REV. A

ADM1025/ADM1025A

–11–

4. 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

thermocouples with a big temperature differential between

them, thermocouple voltages should be much less than 200

µV.

5. Place 0.1

µF bypass and 1 nF input filter capacitors close to

the ADM1025/ADM1025A.

6. If the distance to the remote sensor is more than 8 inches, the

use of twisted pair cable is recommended. This will work up

to about 6 to 12 feet.

7. 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 regis-

ter bit will be set to “1.”

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 one 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 com-

pleted 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, as 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, its 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 sup-

plies together.

As the resistors will form part of the input attenuators, they will

affect the accuracy of the analog measurement if their value is

too high. The analog input channels are calibrated assuming an

external series resistor of 500

Ω, and the accuracy will remain

within specification for any value from zero to 1 k

Ω, so a stan-

dard 510

Ω resistor is suitable.

The worst such accident would be connecting 0 V to 12 V—a

total of 12 V difference, with the series resistors this would draw

a maximum current of approximately 12 mA.

LAYOUT AND GROUNDING

Analog inputs will provide best accuracy when referred to a

clean ground. A separate, low impedance ground plane for

analog ground, which provides a ground point for the voltage

dividers and analog components, will provide best performance

but is not mandatory.

The power supply bypass, the parallel combination of 10

µF

(electrolytic or tantalum) and 0.1

µF (ceramic) bypass capacitors

connected between Pin 9 and ground, should also be located as

close as possible to the ADM1025/ADM1025A.

RST/INT OUTPUT

As previously mentioned, Pin 16 is a multifunction pin. Its state

after power-on is latched to set the lowest two bits of the serial

bus address. During NAND tree board-level connectivity testing

it functions as the output of the NAND tree. It may also be used

as a reset output, or as an interrupt output for out-of-limit tem-

perature/voltage events.

Pin 16 is programmed as a reset output by clearing bit 0 of the

Test Register and setting Bit 7 of the VID Register. A low going,

20 ms, reset output pulse can then be generated by setting Bit 4

of the Configuration Register.

If Bit 7 of the VID Register is cleared, Pin 16 can be programmed

as an interrupt output for out-of-limit temperature/voltage events

(

INT). Desired interrupt operation is achieved by changing the

values of Bits 1 and 0 of the Test Register as shown in Table IV.

Note, however, that Bits 2 to 7 of the Test Register must be

zeros (not don’t cares). If, for example,

INT is programmed for

thermal and voltage interrupts, then if any temperature or volt-

age measurement goes outside its respective high or low limit,

the

INT output will go low. It will remain low until Status Reg-

ister 1 is read, when it will be cleared. If the temperature or

voltage remains out of limit,

INT will be reasserted on the next

monitoring cycle.

INT can also be cleared by issuing an Alert

Response Address Call.