Pentek 6230 Operating Manual page 205

GND
D+
D–
GND
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/
o
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 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. One 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 Hyster-
esis 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 one is written to the
Start Bit (Bit 0), and a zero to the INT_Clear Bit (Bit 3) of
the Configuration Register. INT_Enable (Bit 1) should be set
to one 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.
REV. A
As the ADC will normally be left to free-run in this manner, the
10MIL
time taken to monitor all the analog inputs will normally not be
10MIL
of interest, as the most recently measured value of any input can
be read out at any time.
10MIL
For applications where the monitoring cycle time is important,
10MIL
it can be calculated as follows:
10MIL
10MIL
where:
10MIL
m is the number of inputs configured as analog inputs, plus the
internal V
t is the time taken for an analog input conversion, nominally
1
755 µs.
n is the number of inputs configured as external temperature
inputs.
t is the time taken for a temperature conversion, nominally
2
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 Fan Speed Measurement section.
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 a accidental short such as a probe connect two power
supplies 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 kΩ, so a stan-
dard 510 Ω resistor is suitable.
The worst such accident would be connecting –12 V to +12 V—
a total of 24 V difference, with the series resistors this would draw
a maximum current of approximately 24 mA.
ANALOG OUTPUT
The ADM1024 has a single analog output from a unsigned 8-bit
DAC which produces 0 V–2.5 V. The analog output register
defaults to FF during power-on reset, which produces maximum
fan speed. The analog output may be amplified and buffered
with external circuitry such as an op amp and transistor to provide
fan speed control.
–13–
m × t × n × t
1 2
measurement and internal temperature sensor.
CC
ADM1024