AD ADT7485AARMZ-REEL Sst digital temperature sensor and voltage monitor Datasheet

SST Digital Temperature Sensor
and Voltage Monitor
ADT7485A
FEATURES
GENERAL DESCRIPTION
1 on-chip temperature sensor
1 remote temperature sensor
Monitors up to 5 voltages
Simple Serial Transport™ (SST™) interface
The ADT7485A is a digital temperature sensor and voltage
monitor for use in PC applications with a Simple Serial Transport
(SST) interface. It can monitor its own temperature as well as
the temperature of a remote sensor diode. It can also monitor
four external voltage channels and its own supply voltage. The
ADT7485A is controlled by a single SST bidirectional data line.
This device is a fixed-address SST client where the target address
is chosen by the state of the address pin, ADD.
APPLICATIONS
Personal computers
Portable personal devices
Industrial sensor nets
FUNCTIONAL BLOCK DIAGRAM
ON-CHIP
TEMPERATURE
SENSOR
ADT7485A
OFFSET REGISTERS
5V
VCCP
2.5V
D1+
D1–
INPUT
ATTENUATORS
AND
ANALOG
MULTIPLEXER
A/D
CONVERTER
VOLTAGE
VALUE REGISTERS
GND
SST INTERFACE
ADDRESS
SELECTION
SST
ADD
05197-001
12V
DIGITAL MUX
TEMPERATURE
VALUE REGISTERS
VCC
Figure 1.
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2006 Analog Devices, Inc. All rights reserved.
ADT7485A
TABLE OF CONTENTS
Features .............................................................................................. 1
SST Interface ..................................................................................9
Applications....................................................................................... 1
Voltage Measurement .................................................................... 12
General Description ......................................................................... 1
Analog-to-Digital Converter .................................................... 12
Functional Block Diagram .............................................................. 1
Temperature Measurement ........................................................... 13
Revision History ............................................................................... 2
Temperature Measurement Method ........................................ 13
Specifications..................................................................................... 3
Reading Temperature Measurements...................................... 13
Absolute Maximum Ratings............................................................ 5
SST Temperature Sensor Data Format .................................... 13
Thermal Resistance ...................................................................... 5
Using Discrete Transistors ........................................................ 14
ESD Caution.................................................................................. 5
Layout Considerations............................................................... 14
Pin Configuration and Functional Descriptions.......................... 6
Temperature Offset .................................................................... 14
Typical Performance Characteristics ............................................. 7
Outline Dimensions ....................................................................... 15
Product Description......................................................................... 9
Ordering Guide .......................................................................... 15
REVISION HISTORY
7/06—Revision 0: Initial Version
Rev. 0 | Page 2 of 16
ADT7485A
SPECIFICATIONS
TA = TMIN to TMAX, VCC = VMIN to VMAX, unless otherwise noted.
Table 1.
