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Digital Temperature Sensor with SST Interface ADT7484A/ADT7486A
FEATURES
1 on-chip temperature sensor 1 or 2 remote temperature sensors Simple Serial TransportTM (SSTTM) interface Rev 1 compliant
GENERAL DESCRIPTION
The ADT7484A/ADT7486A are simple digital temperature sensors for use in PC applications with a Simple Serial Transport (SST) interface. These devices can monitor their own temperature as well as the temperature of one (ADT7484A) or two (ADT7486A) remote sensor diodes. The ADT7484A/ADT7486A are controlled by a single SST bidirectional data line. The devices are fixedaddress SST clients where the target address is chosen by the state of the two address pins, ADD0 and ADD1.
APPLICATIONS
Personal computers Portable personal devices Industrial sensor nets
FUNCTIONAL BLOCK DIAGRAM
ON-CHIP TEMPERATURE SENSOR LOCAL TEMPERATURE VALUE REGISTER
DIGITAL MUX
SST INTERFACE
SST
D1+ D1- (ADT7486A ONLY) D2+ D2- ANALOG MUX
A/D CONVERTER
REMOTE TEMPERATURE VALUE REGISTER ADDRESS SELECTION ADD1 ADD0
05198-001
ADT7484A/ ADT7486A
VDD GND
OFFSET REGISTERS
RESERVED
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 (c)2006 Analog Devices, Inc. All rights reserved.
ADT7484A/ADT7486A TABLE OF CONTENTS
Features .............................................................................................. 1 Applications....................................................................................... 1 General Description ......................................................................... 1 Functional Block Diagram .............................................................. 1 Revision History ............................................................................... 2 Specifications..................................................................................... 3 Absolute Maximum Ratings............................................................ 5 Thermal Resistance ...................................................................... 5 ESD Caution.................................................................................. 5 Pin Configurations and Functional Descriptions ........................ 6 Typical Performance Characteristics ............................................. 7 Product Description......................................................................... 9 SST Interface ..................................................................................9 Temperature Measurement ........................................................... 12 Temperature Measurement Method ........................................ 12 Reading Temperature Measurements...................................... 12 SST Temperature Sensor Data Format .................................... 13 Using Discrete Transistors ........................................................ 13 Layout Considerations............................................................... 13 Temperature Offset .................................................................... 14 Application Schematics ............................................................. 14 Outline Dimensions ....................................................................... 15 Ordering Guide .......................................................................... 15
REVISION HISTORY
7/06--Revision 0: Initial Version
Rev. 0 | Page 2 of 16
ADT7484A/ADT7486A 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 Remote Sensor Accuracy +1 Min 3.0 Typ 3.3 2.8 3.8 +1 Max 3.6 5 1.75 4 1 1.75 4 Remote Sensor Source Current 12 80 204 0.016 1.5 12 38 50 2.3 0.8 -1 1 5 1.1 0.4 150 1.1 1.9 1 10 300 Unit V V mA C C C C C A A A C k ms ms ms V V A A pF V V mV V A A mV p-p Test Conditions/Comments
Continuous conversions 40C TA 70C, VCC = 3.3 V 5% -40C TA +100C -40C TD +125C; TA = 25C; VCC = 3.3 V -40C TD +125C; -40 TA 70C, VCC = 3.3 V 5% -40C TD +125C; -40 TA +100C Low level Mid level High level The ADT7484A and ADT7486A cancel 1.5 k in series with the remote thermal diode Averaging enabled Averaging enabled Averaging enabled
Resolution Series Resistance Cancellation Conversion Time (Local Temperature) 1 Conversion Time (Remote Temperature)1 Total Monitoring Cycle Time1 DIGITAL INPUTS (ADD0, ADD1) Input High Voltage, VIH Input Low Voltage, VIL Input High Current, IIH Input Low Current, IIL Pin Capacitance DIGITAL I/O (SST Pin) Input High Voltage, VIH Input Low Voltage, VIL Hysteresis1 Output High Voltage, VOH High Impedance State Leakage, ILEAK High Impedance State Leakage, ILEAK Signal Noise Immunity, VNOISE 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
VIN = VCC VIN = 0
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
0.495 0.6 x tBIT 0.2 x tBIT
0.75 x tBIT 0.25 x tBIT
1.25 x tBIT
2 x tBIT
500 0.8 x tBIT 0.4 x tBIT 0.2 x tBIT 0.5 x tBIT-M 2 x tBIT
s s s s s s
tBIT defined in speed negotiation
See SST Specification Rev 1.0 Device responding to a constant low level driven by originator
Rev. 0 | Page 3 of 16
ADT7484A/ADT7486A
Parameter Time to Respond After a Reset, tRESET Response Time to Speed Negotiation After Power-Up
1 2
Min
Typ 500
Max 0.4
Unit ms s
Test Conditions/Comments Time after power-up when device can participate in speed negotiation
Guaranteed by design, not production tested. Minimum and maximum bit times are relative to tBIT defined in the timing negotiation pulse. 3 Devices compatible with hold time specification as driven by SST originator.
