ADT7484A/ADT7486A

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10

SST Temperature Sensor Data Format

The data for temperature is structured to allow values in

the range of

±512°C 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

0.016

°C resolution.

Table 8. SST Temperature Data Format

Temperature (5C)

Twos Complement

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 0000

0000 0001

0000 0100

0000 1010

0001 0100

0001 1111

1100 0000

0000 0000

0000 0000

0011 1110

1100 0000

1100 0000

0000 0000

0100 0000

0100 0000

1100 0010

0000 0000

0000 0000

0100 0000

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.

Figure 15. Connections for NPN and PNP Transistors

D+

D–

ADT7484A/

ADT7486A

2N3904

NPN

D+

D–

ADT7484A/

ADT7486A

2N3906

PNP

The ADT7484A/ADT7486A show an external temperature

value of 0x8000 if the external diode is an open or short circuit.

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:

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.

with grounded guard tracks on each side. Provide a

ground plane under the tracks if possible.

noise pickup. A 5 mil track minimum width and

spacing is recommended.

Figure 16. Arrangements of Signal Tracks

5MIL

5MIL

5MIL

5MIL

5MIL

5MIL

5MIL

GND

D+

GND

D–

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.

because 1

°C corresponds to about 240 mV, and

thermocouple voltages are about 3

mV/°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.

inches, the use of a twisted-pair cable is recommended.

This works for distances of about 6 to 12 feet.

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 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

W series resistance introduces

about 0.5

°C error.