Parameter
POWER SUPPLY
Supply Voltage, VCC
Undervoltage Lockout Threshold
Average Operating Supply Current, IDD
TEMPERATURE-TO-DIGITAL CONVERTER
Local Sensor Accuracy
Min
Typ
Max
Unit
Test Conditions/Comments
3.0
3.3
2.8
3.8
3.6
5
V
V
mA
Continuous conversions
±1.75
±4
±1
±1.75
°C
°C
°C
°C
±4
°C
μA
μA
μA
°C
kΩ
+1
Remote Sensor Accuracy
+1
Remote Sensor Source Current
12
80
204
0.016
1.5
Resolution
Series Resistance Cancellation
DIGITAL INPUT (ADD)
Input High Voltage, VIH
Input Low Voltage, VIL
Input High Current, IIH
Input Low Current, IIL
Pin Capacitance
ANALOG-TO-DIGITAL CONVERTER
(Including Multiplexer and Attenuators)
Total Unadjusted Error (TUE)
2.3
0.8
−1
1
5
±2
±1.5
±1
Differential Nonlinearity (DNL)
Power Supply Sensitivity
Conversion Time (Voltage Input) 1
Conversion Time (Local Temperature)1
Conversion Time (Remote Temperature)1
Total Monitoring Cycle Time1
Input Resistances
VCCP and 2.5V Channels
5V Channel
12V Channel
DIGITAL I/O (SST Pin)
Input High Voltage , VIH
Input Low Voltage, VIL
Hysteresis1
Output High Voltage, VOH
High Impedance State Leakage, ILEAK
±0.1
11
12
38
145
80
230
180
140
350
280
1.1
±10
μA
300
mV p-p
Rev. 0 | Page 3 of 16
The ADT7485A cancels 1.5 kΩ in series
with the remote thermal diode
VIN = VCC
VIN = 0
12V and 5V channels
For all other channels
10 bits
Averaging enabled
Averaging enabled
Averaging enabled
Averaging enabled
kΩ
kΩ
kΩ
1.9
±1
150
1.1
%
%
LSB
%/V
ms
ms
ms
ms
V
V
mV
V
μA
0.4
High Impedance State Leakage, ILEAK
Signal Noise Immunity, VNOISE
110
290
230
V
V
μA
μA
pF
40°C ≤ TA ≤ 70°C; VCC = 3.3 V ±5%
−40°C ≤ TA ≤ +100°C
−40°C ≤ TD ≤ +125°C; TA = 25°C; VCC = 3.3 V
−40°C ≤ TD ≤ +125°C; −40 ≤ TA ≤ 70°C;
VCC = 3.3 V ±5%
−40°C ≤ TD ≤ +125°C; −40 ≤ TA ≤ +100°C
Low level
Mid level
High level
Between input switching levels
ISOURCE = 6 mA (maximum)
Device powered on SST bus;
VSST = 1.1 V, VCC = 3.3 V
Device unpowered on SST bus;
VSST = 1.1 V, VCC = 0 V
Noise glitches from 10 MHz to 100 MHz;
width up to 50 ns
ADT7485A
Parameter
SST TIMING
Bitwise Period, tBIT
High Level Time for Logic 1, tH1 2
High Level Time for Logic 0, tH02
Time to Assert SST High for Logic 1, tSU, HIGH
Hold Time, tHOLD 3
Stop Time, tSTOP
Time to Respond After a Reset, tRESET
Response Time to Speed Negotiation
After Power-Up
Min
Typ
0.495
0.6 × tBIT
0.2 × tBIT
0.75 × tBIT
0.25 × tBIT
1.25 × tBIT
2 × tBIT
Max
Unit
500
0.8 × tBIT
0.4 × tBIT
0.2 × tBIT
0.5 × tBIT-M
2 × tBIT
μs
μs
μs
μs
μs
μs
0.4
ms
μs
500
1
Guaranteed by design, not production tested.
Minimum and maximum bit times are relative to tBIT defined in the timing negotiation pulse.
3
Device is compatible with hold time specification as driven by SST originator.
2
Rev. 0 | Page 4 of 16
Test Conditions/Comments
tBIT defined in speed negotiation
See SST Specification Rev 1.0
Device responding to a constant low
level driven by originator
Time after power-up when device can
participate in speed negotiation
ADT7485A
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter
Supply Voltage (VCC)
Voltage on 12V Pin
Voltage on 5V Pin
Voltage on 2.5V and VCCP Pins
Voltage on Any Other Pin (Including SST Pin)
Input Current at Any Pin
Package Input Current
Maximum Junction Temperature (TJ max)
Storage Temperature Range
Lead Temperature, Soldering
IR Peak Reflow Temperature
Lead Temperature (10 sec)
ESD Rating
Rating
4V
16 V
7V
3.6 V
−0.3 V to +3.6 V
±5 mA
±20 mA
150°C
−65°C to +150°C
260°C
300°C
1500 V
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.
THERMAL RESISTANCE
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.
Table 3. Thermal Resistance
Package Type
10-Lead MSOP
θJA
206
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.