Rev. 0 | Page 4 of 16
ADT7484A/ADT7486A ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter Supply Voltage (VCC) 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 3.6 V 3.6 V 5 mA 20 mA 150C -65C to +150C 260C 300C 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
JA 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 8-Lead MSOP (ADT7484A) 10-Lead MSOP (ADT7486A) JA 206 206 JC 44 44 Unit C/W C/W
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
ADT7484A/ADT7486A PIN CONFIGURATIONS AND FUNCTIONAL DESCRIPTIONS
VCC 1 GND 2 D1+ 3 D1- 4
8
ADT7484A
TOP VIEW (Not to Scale)
SST ADD0
05198-002
VCC 1 GND 2 D1+ 3 D1- 4 D2+ 5
10
SST ADD0 RESERVED D2-
05198-003
7 6 5
ADT7486A
TOP VIEW (Not to Scale)
9 8 7 6
RESERVED ADD1
ADD1
Figure 2. ADT7484A 8-Lead MSOP
Figure 3. ADT7486A 10-Lead MSOP
Table 4. ADT7484A Pin Function Descriptions
Pin No. 1 2 3 4 5 6 7 8 Mnemonic VCC GND D1+ D1- ADD1 RESERVED ADD0 SST Type Power supply Ground Analog input Analog input Digital input Reserved Digital input Digital input/output Description 3.3 V 10%. Ground Pin. Positive Connection to Remote Temperature Sensor. Negative Connection to Remote Temperature Sensor. SST Address Select. Connect to Ground. SST Address Select. SST Bidirectional Data Line.
Table 5. ADT7486A Pin Function Descriptions
Pin No. 1 2 3 4 5 6 7 8 9 10 Mnemonic VCC GND D1+ D1- D2+ D2- ADD1 RESERVED ADD0 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%. Ground Pin. Positive Connection to Remote 1 Temperature Sensor. Negative Connection to Remote 1 Temperature Sensor. Positive Connection to Remote 2 Temperature Sensor. Negative Connection to Remote 2 Temperature Sensor. SST Address Select. Connect to Ground. SST Address Select. SST Bidirectional Data Line.
