ADT7485AARMZ-REEL7_技术文档

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

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.

Simple Serial Transport™ (SST™) interface

APPLICATIONS

Personal computers

Portable personal devices

Industrial sensor nets

FUNCTIONAL BLOCK DIAGRAM

ADT7485A

ON-CHIP

TEMPERATURE

SENSOR

OFFSET REGISTERS

SST INTERFACE

TEMPERATURE

V

CC

VALUE REGISTERS

12V

5V

INPUT

ATTENUATORS

AND

SST

A/D

CONVERTER

V

CCP

ANALOG

MULTIPLEXER

2.5V

D1+

D1–

VOLTAGE

VALUE REGISTERS

ADDRESS

SELECTION

ADD

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 registeredtrademarks arethe property of their respective owners.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781.329.4700

Fax: 781.461.3113

www.analog.com

ADT7485A

TABLE OF CONTENTS

Features .............................................................................................. 1

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

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

REVISION HISTORY

7/06—Revision 0: Initial Version

Rev. 0 | Page 2 of 16

ADT7485A

SPECIFICATIONS

T_A= T_MINto T_MAX, V_CC= V_MINto V_MAX, unless otherwise noted.

Table 1.

Parameter

Min

Typ

Max

3.6

5

Unit

Test Conditions/Comments

POWER SUPPLY

Supply Voltage, V_CC

Undervoltage Lockout Threshold

Average Operating Supply Current, I_DD

TEMPERATURE-TO-DIGITAL CONVERTER

Local Sensor Accuracy

3.0

3.3

2.8

3.8

V

mA

Continuous conversions

+1

1.ꢀ5

°C

40°C ≤ T_A≤ ꢀ0°C; V_CC= 3.3 V 5ꢁ

−40°C ≤ T_A≤ +100°C

4

Remote Sensor Accuracy

1

−40°C ≤T_D≤ +125°C; T_A= 25°C; V_CC= 3.3 V

+1

1.ꢀ5

−40°C ≤T_D≤ +125°C; −40 ≤T_A≤ ꢀ0°C;

V_CC= 3.3 V 5ꢁ

4

°C

−40°C ≤T_D≤ +125°C; −40 ≤T_A≤ +100°C

Remote Sensor Source Current

12

80

204

0.016

1.5

μA

°C

Low level

Mid level

High level

Resolution

Series Resistance Cancellation

kΩ

The ADTꢀ485A cancels 1.5 kΩ in series

with the remote thermal diode

DIGITAL INPUT (ADD)

Input High Voltage, V_IH

Input Low Voltage, V_IL

Input High Current, I_IH

Input Low Current, I_IL

Pin Capacitance

2.3

−1

V

μA

pF

0.8

V_IN= V_CC

V_IN= 0

1

5

ANALOG-TO-DIGITAL CONVERTER

(Including Multiplexer and Attenuators)

Total Unadjusted Error (TUE)

2

1.5

1

ꢁ

12V and 5V channels

For all other channels

10 bits

Differential Nonlinearity (DNL)

Power Supply Sensitivity

LSB

ꢁ/V

ms

0.1

Conversion Time (Voltage Input)¹

Conversion Time (Local Temperature)¹

Conversion Time (Remote Temperature)¹

Total Monitoring Cycle Time¹

Input Resistances

11

12

38

Averaging enabled

145

V_CCPand 2.5V Channels

5V Channel

12V Channel

80

230

180

110

290

230

140

350

280

kΩ

DIGITAL I/O (SST Pin)

Input High Voltage , V_IH

Input Low Voltage, V_IL

Hysteresis¹

Output High Voltage, V_OH

High Impedance State Leakage, I_LEAK

1.1

V

mV

V

ꢂA

0.4

150

Between input switching levels

I_SOURCE= 6 mA (maximum)

Device powered on SST bus;

V_SST= 1.1 V, V_CC= 3.3 V

1.9

1

High Impedance State Leakage, I_LEAK

Signal Noise Immunity, V_NOISE

10

ꢂA

Device unpowered on SST bus;