Rev. 0 | Page 5 of 16
θJC
44
Unit
°C/W
ADT7485A
VCC 1
10
SST
GND 2
9
ADD
8
2.5V
7
VCCP
6
5V
D1+ 3
D1– 4
12V 5
ADT7485A
TOP VIEW
(Not to Scale)
05197-002
PIN CONFIGURATION AND FUNCTIONAL DESCRIPTIONS
Figure 2. 10-Lead MSOP
Table 4. Pin Function Descriptions
Pin No.
1
2
3
4
5
6
7
8
9
10
Mnemonic
VCC
GND
D1+
D1−
12V
5V
VCCP
2.5V
ADD
SST
Type
Power supply
Ground
Analog input
Analog input
Analog input
Analog input
Analog input
Analog input
Digital input
Digital input/output
Description
3.3 V ± 10%. VCC is also monitored through this pin.
Ground Pin.
Positive Connection to Remote 1 Temperature Sensor.
Negative Connection to Remote 1 Temperature Sensor.
12 V Supply Monitor.
5 V Supply Monitor.
Processor Core Voltage Monitor.
2.5 V Supply Monitor.
SST Address Select.
SST Bidirectional Data Line.
Rev. 0 | Page 6 of 16
ADT7485A
TYPICAL PERFORMANCE CHARACTERISTICS
1.55
1.55
1.50
750Ω (~2mA)
1.50
750Ω (~2mA)
1.45
SST O/P (V)
1.40
270Ω (~5.2mA)
1.35
1.30
120Ω (~10.6mA)
2.8
3.0
3.2
3.4
120Ω (~10.6mA)
1.25
05197-007
1.25
1.20
2.6
1.35
1.20
–50
3.6
0
50
Figure 3. SST O/P Level vs. Supply Voltage
Figure 6. SST O/P Level vs. Temperature
3.9
3.56
3.55
DEV3
3.54
3.7
3.53
DEV2
DEV2
IDD (mA)
3.51
3.50
3.49
3.48
DEV1
05197-008
3.46
–25
–5
15
35
55
75
95
3.5
DEV1
3.3
3.1
3.47
3.45
–45
DEV3
05197-011
IDD (mA)
3.52
2.9
2.65
115
2.85
3.05
TEMPERATURE (°C)
6
6
TEMPERATURE ERROR (°C)
7
5
4
HI SPEC (VCC = 3V)
MEAN (VCC = 3.3V)
1
0
3.65
5
4
3
2
HI SPEC (VCC = 3V)
1
0
MEAN (VCC = 3.3V)
–1
LO SPEC (VCC = 3.6V)
LO SPEC (VCC = 3.6V)
–40
–20
0
20
40
60
80
TEMPERATURE (°C)
100
120
140
–2
–60
05197-019
TEMPERATURE ERROR (°C)
3.45
Figure 7. Supply Current vs. Voltage
7
–1
–60
3.25
VCC (V)
Figure 4. Supply Current vs. Temperature
2
150
100
TEMPERATURE (°C)
VCC (V)
3
05197-010
1.30
270Ω (~5.2mA)
1.40
–40
–20
0
20
40
60
80
100
TEMPERATURE (°C)
Figure 5. Local Temperature Error
Figure 8. Remote Temperature Error
Rev. 0 | Page 7 of 16
120
140
05197-020
SST O/P (V)
1.45
ADT7485A
15
0
10
D+ TO GND
5
DEV1_EXT1
DEV1_EXT2
DEV2_EXT1
DEV2_EXT2
DEV3_EXT1
DEV3_EXT2
–10
–20
–30
–5
–10
D+ TO VCC
–15
–20
DEV1_EXT1
DEV1_EXT2
DEV2_EXT1
ERROR (°C)
ERROR (°C)
0
DEV2_EXT2
DEV3_EXT1
DEV3_EXT2
EXT2
–40
EXT1
–50
–60
–25
–70
–30
20
40
60
80
100
RESISTANCE (MΩ)
Figure 9. Remote Temperature Error vs. PCB Resistance
7
25
6
100mV
15
60mV
10
5
40mV
100k
1M
10M
50
4
3
20mV
2
10mV
100k
1M
10M
100M
1G
NOISE FREQUENCY (°C)
Figure 10. Temperature Error vs. Common-Mode Noise Frequency
Figure 13. Temperature Error vs. Differential-Mode Noise Frequency
20
5
4
TEMPERATURE ERROR (°C)
15
10
5
125mV
0
50mV
–5
100k
1M
10M
100M
3
2
1
125mV
0
50mV
–1
–3
10k
1G
POWER SUPPLY NOISE FREQUENCY (Hz)
05197-018
–2
05197-015
TEMPERATURE ERROR (°C)
40
40mV
NOISE FREQUENCY (°C)
–10
10k
30
5
0