Rev. 0 | Page 6 of 16
ADT7484A/ADT7486A TYPICAL PERFORMANCE CHARACTERISTICS
1.55 1.50 750 (~2mA) 1.45
1.45 1.55 1.50 750 (~2mA)
SST O/P (V)
SST O/P (V)
270 (~5.2mA) 1.40 1.35 1.30 1.25 1.20 -50 120 (~10.6mA)
1.40 270 (~5.2mA) 1.35 1.30 1.25 1.20 2.6
120 (~10.6mA)
05198-009
2.8
3.0
3.2 VCC (V)
3.4
3.6
0
50 TEMPERATURE (C)
100
150
Figure 4. SST O/P Level vs. Supply Voltage
Figure 7. SST O/P Level vs. Temperature
3.56 3.55 3.54 3.53 3.52 3.51 3.50 3.49 3.48 3.47 3.46 DEV1 DEV2 DEV3
3.9
3.7 DEV2 3.5 DEV3 DEV1 3.3
IDD (mA)
IDD (mA)
3.1
TEMPERATURE (C)
VCC (V)
Figure 5. Supply Current vs. Temperature
Figure 8. Supply Current vs. Voltage
7 6 TEMPERATURE ERROR (C) 5 4 3 2 1 0 -1 -60 LO SPEC (VCC = 3.6V)
05198-018
7 6
TEMPERATURE ERROR (C)
5 4 3 2 1 0 -1 LO SPEC (VCC = 3.6V)
05198-019
HI SPEC (VCC = 3V)
HI SPEC (VCC = 3V) MEAN (VCC = 3.3V)
MEAN (VCC = 3.3V)
-40
-20
0
20
40
60
80
100
120
140
-2 -60
-40
-20
0
20
40
60
80
100
120
140
TEMPERATURE (C)
TEMPERATURE (C)
Figure 6. Local Temperature Error
Figure 9. Remote Temperature Error
Rev. 0 | Page 7 of 16
05198-012
-25
-5
15
35
55
75
95
115
05198-010
3.45 -45
2.9 2.65
2.85
3.05
3.25
3.45
3.65
05198-011
ADT7484A/ADT7486A
15 10 5 0
ERROR (C) ERROR (C)
0 D+ TO GND DEV1_EXT1 DEV1_EXT2 DEV2_EXT1 DEV2_EXT2 DEV3_EXT1 DEV3_EXT2 -10 -20 -30 EXT2 -40 -50 -60 -70 -80
05198-020 05198-015 05198-017 05198-016
-5 -10 -15 -20 -25 -30 -35 -40 0 20 40 60 80 100 D+ TO VCC DEV1_EXT1 DEV1_EXT2 DEV2_EXT1 DEV2_EXT2 DEV3_EXT1 DEV3_EXT2
EXT1
-90
0
10
20
30
40
50
RESISTANCE (M)
CAPACITANCE (nF)
Figure 10. Remote Temperature Error vs. PCB Resistance
Figure 13. Remote Temperature Error vs. Capacitance Between D1+ and D1-
30 25 TEMPERATURE ERROR (C) 20 15 60mV 10 5 0 -5 10k TEMPERATURE ERROR (C) 100mV
7 6 5 4 3 2 1
40mV
20mV
40mV
10mV
100k 1M 10M 100M
05198-013
1G
0 10k
100k
1M
10M
100M
1G
NOISE FREQUENCY (C)
NOISE FREQUENCY (C)
Figure 11. Temperature Error vs. Common-Mode Noise Frequency
Figure 14. Temperature Error vs. Differential-Mode Noise Frequency
20
5 4 TEMPERATURE ERROR (C) 3 2 1 0 50mV -1 -2 -3 10k
15 TEMPERATURE ERROR (C)
10
5 125mV 0 50mV -5
125mV
05198-014
-10 10k
100k
1M
10M
100M
1G
100k
1M
10M
100M
1G
POWER SUPPLY NOISE FREQUENCY (Hz)
POWER SUPPLY NOISE FREQUENCY (Hz)
Figure 12. Local Temperature Error vs. Power Supply Noise
Figure 15. Remote Temperature Error vs. Power Supply Noise
Rev. 0 | Page 8 of 16
ADT7484A/ADT7486A PRODUCT DESCRIPTION
The ADT7484A is a single remote temperature sensor, and the ADT7486A is a dual temperature sensor for use in PC applications. The ADT7484A/ADT7486A accurately measure local and remote temperature and communicate over a one-wire Simple Serial Transport (SST) bus interface.
