ADM1024ARU View Datasheet(PDF) - Analog Devices
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ADM1024ARU Datasheet PDF : 32 Pages
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ADM1024
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments, and care
must be taken to protect the analog inputs from noise, particu-
larly when measuring the very small voltages from a remote
diode sensor. The following precautions should be taken:
1. Place the ADM1024 as close as possible to the remote sens-
ing diode. Provided that the worst noise sources such as
clock generators, data/address buses, and CRTs are avoided,
this distance can be 4 inches to 8 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. A 10 mil track minimum width and spacing is
recommended.
GND
D+
D–
GND
10MIL
10MIL
10MIL
10MIL
10MIL
10MIL
10MIL
Figure 8. Arrangement of Signal Tracks
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 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 mV.
5. Place 0.1 µF bypass and 2200 pF input filter capacitors close
to the ADM1024.
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 feet 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 ADM1024. Leave the remote end of the shield uncon-
nected 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 Ω series resis-
tance introduces about 0.5°C error.
LIMIT VALUES
Limit values for analog measurements are stored in the appro-
priate limit registers. In the case of voltage measurements, high
and low limits can be stored so that an interrupt request will be
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, which will usually be some degrees lower. This
can be useful as 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.
MONITORING CYCLE TIME
The monitoring cycle begins when a 1 is written to the Start Bit
(Bit 0), and a 0 to the INT_Clear Bit (Bit 3) 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; 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, as 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 be calculated as follows:
m × t1 + n × t2
where:
m is the number of inputs configured as analog inputs, plus the
internal VCC measurement and internal temperature sensor.
t1 is the time taken for an analog input conversion, nominally
6.044 ms.
n is the number of inputs configured as external temperature
inputs.
t2 is the time taken for a temperature conversion, nominally
33.24 ms.
This rapid sampling of the analog inputs ensures a quick response
in the event of any input going out of limits, unlike other moni-
toring chips that employ slower ADCs.
FAN MONITORING CYCLE TIME
When a monitoring cycle is started, monitoring of the fan speed
inputs begins at the same time as monitoring of the analog inputs.
However, the two monitoring cycles are not synchronized in any
way. The monitoring cycle time for the fan inputs is dependent
on fan speed and is much slower than for the analog inputs. For
more details, see the Fan Speed Measurement section.
–14–
REV. B
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