ADM1021A

http://onsemi.com

14

Where copper/solder joints are used, ensure they

are in both the D+ and D− paths and at the same

temperature.

Thermocouple effects should not be a major

problem as 1

°C corresponds to about 240 mV, and

thermocouple voltages are about 3

mV/°C of

temperature difference. Unless there are two

thermocouples with a big temperature differential

between them, thermocouple voltages should be

much less than 240

mV.

5. Place a 0.1

pin, and 2200 pF input filter capacitors across D+,

D− close to the ADM1021A.

6. If the distance to the remote sensor is more than

eight inches, the use of twisted pair cable is

recommended. This works up to about 6 to 12 feet.

7. For very long distances (up to 100 feet), use

shielded twisted pair, such as Belden #8451

microphone cable. Connect the twisted pair to D+

and D− and the shield to GND close to the

ADM1021A. Leave the remote end of the shield

unconnected to avoid ground loops.

Figure 18. Arrangement of Signal Tracks

10MIL

GND

D+

GND

D–

10MIL

10MIL

10MIL

10MIL

10MIL

10MIL

10MIL

10MIL

10MIL

10MIL

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 series

resistance of 1

W introduces about 1°C error.

Application Circuits

Figure 19 shows a typical application circuit for the

ADM1021A, using a discrete sensor transistor connected

via a shielded, twisted pair cable. The pullups on SCLK,

SDATA, and ALERT are required only if they are not

already provided elsewhere in the system.

The SCLK and SDATA pins of the ADM1021A can be

interfaced directly to the SMBus of an I/O chip. Figure 20

shows how the ADM1021A might be integrated into a

system using this type of I/O controller.

Figure 19. Typical Application Circuit

STBY

SCLK

SDATA

ALERT

ADD0

ADD1

GND

D+

D–

0.1µ F

ALL

3.3 V

TO CONTROL

CHIP

SET TO REQUIRED

ADDRESS

IN

OUT

I/O

C1*

SHIELD

2N3904

* C1 IS OPTIONAL

ADM1021A

10k

Figure 20. Typical System Using ADM1021A

USB

PROCESSOR

DISPLAY

SYSTEM BUS

DISPLAY

CACHE

SYSTEM

MEMORY

GMCH

FWH

(FIRMWARE HUB)

ADM1021A

D–

D+

ALERT

SCLK

SDATA

SUPER

I/O

SMBus

PCI BUS

PCI SLOTS

2 USB PORTS

CD−ROM

HARD

DISK

2 IDE PORTS

ICH

I/O CONTROLLER

HUB

USB

ORDERING INFORMATION

Device Number

Temperature Range

Package Type

Package Option

ADM1021AARQZ

0

°C to +100°C

16−Lead QSOP

RQ−16

98 Tube

ADM1021AARQZ−R

0

°C to +100°C

16−Lead QSOP

RQ−16

2500 Tape & Reel

ADM1021AARQZ−R7

0

°C to +100°C

16−Lead QSOP

RQ−16

1000 Tape & Reel

†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging

Specifications Brochure, BRD8011/D.

*The “Z’’ suffix indicates Pb−Free part.