ADT7484A/ADT7486A Client Address
The client address for the ADT7484A/ADT7486A is selected using the address pin. The address pin is connected to a float detection circuit, which allows the ADT7484A/ADT7486A to distinguish between three input states: high, low (GND), and floating. The address range for fixed address, discoverable devices is 0x48 to 0x50. Table 6. ADT7484A/ADT7486A Selectable Addresses
ADD1 Low (GND) Low (GND) Low (GND) Float Float Float High High High ADD0 Low (GND) Float High Low (GND) Float High Low (GND) Float High Address Selected 0x48 0x49 0x4A 0x4B 0x4C 0x4D 0x4E 0x4F 0x50
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 ADT7484A/ADT7486A support SST specification Rev 1. 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 sst_licensing@analog.com 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
ADT7484A/ADT7486A
Command Summary
Table 7 summarizes the commands supported by the ADT7484A/ADT7486A devices when directed at the target address selected by the fixed address pins. It contains the command name, command code (CC), write data length (WL), read data length (RL), and a brief description. Table 7. Command Code Summary
Command Ping() GetIntTemp() GetExt1Temp() GetExt2Temp() GetAllTemps() Command Code, CC 0x00 0x00 0x01 0x02 0x00 Write Length, WL 0x00 0x01 0x01 0x01 0x01 Read Length, RL 0x00 0x02 0x02 0x02 0x04 (ADT7484A) 0x06 (ADT7486A) 0x00 0x02 0x00 0x02 0x00 Description Shows a nonzero FCS over the header if present. Shows the temperature of the device's internal thermal diode. Shows the temperature of External Thermal Diode 1. Shows the temperature of External Thermal Diode 2 (ADT7486A only). Shows a 4- or 6-byte block of data (ADT7484A: GetIntTemp, GetExt1Temp; ADT7486A: GetIntTemp, GetExt1Temp, GetExt2Temp). Sets the offset used to correct errors in External Diode 1. Shows the offset that the device is using to correct errors in External Diode 1. Sets the offset used to correct errors in External Diode 2 (ADT7486A only). Shows the offset that the device is using to correct errors in External Diode 2 (ADT7486A only). Functional reset. The ADT7484A/ADT7486A also respond 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.
SetExt1Offset() GetExt1Offset() SetExt2Offset() GetExt2Offset() ResetDevice()
0xe0 0xe0 0xe1 0xe1 0xf6
0x03 0x01 0x03 0x01 0x01
GetDIB()
0xf7 0xf7
0x01 0x01
0x08 0x10
Rev. 0 | Page 10 of 16
ADT7484A/ADT7486A
Command Code Details ADT7484A/ADT7486A Device Identifier Block
The GetDIB() command retrieves the device identifier block (DIB), which provides information to identify the capabilities of the ADT7484A/ADT7486A. The data returned can be in 8- or 16-byte format. The full 16 bytes of DIB is detailed in Table 8. 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 8. 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 8. DIB Byte Details
Byte 0 1 2, 3 Name Device Capabilities Version/Revision Vendor ID Value 0xc0 0x10 00x11d4 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 Reserved Reserved Reserved Reserved Reserved Reserved Contains revision ID Dependent on the state of the address pins
GetIntTemp()
The ADT7484A/ADT7486A show 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 ADT7484A/ ADT7486A show the temperature of the remote diode in little endian, 16-bit, twos complement format. The ADT7484A/ ADT7486A show 0x8000 in response to this command if the external diode is an open or short circuit.
GetAllTemps()
The ADT7484A shows the local and remote temperatures in a 4-byte block of data (internal temperature first, followed by External Temperature 1) in response to a GetAllTemps() command. The ADT7486A shows the local and remote temperatures in a 6-byte block of data (internal temperature first, followed by External Temperature 1 and External Temperature 2) in response to this command.
4, 5
Device ID
0x7484 or 0x7486 0x01 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x05 0x48 to 0x50
SetExtOffset()
This command sets the offset that the ADT7484A/ADT7486A 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 128C with +0.25C resolution.
