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SST Digital Temperature Sensor and Voltage Monitor ADT7485A
FEATURES
1 on-chip temperature sensor 1 remote temperature sensor Monitors up to 5 voltages Simple Serial TransportTM (SSTTM) interface
GENERAL DESCRIPTION
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
ADT7485A
OFFSET REGISTERS
ON-CHIP TEMPERATURE SENSOR
VCC 12V 5V VCCP 2.5V D1+ D1- INPUT ATTENUATORS AND ANALOG MULTIPLEXER
TEMPERATURE VALUE REGISTERS
DIGITAL MUX
SST INTERFACE
SST
A/D CONVERTER
VOLTAGE VALUE REGISTERS
ADDRESS SELECTION
ADD
05197-001
GND
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.
ADT7485A 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 Configuration and Functional Descriptions.......................... 6 Typical Performance Characteristics ............................................. 7 Product Description......................................................................... 9 SST Interface ..................................................................................9 Voltage Measurement .................................................................... 12 Analog-to-Digital Converter .................................................... 12 Temperature Measurement ........................................................... 13 Temperature Measurement Method ........................................ 13 Reading Temperature Measurements...................................... 13 SST Temperature Sensor Data Format .................................... 13 Using Discrete Transistors ........................................................ 14 Layout Considerations............................................................... 14 Temperature Offset .................................................................... 14 Outline Dimensions ....................................................................... 15 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 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 Unit V V mA C C C C C A A A C k 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 ADT7485A cancels 1.5 k in series with the remote thermal diode
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) 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 High Impedance State Leakage, ILEAK Signal Noise Immunity, VNOISE 300 80 230 180 1.1
2.3 0.8 -1 1 5
V V A A pF
VIN = VCC VIN = 0
2 1.5 1 0.1 11 12 38 145 110 290 230 140 350 280
% % LSB %/V ms ms ms ms k k k V V mV V A A mV p-p
12V and 5V channels For all other channels 10 bits Averaging enabled Averaging enabled Averaging enabled Averaging enabled
0.4 150 1.1 1.9 1 10
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
Rev. 0 | Page 3 of 16
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
1 2
Min 0.495 0.6 x tBIT 0.2 x tBIT
Typ
Max 500 0.8 x tBIT 0.4 x tBIT 0.2 x tBIT 0.5 x tBIT-M 2 x tBIT 0.4
Unit s s s s s s ms s
Test Conditions/Comments
0.75 x tBIT 0.25 x tBIT
tBIT defined in speed negotiation
1.25 x tBIT
2 x tBIT
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
500
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.
Rev. 0 | Page 4 of 16
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 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 10-Lead MSOP JA 206 JC 44 Unit 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
ADT7485A PIN CONFIGURATION AND FUNCTIONAL DESCRIPTIONS
VCC 1 GND 2 D1+ 3 D1- 4 12V 5
10
SST ADD 2.5V 5V
05197-002
ADT7485A
TOP VIEW (Not to Scale)
9 8 7 6
VCCP
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 1.50 750 (~2mA) 1.45
SST O/P (V) SST O/P (V)
1.55 1.50 1.45 270 (~5.2mA) 1.40 1.35 1.30 1.25 1.20 -50 120 (~10.6mA) 750 (~2mA)
1.40 270 (~5.2mA) 1.35 1.30 1.25 1.20 2.6
120 (~10.6mA)
05197-007
2.8
3.0
3.2 VCC (V)
3.4
3.6
0
50 TEMPERATURE (C)
100
150
Figure 3. SST O/P Level vs. Supply Voltage
Figure 6. 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
05197-008
3.9
DEV3
3.7
DEV2
DEV2 3.5 DEV3 DEV1 3.3
DEV1
IDD (mA)
IDD (mA)
3.1
05197-011
3.46 3.45 -45 -25 -5 15 35 55 75 95 115
2.9 2.65
2.85
3.05 VCC (V)
3.25
3.45
3.65
TEMPERATURE (C)
Figure 4. Supply Current vs. Temperature
Figure 7. Supply Current vs. Voltage
7 6 TEMPERATURE ERROR (C) 5 4 3 2 1 0 -1 -60 LO SPEC (VCC = 3.6V) -40 -20 0 20 40 60 80 100 120 140
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7 6
TEMPERATURE ERROR (C)
5 4 3 2 1 0 -1 LO SPEC (VCC = 3.6V) -40 -20 0 20 40 60 80 100 120 140
05197-020
HI SPEC (VCC = 3V)
HI SPEC (VCC = 3V) MEAN (VCC = 3.3V)
MEAN (VCC = 3.3V)
-2 -60
TEMPERATURE (C)
TEMPERATURE (C)
Figure 5. Local Temperature Error
Figure 8. Remote Temperature Error
Rev. 0 | Page 7 of 16
05197-010
ADT7485A
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
05197-016
-5 -10 -15 -20 -25 -30 -35 0 20 40 60 80 100
05197-021
D+ TO VCC
DEV1_EXT1 DEV1_EXT2 DEV2_EXT1
DEV2_EXT2 DEV3_EXT1 DEV3_EXT2
EXT1
-80 -90
-40
0
10
20
30
40
50
RESISTANCE (M)
CAPACITANCE (nF)
Figure 9. Remote Temperature Error vs. PCB Resistance
Figure 12. Remote Temperature Error vs. Capacitance Between D1+ and D1-
30 25 20 15 60mV 10 5 0 -5 10k
TEMPERATURE ERROR (C)
TEMPERATURE ERROR (C)
7 6
40mV
100mV
5 4 3 2 1
20mV
40mV
05197-014
10mV
0 10k 100k 1M 10M 100M 1G
100k
1M
10M
100M
1G
NOISE FREQUENCY (C)
NOISE FREQUENCY (C)
Figure 10. Temperature Error vs. Common-Mode Noise Frequency
Figure 13. Temperature Error vs. Differential-Mode Noise Frequency
20
5 4
15
TEMPERATURE ERROR (C)
TEMPERATURE ERROR (C)
3 2 1 0 50mV -1
05197-018
10
5 125mV 0 50mV -5
05197-015
125mV
-2 -3 10k
-10 10k
100k
1M
10M
100M
1G
100k
1M
10M
100M
1G
POWER SUPPLY NOISE FREQUENCY (Hz)
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
05197-017
ADT7485A PRODUCT DESCRIPTION
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.