10k
1G
100M
20
1
05197-014
0
–5
10k
10
Figure 12. Remote Temperature Error vs. Capacitance Between D1+ and D1−
30
20
0
CAPACITANCE (nF)
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR (°C)
–90
05197-017
0
05197-021
–40
05197-016
–80
–35
100k
1M
10M
100M
1G
POWER SUPPLY NOISE FREQUENCY (Hz)
Figure 11. Local Temperature Error vs. Power Supply Noise
Figure 14. Remote Temperature Error vs. Power Supply Noise
Rev. 0 | Page 8 of 16
ADT7485A
PRODUCT DESCRIPTION
ADT7485A Client Address
The ADT7485A is a temperature- and voltage-monitoring
device. The ADT7485A can monitor the temperature of one
remote sensor diode, plus its own internal temperature. It can
also monitor up to five voltage channels, including its own
supply voltage.
The client address for the ADT7485A is selected using the
address pin. The address pin is connected to a float detection
circuit, which allows the ADT7485A to distinguish between
three input states: high, low (GND), and floating. The address
range for fixed address, discoverable devices is 0x54 to 0x56.
SST INTERFACE
Simple Serial Transport (SST) is a one-wire serial bus and a
communications protocol between components intended for
use in personal computers, personal handheld devices, or other
industrial sensor nets. The ADT7485A supports SST Rev 0.9.
Table 5. ADT7485A Selectable Addresses
ADD
Low (GND)
Float
High
SST is a licensable bus technology from Analog Devices, Inc., and
Intel Corporation. To inquire about obtaining a copy of the Simple
Serial Transport Specification or an SST technology license,
please email Analog Devices at [email protected] or
write to Analog Devices, 3550 North First Street, San Jose, CA
95134, Attention: SST Licensing, M/S B7-24.
Rev. 0 | Page 9 of 16
Address Selected
0x48
0x49
0x4A
ADT7485A
Command Summary
Table 6 summarizes the commands supported by the ADT7485A device when directed at the target address selected by the fixed address
pin. It contains the command name, command code (CC), write data length (WL), read data length (RL), and a brief description.
Table 6. Command Code Summary
Command
Ping()
Command Code, CC
0x00
Write Length, WL
0x00
Read Length, RL
0x00
Description
Shows a nonzero FCS over the header if present.
GetIntTemp()
0x00
0x01
0x02
GetExtTemp()
0x01
0x01
0x02
Shows the temperature of the device’s internal
thermal diode.
Shows the temperature of the external thermal diode.
GetAllTemps()
0x00
0x01
0x04
Shows a 4-byte block of data (GetIntTemp, GetExtTemp).
GetVolt12V()
GetVolt5V()
GetVoltVCC()
0x10
0x11
0x12
0x01
0x01
0x01
0x02
0x02
0x02
Shows the voltage attached to 12V input.
Shows the voltage attached to 5V input.
Shows the voltage attached to VCC input.
GetVolt2.5V()
0x13
0x01
0x01
Shows the voltage attached to 2.5V input.