6 7 8 9 10 11 12 13 14 15
Device Interface Function Interface Reserved Reserved Reserved Reserved Reserved Reserved Revision ID Client Device Address
GetExtOffset()
This command causes the ADT7484A/ADT7486A to show the offset that they are using to correct errors in the external diode. The offset value is returned in little endian format, that is, LSB before MSB.
Ping()
The Ping() command verifies if a device is responding at a particular address. The ADT7484A/ADT7486A show a valid nonzero FCS in response to the Ping() command when correctly addressed. Table 9. Ping() Command
Target Address Device Address Write Length 0x00 Read Length 0x00 FCS
ADT7484A/ADT7486A Response to Unsupported Commands
A full list of command codes supported by the ADT7484A/ ADT7486A is given in Table 7. The offset registers (Command Codes 0xe0 and 0xe1) are the only registers that the user can write to. The other defined registers are read only. Writing to Register Addresses 0x03 to 0xdf shows a valid FSC, but no action is taken by the ADT7484A/ADT7486A. The ADT7484A/ADT7486A show an invalid FSC if the user attempts to write to the devices between Command Codes 0xe2 to 0xee and no data is written to the device. These registers are reserved for the manufacturer's use only, and no data can be written to the device via these addresses.
ResetDevice()
This command resets the register map and conversion controller. The reset command can be global or directed at the client address of the ADT7484A/ADT7486A. Table 10. ResetDevice() Command
Target Address Device Address Write Length 0x01 Read Length 0x00 Reset command 0xf6 FCS
Rev. 0 | Page 11 of 16
ADT7484A/ADT7486A TEMPERATURE MEASUREMENT
The ADT7484A/ADT7486A each have 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. The ADT7484A monitors one local and one remote temperature channel, whereas the ADT7486A monitors one local and two remote temperature channels. Monitoring of each of the channels is done in a round-robin sequence. The monitoring sequence is in the order shown in Table 11. Table 11. Temperature Monitoring Sequence
Channel Number 0 1 2 Measurement Local temperature Remote Temperature 1 Remote Temperature 2 (ADT7486A only) Conversion Time (ms) 12 38 38
To measure VBE, the operating current through the sensor is switched between three related currents. Figure 16 shows N1 x I and N2 x I as different multiples of the current I. The currents through the temperature diode are switched between I and N1 x I, giving VBE1, and then between I and N2 x I, giving VBE2. The temperature can then be calculated using the two VBE measurements. This method can also cancel the effect of series resistance on the temperature measurement. The resulting VBE 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 VBE. The ADC digitizes this voltage, and a temperature measurement is produced. To reduce the effects of noise, digital filtering is performed by averaging the results of 16 measurement cycles for low conversion rates. Signal conditioning and measurement of the internal temperature sensor is performed in the same manner.
VDD I N1 x I N2 x I IBIAS
TEMPERATURE MEASUREMENT METHOD
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. The technique used in the ADT7484A/ADT7486A measures the change in VBE when the device is operated at three different currents. 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.
D+ REMOTE SENSING TRANSISTOR C1* D- BIAS DIODE LOW-PASS FILTER fC = 65kHz VOUT+ TO ADC VOUT-
*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
Figure 16. Signal Conditioning for Remote Diode Temperature Sensors
READING TEMPERATURE MEASUREMENTS
The temperature measurement command codes are detailed in Table 12. 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 12. Table 12. Temperature Channel Command Codes
Temp Channel Internal External 1 External 2 All Temps Command Code 0x00 0x01 0x02 0x00 Returned data LSB, MSB LSB, MSB LSB, MSB Internal LSB, Internal MSB; External 1 LSB, External 1 MSB; External 2 LSB, External 2 MSB
Rev. 0 | Page 12 of 16
05198-004
ADT7484A/ADT7486A
SST TEMPERATURE SENSOR DATA FORMAT
The data for temperature is structured to allow values in the range of 512C 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 0.016C resolution. Table 13. SST Temperature Data Format
Temperature (C) -125 -80 -40 -20 -5 -1 0 +1 +5 +20 +40 +80 +125 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
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 device 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.