ADT7485A Client Address
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. Table 5. ADT7485A Selectable Addresses
ADD Low (GND) Float High Address Selected 0x48 0x49 0x4A
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. 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
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() GetIntTemp() GetExtTemp() GetAllTemps() GetVolt12V() GetVolt5V() GetVoltVCC() GetVolt2.5V() GetVoltVCCP() GetAllVolts() SetExtOffset() GetExtOffset() ResetDevice() GetDIB() Command Code, CC 0x00 0x00 0x01 0x00 0x10 0x11 0x12 0x13 0x14 0x10 0xe0 0xe0 0xf6 0xf7 0xf7 Write Length, WL 0x00 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x02 0x01 0x01 0x01 0x01 Read Length, RL 0x00 0x02 0x02 0x04 0x02 0x02 0x02 0x01 0x02 0x10 0x00 0x01 0x00 0x08 0x10 Description Shows a nonzero FCS over the header if present. Shows the temperature of the device's internal thermal diode. Shows the temperature of the external thermal diode. Shows a 4-byte block of data (GetIntTemp, GetExtTemp). Shows the voltage attached to 12V input. Shows the voltage attached to 5V input. Shows the voltage attached to VCC input. Shows the voltage attached to 2.5V input. Shows the voltage attached to VCCP input. Shows all voltage measurement values. Sets the offset used to correct errors in the external diode. 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
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 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 pin
GetIntTemp()
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 128C with +0.25C resolution.
4, 5
Device ID
0x7485
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
0x01 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x05 0x48 to 0x4a
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.
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. 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
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.
Voltage Measurement Command Codes
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. Table 11. Voltage Measurement Command Codes
Voltage Channel 12V 5V VCC 2.5V VCCP Command Code 0x10 0x11 0x12 0x13 0x14 Returned Data LSB, MSB LSB, MSB LSB, MSB LSB, MSB LSB, MSB
ANALOG-TO-DIGITAL CONVERTER
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. 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. Table 10. Maximum Reported Input Voltages
Voltage Channel 12V 5V VCC 2.5V VCCP Full-Scale Voltage 16 V 8V 4V 4V 4V
Voltage Data Format
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. Table 12. Analog-to-Digital Output Code vs. VIN
Voltage 12 5 3.3 3 2.5 1 0 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
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.
12V IN 120k 20k 93k 47k 68k 71k 45k 94k 17.5k 52.5k 35pF 30pF 30pF MUX 30pF 30pF
5VIN
3.3VIN
2.5VIN
VCCP
Figure 15. Internal Structure of Analog Inputs
05197-003
Rev. 0 | Page 12 of 16
ADT7485A TEMPERATURE MEASUREMENT
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. 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 1 Measurement Local temperature Remote temperature Conversion Time (ms) 12 38
D1+ REMOTE SENSING TRANSISTOR C11 D1- BIAS DIODE LOW-PASS FILTER fC = 65kHz
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
VOUT+ TO ADC VOUT-
1CAPACITOR
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 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. Table 14. Temperature Channel Command Codes
Temp Channel Internal External All Temps Command Code 0x00 0x01 0x00 Returned data LSB, MSB LSB, MSB Internal LSB, Internal MSB; External LSB, External MSB
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 ADT7485A 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. 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,
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 15. 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
Rev. 0 | Page 13 of 16
05197-004
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.
2N3904 NPN
*
*
* *
ADT7485A
D1+ D1- 2N3906 PNP
D1+ D1-
ADT7485A
05197-005
*
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.
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 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.
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.
GND 5mil 5mil D1+ 5mil 5mil
D1-
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.
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 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 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.
*
*
5mil 5mil
GND
5mil
Figure 18. Arrangements of Signal Tracks
05197-006
Rev. 0 | Page 14 of 16
ADT7485A OUTLINE DIMENSIONS
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 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 -40C to +125C -40C to +125C
Package Description 10-Lead MSOP 10-Lead MSOP
Package Option RM-10 RM-10
Branding T21 T21
Z = Pb-free part.
Rev. 0 | Page 15 of 16
ADT7485A NOTES
(c)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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