GetVoltVCCP()
0x14
0x01
0x02
Shows the voltage attached to VCCP input.
GetAllVolts()
0x10
0x01
0x10
Shows all voltage measurement values.
SetExtOffset()
0xe0
0x02
0x00
Sets the offset used to correct errors in the external diode.
GetExtOffset()
0xe0
0x01
0x01
ResetDevice()
0xf6
0x01
0x00
GetDIB()
0xf7
0xf7
0x01
0x01
0x08
0x10
Shows the offset that the device is using to correct
errors in the external diode.
Functional reset. The ADT7485A also responds to this
command when directed to the Target Address 0x00.
Shows information used by SW to identify the device’s
capabilities. Can be in 8- or 16-byte format.
Rev. 0 | Page 10 of 16
ADT7485A
GetIntTemp()
Command Code Details
ADT7485A Device Identifier Block
The GetDIB() command retrieves the device identifier block
(DIB), which provides information to identify the capabilities of
the ADT7485A. The data returned can be in 8- or 16-byte format.
The full 16 bytes of DIB is detailed in Table 7. The 8-byte format
involves the first eight bytes described in this table. Byte-sized
data is returned in the respective fields as it appears in Table 7.
Word-sized data, including vendor ID, device ID, and data values
use little endian format, that is, the LSB is returned first, followed
by the MSB.
Table 7. 16-Byte DIB Details
Byte
0
1
Name
Device Capabilities
Version/Revision
Value
0xc0
0x10
2, 3
Vendor ID
00x11d4
4, 5
Device ID
0x7485
6
7
Device Interface
Function
Interface
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Revision ID
Client Device
Address
0x01
0x00
Description
Fixed address device
Meets Version 1 of the
SST specification
Contains company ID
number in little endian
format
Contains device ID
number in little
endian format
SST device
Reserved
0x00
0x00
0x00
0x00
0x00
0x00
0x05
0x48 to
0x4a
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Contains revision ID
Dependent on the state
of the address pin
8
9
10
11
12
13
14
15
Ping()
The Ping() command verifies if a device is responding at a
particular address. The ADT7485A shows a valid nonzero FCS
in response to the Ping() command when correctly addressed.
The ADT7485A shows the local temperature of the device in
response to the GetIntTemp() command. The data has a little
endian, 16-bit, twos complement format.
GetExtTemp()
Prompted by the GetExtTemp() command, the ADT7485A shows
the temperature of the remote diode in little endian, 16-bit, twos
complement format. The ADT7485A shows 0x8000 in response
to this command if the external diode is an open or short circuit.
GetAllTemps()
The ADT7485A shows the local and remote temperatures in a
4-byte block of data (internal temperature first, followed by external
temperature) in response to a GetAllTemps() command.
SetExtOffset()
This command sets the offset that the ADT7485A will use to
correct errors in the external diode. The offset is set in little
endian, 16-bit, twos complement format. The maximum offset
is ±128°C with +0.25°C resolution.
GetExtOffset()
This command causes the ADT7485A to show the offset that it
is using to correct errors in the external diode. The offset value
is returned in little endian format, that is, LSB before MSB.
ADT7485A Response to Unsupported Commands
A full list of command codes supported by the ADT7485A is
given in Table 6. The offset registers (Command Code 0xe0) are
the only registers that the user can write to. The other defined
registers are read only. Writing to Register Addresses 0x02,
0x09, and 0x15 to 0xdf shows a valid FSC, but no action is taken
by the ADT7485A. The ADT7485A shows an invalid FSC if the
user attempts to write to the device between Command Codes
0xe2 to 0xee. These registers are reserved for the manufacturer’s
use only, and no data can be written to the device via these
addresses.
Table 8. Ping() Command
Target Address
(Not necessary)
Write Length
0x00
Read Length
0x00
FCS
ResetDevice()
This command resets the register map and conversion
controller. The reset command can be global or directed at the
client address of the ADT7485A.
