactorScaleCC
Conversion Result = VDD/(Scale Factor × LSB Size) mV197.211.5×=
DBh2=
Table 5. VDD Data Format, VREF = 2.25 V
On-Chip Reference The ADT7411 has an on-chip 1.125 V band gap reference that is
gained up by a switched capacitor amplifier to give an output of
2.25 V. The amplifier is powered up for the duration of the
device monitoring phase and is powered down once monitoring
is disabled. This saves on current consumption. The internal
reference is used as the reference for the ADC.
Round Robin Measurement Upon power-up, the ADT7411 goes into round robin mode, but
monitoring is disabled. Setting Bit C0 of the Configuration 1
register to 1 enables conversions. It sequences through all
available channels, taking a measurement from each in the
following order: VDD, internal temperature sensor, external
temperature sensor/(AIN1 and AIN2), AIN3, AIN4, AIN5,
AIN6, AIN7, and AIN8. Pin 7 and Pin 8 can be configured as
either external temperature sensor pins or standalone analog
input pins. Once conversion is completed on the AIN8 channel,
the device loops around for another measurement cycle. This
method of taking a measurement on all the channels in one
cycle is called round robin. Setting Bit 4 of the Control
Configuration 2 register (Address 19h) disables the round robin
mode and in turn sets up the single-channel mode. The single-
channel mode is where only one channel, e.g., the internal
temperature sensor, is measured in each conversion cycle.
The time taken to monitor all channels will normally not be of
interest, as the most recently measured value can be read at any
time. For applications where the round robin time is important,
typical times at 25°C are given in the specification pages.
Single-Channel Measurement Setting Bit C4 of the Control Configuration 2 register enables
the single-channel mode and allows the ADT7411 to focus on
one channel only. A channel is selected by writing to Bits C0:C3
in the Control Configuration 2 register. For example, to select
the VDD channel for monitoring, write to the Control
Configuration 2 register and set C4 to 1 (if not done so already),
then write all 0s to Bits C0 to C3. All subsequent conversions
will be done on the VDD channel only. To change the channel
selection to the internal temperature channel, write to the
Control Configuration 2 register and set C0 = 1. When
measuring in single-channel mode, conversions on the channel
selected occur directly after each other. Any communication to
the ADT7411 stops the conversions, but they are restarted once
the read or write operation is completed.
Temperature Measurement Method
Internal Temperature Measurement The ADT7411 contains an on-chip, band gap temperature
sensor whose output is digitized by the on-chip ADC. The
temperature data is stored in the internal temperature value
register. As both positive and negative temperatures can be
measurement sensor could change and therefore an offset is
added to the measured value to enable the transfer function to
match the thermal characteristics. This offset is added before
the temperature data is stored. The offset value used is stored in
the internal temperature offset register.
External Temperature Measurement The ADT7411 can measure the temperature of one external
diode sensor or diode-connected transistor.
The forward voltage of a diode or diode-connected transistor,
operated at a constant current, exhibits a negative temperature
coefficient of about −2 mV/°C. Unfortunately, the absolute
value of VBE varies from device to device, and individual
calibration is required to null this out, so the technique is
unsuitable for mass production.
The technique used in the ADT7411 is to measure the change in
VBE when the device is operated at two different currents.
This is given by: NInKTVBE×=∆
where:
K is Boltzmann’s constant
q is the charge on the carrier
T is the absolute temperature in Kelvin
N is the ratio of the two currents
Figure 23 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, provided for temp-
erature monitoring on some microprocessors, but it could
equally well be a discrete transistor.
If a discrete transistor is used, the collector will not be
grounded, and should be linked to the base. If a PNP transistor
is used, the base is connected to the D− input and the emitter to
the D+ input. If an NPN transistor is used, the emitter is con-
nected to the D− input and the base to the D+ input. A 2N3906
is recommended as the external transistor.
To prevent ground noise from interfering with the measure-
ment, 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. As the sensor is operating in a noisy environment, C1
is provided as a noise filter. See the Layout Considerations
section for more information on C1.
To measure ∆VBE, the sensor is switched between operating
currents of I, and N × I. The resulting waveform is passed
through a low-pass filter to remove noise, then to a chopper-
stabilized amplifier that performs the functions of amplification
and rectification of the waveform to produce a dc voltage
further reduce the effects of noise, digital filtering is performed
by averaging the results of 16 measurement cycles.
Layout Considerations Digital boards can be electrically noisy environments, and care
must be taken to protect the analog inputs from noise, particu-
larly when measuring the very small voltages from a remote
diode sensor. The following precautions should be taken:
1. Place the ADT7411 as close as possible to the remote
sensing diode. Provided that the worst noise sources such
as 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. Provide a ground
plane under the tracks if possible.
3. Use wide tracks to minimize inductance and reduce noise
pickup. A 10 mil track minimum width and spacing is
recommended (see Figure 28).
02882-A
GND
GND
Figure 28. Arrangement of Signal Tracks
4. Try to minimize the number of copper/solder joints, which
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 240 µV, and thermocouple
voltages are about 3 µV/°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.
5. Place 0.1 µF bypass and 2200 pF input filter capacitors
close to the ADT7411.
6. 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.
7. For long distances (up to 100 feet) use shielded twisted-
pair cable, such as Belden #8451 microphone cable.
Connect the twisted pair to D+ and D− and the shield to
GND close to the ADT7411. Leave the remote end of the