V_SST= 1.1 V, V_CC= 0 V

Noise glitches from 10 MHz to 100 MHz;

width up to 50 ns

300

mV p-p

Rev. 0 | Page 3 of 16

ADT7485A

Parameter

Min

Typ

Max

Unit

Test Conditions/Comments

t_BITdefined in speed negotiation

See SST Specification Rev 1.0

SST TIMING

Bitwise Period, t_BIT

High Level Time for Logic 1, t_H1

High Level Time for Logic 0, t_H0

Time to Assert SST High for Logic 1, t_{SU, HIGH}

0.495

0.6 × t_BIT

0.2 × t_BIT

500

ꢂs

2

0.ꢀ5 × t_BIT

0.25 × t_BIT

0.8 × t_BIT

0.4 × t_BIT

0.2 × t_BIT

0.5 × t_BIT-M

2 × t_BIT

3

Hold Time, t_HOLD

Stop Time, t_STOP

1.25 × t_BIT

2 × t_BIT

500

Device responding to a constant low

level driven by originator

Time to Respond After a Reset, t_RESET

Response Time to Speed Negotiation

After Power-Up

0.4

ms

ꢂs

Time after power-up when device can

participate in speed negotiation

¹Guaranteed by design, not production tested.

²Minimum and maximum bit times are relative to t_BITdefined in the timing negotiation pulse.

³Device 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 (V_CC)

Voltage on 12V Pin

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.

Rating

4 V

16 V

ꢀ V

3.6 V

−0.3 V to +3.6 V

5 mA

20 mA

150°C

−65°C to +150°C

Voltage on 5V Pin

Voltage on 2.5V and V_CCPPins

Voltage on Any Other Pin (Including SST Pin)

Input Current at Any Pin

Package Input Current

Maximum Junction Temperature (T_Jmax)

Storage Temperature Range

Lead Temperature, Soldering

IR Peak Reflow Temperature

Lead Temperature (10 sec)

ESD Rating

THERMAL RESISTANCE

θ_JAis specified for the worst-case conditions, that is, a device

soldered in a circuit board for surface-mount packages.

Table 3. Thermal Resistance

260°C

300°C

1500 V

Package Type

θ_JA

θ_JC

Unit

10-Lead MSOP

206

44

°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

V

1

2

3

4

5

10 SST

CC

GND

D1+

D1–

12V

9

8

7

6

ADD

2.5V

ADT7485A

TOP VIEW

(Not to Scale)

V

CCP

5V

Figure 2. 10-Lead MSOP

Table 4. Pin Function Descriptions

Pin No.

Mnemonic

Type

Description

1

2

3

4

5

6

ꢀ

8

9

10

V_CC

Power supply

Ground

3.3 V 10ꢁ. V_CCis 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.

GND

D1+

D1−

12V

5V

V_CCP

2.5V

ADD

SST

Analog input

Digital input

Digital input/output

Rev. 0 | Page 6 of 16

ADT7485A

TYPICAL PERFORMANCE CHARACTERISTICS

1.55

1.50

1.45

1.40

1.35

1.30

1.25

1.20

750Ω (~2mA)

1.50

750Ω (~2mA)

1.45

270Ω (~5.2mA)

1.40

270Ω (~5.2mA)

1.35

1.30

120Ω (~10.6mA)

1.25

1.20

–50

0

50

100

150

2.6

2.8

3.0

3.2

(V)

3.4

3.6

TEMPERATURE (°C)

V

CC

Figure 3. SST O/P Level vs. Supply Voltage

Figure 6. SST O/P Level vs. Temperature

3.56

3.9

3.7

3.5

3.3

3.1

2.9

3.55

3.54

3.53

3.52

3.51

3.50

3.49

3.48

3.47

3.46

3.45

DEV3

DEV2

DEV3

DEV1

–45

–25

–5

15

35

55

75

95

115

2.65

2.85

3.05

3.25

3.45

3.65

TEMPERATURE (°C)

V

(V)

CC

Figure 4. Supply Current vs. Temperature

Figure 7. Supply Current vs. Voltage

7

6

7

6

5

4

3

HI SPEC (V = 3V)

CC

3

2

HI SPEC (V = 3V)

CC

2

MEAN (V

= 3.3V)

CC

1

MEAN (V

= 3.3V)

CC

1

0

–1

–2

LO SPEC (V = 3.6V)

CC

LO SPEC (V = 3.6V)

CC

–1

–60 –40 –20

0

20

40

60

80

100 120 140

–60 –40 –20

0

20

40

60

80

100 120 140

TEMPERATURE (°C)