Table 9. ResetDevice() Command
Target Address
Device Address
Write
Length
0x01
Read
Length
0x00
Reset
command
0xf6
FCS
Rev. 0 | Page 11 of 16
ADT7485A
VOLTAGE MEASUREMENT
Voltage Measurement Command Codes
The ADT7485A has four external voltage measurement channels.
It can also measure its own supply voltage, VCC. Pins 5 to 8 can
measure the supplies of the 12V, 5V, processor core voltage (VCCP),
and 2.5V pins, respectively. The VCC supply voltage measurement is
carried out through the VCC pin (Pin 1). The 2.5V pin can be
used to monitor a chipset supply voltage in a computer system.
The voltage measurement command codes are detailed in Table 11.
Each voltage measurement has a read length of two bytes in
little endian format (LSB followed by MSB). All voltages can be
read together by addressing Command Code 0x10 with a read
length of 0x10. The data is retrieved in the order listed in Table 11.
ANALOG-TO-DIGITAL CONVERTER
Table 11. Voltage Measurement Command Codes
All analog inputs are multiplexed into the on-chip, successiveapproximation, analog-to-digital converter (ADC). This has a
resolution of 10 bits. The basic input range is 0 V to 2.25 V, but
the inputs have built-in attenuators to allow measurement of 2.5 V,
3.3 V, 5 V, 12 V, and the processor core voltage (VCCP) without
any external components.
Voltage Channel
12V
Command Code
0x10
Returned Data
LSB, MSB
5V
0x11
LSB, MSB
VCC
0x12
LSB, MSB
To allow for the tolerance of these supply voltages, the ADC
produces a specific output for each nominal input voltage and
therefore has adequate headroom to cope with overvoltages.
The full-scale voltage that can be recorded for each channel is
shown in Table 10.
2.5V
0x13
LSB, MSB
VCCP
0x14
LSB, MSB
Table 10. Maximum Reported Input Voltages
The returned voltage value is in twos complement, 16-bit,
binary format. The format is structured so that voltages in the
range of ±32 V can be reported. In this way, the reported value
represents the number of 1/1024 V in the actual reading,
allowing a resolution of approximately 1 mV.
Voltage Channel
12V
5V
VCC
2.5V
VCCP
Full-Scale Voltage
16 V
8V
4V
4V
4V
Table 12. Analog-to-Digital Output Code vs. VIN
Input Circuitry
The internal structure for the analog inputs is shown in
Figure 15. The input circuit consists of an input protection
diode and an attenuator, plus a capacitor that forms a firstorder, low-pass filter to provide input immunity to high
frequency noise.
5VIN
3.3VIN
2.5VIN
VCCP
120kΩ
20kΩ
30pF
47kΩ
30pF
71kΩ
30pF
94kΩ
30pF
Voltage
12
5
3.3
3
2.5
1
0
93kΩ
68kΩ
MUX
45kΩ
17.5kΩ
52.5kΩ
35pF
05197-003
12V IN
Voltage Data Format
Figure 15. Internal Structure of Analog Inputs
Rev. 0 | Page 12 of 16
Twos Complement
MSB
LSB
0011 0000
0000 0000
0001 0100
0000 0000
0000 1101
0011 0011
0000 1100
0000 0000
0000 1010
0000 0000
0000 0100
0000 0000
0000 0000
0000 0000
ADT7485A
TEMPERATURE MEASUREMENT
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.
Signal conditioning and measurement of the internal
temperature sensor is performed in the same manner.
The ADT7485A monitors one local and one remote
temperature channel. Monitoring of each of the channels is
done in a round-robin sequence. The monitoring sequence is in
the order shown in Table 13.
