ADM1021AARQZ-R datasheet(14/16 Pages) ONSEMI

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ADM1021A

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Layout Considerations

Digital boards can be electrically noisy environments, and

because the ADM1021A is measuring very small voltages

from the remote sensor, care must be taken to minimize

noise induced at the sensor inputs. The following

precautions should be taken:

1. Place the ADM1021A 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.

2. Route the D+ and D− tracks close together, in

parallel, with grounded guard tracks on each side.

Provide a ground plane under the tracks, if

possible.

3. Use wide tracks to minimize inductance and

reduce noise pickup. 10 mil track minimum width

and spacing is recommended.

4. Try to minimize the number of copper/solder

joints, which can cause thermocouple effects.

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

mF bypass capacitor close to the VDD

pin, and 2,200 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

10 MIL

GND

D−

D+

GND

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

TO CONTROL

CHIP

3.3 V

ALL 10 kW

0.1 mF

GND

2N3904

ADM1021A

SCLK

SDATA

ALERT

VDD

D−

D+

ADD1

ADD0

STBY

SET TO REQUIRED

ADDRESS

SHIELD

C1*

OUT

I/O

* C1 IS OPTIONAL

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