GND 5MIL 5MIL D+ 5MIL 5MIL
D-
*
*
5MIL 5MIL
GND
5MIL
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 ADT7484A/ADT7486A 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.
2N3904 NPN D+ D-
Figure 18. Arrangements of Signal Tracks
*
*
ADT7484A/ ADT7486A
2N3906 PNP
D+ D-
ADT7484A/ ADT7486A
05198-005
* *
Figure 17. Connections for NPN and PNP Transistors
*
The ADT7484A/ADT7486A show an external temperature value of 0x8000 if the external diode is an open or short circuit.
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 1C corresponds to about 240 V, and thermocouple voltages are about 3 V/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 F bypass capacitor close to the device. 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 twistedpair cables, such as Belden #8451 microphone cables. Connect the twisted-pair cable to D1+ and D1- and the shield to GND, close to the device. 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 series resistance introduces about 0.5C error.
Rev. 0 | Page 13 of 16
05198-006
ADT7484A/ADT7486A
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 ADT7484A/ADT7486A have 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 ADT7484A/ADT7486A. The offset is automatically added to every temperature measurement. The maximum offset is 128C with 0.25C resolution. The offset format is the same as the temperature data format--16-bit, twos complement notation, as shown in Table 13. 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.
APPLICATION SCHEMATICS
VCC
1 2
ADT7484A
VCC GND SST ADD0
8 7 6 5
05198-007
SST
2N3904 OR CPU THERMAL DIODE
3 4
D1+ RESERVED D1- ADD1
Figure 19. ADT7484A Typical Application Schematic
VCC
1 2
ADT7486A
VCC GND SST 10 ADD0
9 8 7 6
05198-008
SST
2N3904 NPN
3 4 5
D1+ RESERVED D1- D2+ ADD1 D2-
CPU THERMAL DIODE
Figure 20. ADT7486A Typical Application Schematic
Rev. 0 | Page 14 of 16
ADT7484A/ADT7486A OUTLINE DIMENSIONS
3.20 3.00 2.80
3.20 3.00 2.80 PIN 1
8
5
1
5.15 4.90 4.65
4
0.65 BSC 0.95 0.85 0.75 0.15 0.00 0.38 0.22 SEATING PLANE 1.10 MAX 8 0 0.80 0.60 0.40
0.23 0.08
COPLANARITY 0.10
COMPLIANT TO JEDEC STANDARDS MO-187-AA
Figure 21. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters
3.10 3.00 2.90 3.10 3.00 2.90 PIN 1 0.50 BSC 0.95 0.85 0.75 0.15 0.05 0.33 0.17 COPLANARITY 0.10 COMPLIANT TO JEDEC STANDARDS MO-187-BA 1.10 MAX 8 0 0.80 0.60 0.40
10 6
1
5
5.15 4.90 4.65
SEATING PLANE
0.23 0.08
Figure 22. 10-Lead Mini Small Outline Package [MSOP] (RM-10) Dimensions shown in millimeters
ORDERING GUIDE
Model ADT7484AARMZ-REEL 1 ADT7484AARMZ-REEL71 ADT7486AARMZ-REEL1 ADT7486AARMZ-REEL71
1
Temperature Range -40C to +125C -40C to +125C -40C to +125C -40C to +125C
Package Description 8-Lead MSOP 8-Lead MSOP 10-Lead MSOP 10-Lead MSOP
Package Option RM-8 RM-8 RM-10 RM-10
Branding T20 T20 T22 T22
Z = Pb-free part.
Rev. 0 | Page 15 of 16
ADT7484A/ADT7486A NOTES
(c)2006 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D05198-0-7/06(0)
Rev. 0 | Page 16 of 16

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