Table 13. Temperature Monitoring Sequence
Channel
Number
0
Measurement
Local temperature
VDD
I
REMOTE
SENSING
TRANSISTOR
Conversion Time (ms)
12
N1 × I
N2 × I
IBIAS
D1+
VOUT+
C11
TO ADC
D1–
1CAPACITOR
BIAS
DIODE
LOW-PASS FILTER
fC = 65kHz
VOUT–
C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
05197-004
The ADT7485A has two dedicated temperature measurement
channels: one for measuring the temperature of an on-chip band
gap temperature sensor, and one for measuring the temperature
of a remote diode, usually located in the CPU or GPU.
Figure 16. Signal Conditioning for Remote Diode Temperature Sensors
1
Remote temperature
38
READING TEMPERATURE MEASUREMENTS
TEMPERATURE MEASUREMENT METHOD
The temperature data returned is two bytes in little endian
format, that is, LSB before MSB. All temperatures can be read
together by using Command Code 0x00 with a read length of
0x04. The command codes and returned data are described in
Table 14.
A simple method for measuring temperature is to exploit the
negative temperature coefficient of a diode by measuring the
base-emitter voltage (VBE) of a transistor operated at constant
current. Unfortunately, this technique requires calibration to
null the effect of the absolute value of VBE, which varies from
device to device.
Table 14. Temperature Channel Command Codes
Temp Channel
Internal
Command Code
0x00
Returned data
LSB, MSB
The technique used in the ADT7485A measures the change in
VBE when the device is operated at three different currents.
External
All Temps
0x01
0x00
LSB, MSB
Internal LSB, Internal MSB;
External LSB, External MSB
Figure 16 shows the input signal conditioning used to measure
the output of a remote temperature sensor. This figure shows
the remote sensor as a substrate transistor, which is provided for
temperature monitoring on some microprocessors, but it could
also be a discrete transistor. If a discrete transistor is used, the
collector is not grounded and should be linked to the base. To
prevent ground noise from 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
D1− input. If the sensor is operating in an extremely noisy
environment, C1 can be added as a noise filter. Its value should
not exceed 1000 pF.
To measure ΔVBE, the operating current through the sensor is
switched between three related currents. Figure 16 shows N1 × I
and N2 × I as different multiples of the current I. The currents
through the temperature diode are switched between I and
N1 × I, giving ΔVBE1, and then between I and N2 × I, giving
ΔVBE2. The temperature can then be calculated using the two
ΔVBE measurements. This method can also cancel the effect of
series resistance on the temperature measurement. The
resulting ΔVBE waveforms are passed through a 65 kHz low-pass
filter to remove noise and then through a chopper-stabilized
amplifier to amplify and rectify the waveform, producing a dc
voltage proportional to ΔVBE. The ADC digitizes this voltage,
SST TEMPERATURE SENSOR DATA FORMAT
The data for temperature is structured to allow values in the
range of ±512°C to be reported. Thus, the temperature sensor
format uses a twos complement, 16-bit binary value to represent
values in this range. This format allows temperatures to be
represented with approximately a 0.016°C resolution.
Table 15. SST Temperature Data Format
Temperature (°C)
−125
−80
−40
−20
−5
−1
0
+1
+5
+20
+40
+80
+125
Rev. 0 | Page 13 of 16
Twos Complement
MSB
LSB
1110 0000
1100 0000
1110 1100
0000 0000
1111 0110
0000 0000
1111 1011
0011 1110
1111 1110
1100 0000
1111 1111
1100 0000
0000 0000
0000 0000
0000 0000
0100 0000
0000 0001
0100 0000
0000 0100
1100 0010
0000 1010
0000 0000
0001 0100
0000 0000
0001 1111
0100 0000
ADT7485A
•
USING DISCRETE TRANSISTORS
If a discrete transistor is used, the collector is not grounded and
should be linked to the base. If a PNP transistor is used, the
base is connected to the D1− input and the emitter is connected
to the D1+ input. If an NPN transistor is used, the emitter is
connected to the D1− input and the base is connected to the
D1+ input. Figure 17 shows how to connect the ADT7485A to
an NPN or PNP transistor for temperature measurement. To
prevent ground noise from 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
D1− input.
