ADM1025ARQZ-R7 datasheet(14/21 Pages) ONSEMI

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ADM1025/ADM1025A

Preliminary Technical Data

Rev. P5 | Page 14 of 21| www.onsemi.com

LAYOUT CONSIDERATIONS

Digital boards can be electrically noisy environments and care

must be taken to protect the analog inputs from noise,

particularly when measuring the very small voltages from a

remote diode sensor. The following precautions should be

taken:

Place the ADM1025/ADM1025A 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 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. 10 mil track minimum width and spacing is

recommended.

Figure 16. Arrangement of Signal Tracks

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− path and at the same temperature.

Thermocouple effects should not be a major problem as

1°C corresponds to about 240 μV, and thermocouple

voltages are about 3 μV/°C of temperature difference.

Unless there are two thermocouples with a big temperature

differential between them, thermocouple voltages should

be much less than 200 μV.

Place 0.1 μF bypass and 1 nF input filter capacitors close to

the ADM1025/ADM1025A.

If the distance to the remote sensor is more than eight

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

will work up to about 6 to 12 feet.

For really 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 ADM1025/ADM1025A. 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. 1 Ω series resistance

introduces about 0.5°C error.

LIMIT VALUES

High and low limit values for each measurement channel are

stored in the appropriate limit registers. As each channel is

measured, the measured value is stored and compared with the

programmed limit.

STATUS REGISTERS

The results of limit comparisons are stored in Status Registers 1

and 2. The Status Register bit for a particular measurement

channel reflects the status of the last measurement and limit

comparison on that channel. If a measurement is within limits,

the corresponding Status Register bit will be cleared to “0.” If

the measurement is out of limits, the corresponding status

The state of the various measurement channels may be polled

by reading the Status Registers over the serial bus. Reading the

Status Registers does not affect their contents. Out-of-limit

temperature/voltage events may also be used to generate an

interrupt so that remedial action, such as turning on a cooling

fan, may be taken immediately. This is described in the section

on RST and INT.

MONITORING CYCLE TIME

The monitoring cycle begins when a 1 is written to the Start Bit

(Bit 0) of the Configuration Register. The ADC measures each

analog input in turn and 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.

As the ADC will normally be left to free-run in this manner, the

time taken to monitor all the analog inputs will normally not be

of interest, since the most recently measured value of any input

can be read out at any time.

INPUT SAFETY

Scaling of the analog inputs is performed on-chip, so external

attenuators are normally not required. However, since the

power supply voltages will appear directly at the pins, it is

advisable to add small external resistors in series with the

supply traces to the chip to prevent damaging the traces or

power supplies should an accidental short such as a probe

connect two power supplies together.

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