Figure 5. Local Temperature Error

Figure 8. Remote Temperature Error

Rev. 0 | Page ꢀ of 16

ADT7485A

15

10

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

DEV1_EXT1

DEV1_EXT2

DEV2_EXT1

DEV2_EXT2

DEV3_EXT1

DEV3_EXT2

D+ TO GND

5

0

–5

EXT2

–10

–15

–20

–25

–30

–35

–40

DEV1_EXT1

DEV1_EXT2

DEV2_EXT1

DEV2_EXT2

DEV3_EXT1

DEV3_EXT2

EXT1

D+ TO V

CC

0

20

40

60

80

100

0

10

20

30

40

50

RESISTANCE (MΩ)

CAPACITANCE (nF)

Figure 9. Remote Temperature Error vs. PCB Resistance

Figure 12. Remote Temperature Error vs. Capacitance Between D1+ and D1−

30

7

40mV

25

20

15

10

5

6

5

4

3

100mV

60mV

20mV

2

40mV

0

1

10mV

–5

10k

0

10k

100k

1M

10M

100M

1G

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

15

10

5

4

3

2

1

125mV

0

50mV

–1

–5

–2

–3

–10

10k

100k

1M

10M

100M

1G

10k

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

Table 5. ADT7485A Selectable Addresses

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.

ADD

Address Selected

Low (GND)

Float

High

0x48

0x49

0x4A

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

Command Code, CC Write Length, WL Read Length, RL Description

Ping()

0x00

Shows a nonzero FCS over the header if present.

GetIntTemp()

0x00

0x01

0x02

Shows the temperature of the device’s internal

thermal diode.

GetExtTemp()

GetAllTemps()

0x01

0x00

0x01

0x02

0x04

Shows the temperature of the external thermal diode.

Shows a 4-byte block of data (GetIntTemp, GetExtTemp).

GetVolt12V()

GetVolt5V()

GetVoltVCC()

0x10

0x11

0x12

0x01

0x02

Shows the voltage attached to 12V input.

Shows the voltage attached to 5V input.

Shows the voltage attached to V_CCinput.

GetVolt2.5V()

GetVoltVCCP()

GetAllVolts()

SetExtOffset()

GetExtOffset()

0x13

0x14

0x10

0xe0

0x01

0x02

0x01

0x02

0x10

0x00

0x01

Shows the voltage attached to 2.5V input.

Shows the voltage attached to V_CCPinput.

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.

ResetDevice()

GetDIB()

0xf6

0x01

0x00

Functional reset. The ADTꢀ485A 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.

0xfꢀ

0x01

0x08

0x10

Rev. 0 | Page 10 of 16

ADT7485A

GetIntTemp()

Command Code Details

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.

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.

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()

Table 7. 16-Byte DIB Details

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.

Byte Name

Value

Description

0

1

Device Capabilities 0xc0

Fixed address device

Meets Version 1 of the

SST specification

Version/Revision

Vendor ID

0x10

SetExtOffset()

2, 3

4, 5

00x11d4 Contains company ID

number in little endian

format

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 128ꢀC with +0.25ꢀC resolution.

Device ID

0xꢀ485

Contains device ID

number in little

endian format

SST device

Reserved

GetExtOffset()

6

ꢀ

Device Interface

Function

Interface

0x01

0x00

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.

8

9

Reserved

Revision ID

0x00

0x05

Reserved

Contains revision ID

Dependent on the state

of the address pin

ADT7485A Response to Unsupported Commands

10

11

12

13

14

15

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.

Client Device

Address

0x48 to

0x4a

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

Write Length

Read Length

FCS

(Not necessary)

0x00

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

Write

Length

Read

Length

Reset

command

Target Address

FCS

Device Address

0x01

0x00

0xf6

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, V_CC. Pins 5 to 8 can

measure the supplies of the 12V, 5V, processor core voltage (V_CCP),

and 2.5V pins, respectively. The V_CCsupply voltage measurement is

carried out through the V_CCpin (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.

Table 11. Voltage Measurement Command Codes

ANALOG-TO-DIGITAL CONVERTER

All analog inputs are multiplexed into the on-chip, successive-

approximation, 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 (V_CCP) without

any external components.

Voltage Channel Command Code

Returned Data

12V

0x10

0x11

0x12

0x13

0x14

LSB, MSB

5V

LSB, MSB

V_CC

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

V_CCP

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 10. Maximum Reported Input Voltages

Voltage Channel

Full-Scale Voltage

12V

5V

V_CC

2.5V

V_CCP

16 V

8 V

4 V

Table 12. Analog-to-Digital Output Code vs. V_IN

Twos Complement

Voltage

MSB

LSB

Input Circuitry

12

5

3.3

3

2.5

1

0

0011 0000

0001 0100

0000 1101

0000 1100

0000 1010

0000 0100

0000 0000

0011 0011

0000 0000

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 first-

order, low-pass filter to provide input immunity to high

frequency noise.