D1+
D1+
D1–
2N3906
PNP
•
•
ADT7485A
05197-005
ADT7485A
2N3904
NPN
•
D1–
•
Figure 17. Connections for NPN and PNP Transistors
The ADT7485A shows an external temperature value of 0x8000
if the external diode is an open or short circuit.
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments. Take the
following precautions to protect the analog inputs from noise,
particularly when measuring the very small voltages from a
remote diode sensor:
•
•
Place the ADT7485A 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 four to eight inches.
Route the D1+ and D1− tracks close together in parallel
with grounded guard tracks on each side. Provide a ground
plane under the tracks if possible.
Use wide tracks to minimize inductance and reduce noise
pickup. A 5 mil track minimum width and spacing is
recommended.
5mil
GND
5mil
D1+
5mil
5mil
5mil
D1–
5mil
5mil
GND
05197-006
•
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 D1+
and D1− paths and are at the same temperature.
Thermocouple effects should not be a major problem because
1°C corresponds to about 240 μV, and thermocouple voltages
are about 3 μV/°C of the temperature difference. Unless there
are two thermocouples with a big temperature differential
between them, thermocouple voltages should be much less
than 200 mV.
Place a 0.1 μF bypass capacitor close to the ADT7485A.
If the distance to the remote sensor is more than eight
inches, the use of a twisted-pair cable is recommended.
This works for distances of about 6 to 12 feet.
For very long distances (up to 100 feet), use shielded
twisted-pair cables, such as Belden #8451 microphone
cables. Connect the twisted-pair cable to D1+ and D1− and
the shield to GND, close to the ADT7485A. Leave the remote
end of the shield unconnected to avoid ground loops.
Because the measurement technique uses switched current
sources, excessive cable and/or filter capacitance can affect the
measurement. When using long cables, the filter capacitor can
be reduced or removed. Cable resistance can also introduce
errors. A 1 Ω series resistance introduces about 0.5°C error.
TEMPERATURE OFFSET
As CPUs run faster, it is more difficult to avoid high frequency
clocks when routing the D1+ and D1− tracks around a system
board. Even when the recommended layout guidelines are
followed, there may still be temperature errors, attributed to noise
being coupled on to the D1+ and D1− lines. High frequency
noise generally has the effect of producing temperature measurements that are consistently too high by a specific amount.
The ADT7485A has a temperature offset command code of 0xe0
through which a desired offset can be set. By doing a one-time
calibration of the system, the offset caused by system board noise
can be calculated and nulled by specifying it in the ADT7485A.
The offset is automatically added to every temperature measurement. The maximum offset is ±128°C with 0.25°C resolution. The
offset format is the same as the temperature data format—16-bit,
twos complement notation, as shown in Table 15. The offset
should be programmed in little endian format, that is, LSB
before MSB. The offset value is also returned in little endian
format when read.
Figure 18. Arrangements of Signal Tracks
Rev. 0 | Page 14 of 16
ADT7485A
OUTLINE DIMENSIONS
3.10
3.00
2.90
10
3.10
3.00
2.90
1
6
5
5.15
4.90
4.65
PIN 1
0.50 BSC
0.95
0.85
0.75
0.15
0.05
1.10 MAX
0.33
0.17
SEATING
PLANE
0.23
0.08
8°
0°
0.80
0.60
0.40
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-187-BA
Figure 19. 10-Lead Mini Small Outline Package [MSOP]
(RM-10)
Dimensions shown in millimeters
ORDERING GUIDE
Model
ADT7485AARMZ-REEL 1
ADT7485AARMZ-REEL71
1
Temperature Range
–40°C to +125°C
–40°C to +125°C
Package Description
10-Lead MSOP
10-Lead MSOP
Z = Pb-free part.
Rev. 0 | Page 15 of 16
Package Option
RM-10
RM-10
Branding
T21
T21
ADT7485A
NOTES
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D05197-0-7/06(0)
Rev. 0 | Page 16 of 16
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