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:
1.
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.
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. 10 mil track minimum width and spacing is
recommended.
Figure 16. 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 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 eight
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
register 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 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.