120kΩ

12V

IN

20kΩ

47kΩ

71kΩ

94kΩ

30pF

35pF

93kΩ

68kΩ

45kΩ

5V

IN

3.3V

2.5V

IN

MUX

IN

17.5kΩ

52.5kΩ

V

CCP

Figure 15. Internal Structure of Analog Inputs

Rev. 0 | Page 12 of 16

ADT7485A

TEMPERATURE MEASUREMENT

and a temperature measurement is produced. To reduce the

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.

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.

V

DD

I

BIAS

I

N1 × I

N2 × I

V

OUT+

D1+

Table 13. Temperature Monitoring Sequence

1

TO ADC

REMOTE

SENSING

TRANSISTOR

C1

D1–

BIAS

DIODE

V

OUT–

Channel

LOW-PASS FILTER

f_C= 65kHz

Number

Measurement

ConversionTime (ms)

1

CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.

0

Local temperature

12

Figure 16. Signal Conditioning for Remote Diode Temperature Sensors

1

Remote temperature

38

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.

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 (V_BE) of a transistor operated at constant

current. Unfortunately, this technique requires calibration to

null the effect of the absolute value of V_BE, which varies from

device to device.

Table 14. Temperature Channel Command Codes

Temp Channel Command Code Returned data

Internal

0x00

LSB, MSB

The technique used in the ADT7485A measures the change in

External

All Temps

0x01

0x00

LSB, MSB

Internal LSB, Internal MSB;

External LSB, External MSB

V_BEwhen 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.

SST TEMPERATURE SENSOR DATA FORMAT

The data for temperature is structured to allow values in the

range of 512ꢀC 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 0.016ꢀC resolution.

Table 15. SST Temperature Data Format

Twos Complement

Temperature (°C)

MSB

LSB

−125

−80

−40

−20

−5

−1

0

+1

+5

+20

+40

+80

+125

1110 0000

1110 1100

1111 0110

1111 1011

1111 1110

1111 1111

0000 0000

0000 0001

0000 0100

0000 1010

0001 0100

0001 1111

1100 0000

0000 0000

0011 1110

1100 0000

0000 0000

0100 0000

1100 0010

0000 0000

0100 0000

To measure ΔV_BE, 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 ΔV_BE1, and then between I and N2 × I, giving

ΔV_BE2. The temperature can then be calculated using the two

ΔV_BEmeasurements. This method can also cancel the effect of

series resistance on the temperature measurement. The

resulting ΔV_BEwaveforms 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 ΔV_BE. The ADC digitizes this voltage,

Rev. 0 | Page 13 of 16

ADT7485A

•

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

1ꢀC 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.

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.

•

ADT7485A

2N3904

NPN

D1+

D1–

D1+

D1–

•

2N3906

PNP

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.

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 0.5ꢀC error.

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:

TEMPERATURE OFFSET

•

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.

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 mea-

surements 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 measure-

ment. The maximum offset is 128ꢀC with 0.25ꢀC 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

GND

D1+

D1–

GND

Figure 18. Arrangements of Signal Tracks

Rev. 0 | Page 14 of 16

ADT7485A

OUTLINE DIMENSIONS

3.10

3.00

2.90

10

6

5.15

4.90

4.65

3.10

3.00

2.90

1

5

PIN 1

0.50 BSC

0.95

0.85

0.75

1.10 MAX

0.80

0.60

0.40

8°

0°

0.15

0.05

0.33

0.17

SEATING

PLANE

0.23

0.08

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

ADTꢀ485AARMZ-REEL¹

ADTꢀ485AARMZ-REELꢀ¹

Temperature Range

–40°C to +125°C

Package Description

10-Lead MSOP

Package Option

RM-10

Branding

T21

¹Z = Pb-free part.

Rev. 0 | Page 15 of 16

ADT7485A

NOTES

registered trademarks are the property of their respective owners.

D05197-0-7/06(0)

Rev. 0 | Page 16 of 16


型号：	ADT7485AARMZ-REEL7
厂家：	ADI
描述：	SST Digital Temperature Sensor and Voltage Monitor SST数字温度传感器和电压监视器传感器温度传感器监视器
文件：	总16页 (文件大小：401K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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