ADM1026JSTZ-R7 datasheet(20/55 Pages) ONSEMI

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ADM1026

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The results of external temperature measurements are

stored in 8-bit, twos complement format, as illustrated in

Table 8.

Table 8. TEMPERATURE DATA FORMAT

Temperature

Digital Output

Hex

−128C

1000 0000

−125C

1000 0011

−100C

1001 1100

−75C

1011 0101

−50C

1100 1110

−25C

1110 0111

−10C

1111 0110

0C

0000 0000

10C

0000 1010

25C

0001 1001

50C

0011 0010

75C

0100 1011

100C

0110 0100

125C

0111 1101

127C

0111 1111

Layout Considerations

Digital boards can be electrically noisy environments.

Take these precautions to protect the analog inputs from

noise, particularly when measuring the very small voltages

from a remote diode sensor.

 Place the ADM1026 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 4 to 8 inches.

 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.

 Use wide tracks to minimize inductance and reduce noise

pickup. A 10 mil track minimum width and spacing is

recommended.

Figure 34. Arrangement of Signal Tracks

10MIL

GND

D+

GND

D–

10MIL

 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 D+ and D− paths and are at the same temperature.

 Thermocouple effects should not be a major problem

because 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 200 mV.

 Place a 0.1 mF bypass capacitor close to the ADM1026.

 If the distance to the remote sensor is more than eight

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

This works from about 6 to 12 feet.

 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 ADM1026. 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 may be reduced or removed. Cable resistance

can also introduce errors. A 1

W series resistance introduces

about 0.5C error.

Limit Values

Limit values for analog measurements are stored in the

appropriate limit registers. In the case of voltage

measurements, high and low limits can be stored so that an

interrupt request is generated if the measured value goes

above or below acceptable values. In the case of

temperature, a hot temperature or high limit can be

programmed, and a hot temperature hysteresis or low limit

can be programmed, which is usually some degrees lower.

This can be useful because it allows the system to be shut

down when the hot limit is exceeded, and restarted

automatically when it has cooled down to a safe

temperature.

Analog Monitoring Cycle Time

The analog monitoring cycle begins when a 1 is written to

the start bit (Bit 0), and a 0 to the INT_Clear bit (Bit 2) of the

configuration register. INT_Enable (Bit 1) should be set to

1 to enable the INT output. The ADC measures each analog

input in turn, starting with Remote Temperature Channel 1

and ending with local temperature. As each measurement is

completed, the result is automatically stored in the

appropriate value register. This round-robin monitoring

cycle continues until it is disabled by writing a 0 to Bit 0 of

the configuration register. Because the ADC is typically left

to free-run in this way, the most recently measured value of

any input can be read out at any time.

For applications where the monitoring cycle time is

important, it can easily be calculated.

The total number of channels measured is:

 Five Dedicated Supply Voltage Inputs

 Ten General-purpose Analog Inputs

 3.3 V MAIN

 3.3 V STBY

 Local Temperature

 Two Remote Temperature

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