Remote/Local Temperature Sensor
with SMBus Serial Interface
PC Board Layout
1) Place the MAX1617A as close as practical to the
remote diode. In a noisy environment, such as a
computer motherboard, this distance can be 4 in. to
8 in. (typical) or more as long as the worst noise
sources (such as CRTs, clock generators, memory
buses, and ISA/PCI buses) are avoided.
2) Do not route the DXP–DXN lines next to the deflec-
tion coils of a CRT. Also, do not route the traces
across a fast memory bus, which can easily intro-
duce +30°C error, even with good filtering.
Otherwise, most noise sources are fairly benign.
3) Route the DXP and DXN traces in parallel and in
close proximity to each other, away from any high-
voltage traces such as +12V
DC
. Leakage currents
from PC board contamination must be dealt with
carefully, since a 20MΩ leakage path from DXP to
ground causes about +1°C error.
4) Connect guard traces to GND on either side of the
DXP–DXN traces (Figure 2). With guard traces in
place, routing near high-voltage traces is no longer
an issue.
5) Route through as few vias and crossunders as possi-
ble to minimize copper/solder thermocouple effects.
6) When introducing a thermocouple, make sure that
both the DXP and the DXN paths have matching
thermocouples. In general, PC board-induced ther-
mocouples are not a serious problem. A copper-sol-
der thermocouple exhibits 3µV/°C, and it takes
about 200µV of voltage error at DXP–DXN to cause
a +1°C measurement error. So, most parasitic ther-
mocouple errors are swamped out.
7) Use wide traces. Narrow ones are more inductive
and tend to pick up radiated noise. The 10 mil
widths and spacings recommended in Figure 2
aren’t absolutely necessary (as they offer only a
minor improvement in leakage and noise), but try to
use them where practical.
8) Keep in mind that copper can’t be used as an EMI
shield, and only ferrous materials, such as steel, work
well. Placing a copper ground plane between the
DXP-DXN traces and traces carrying high-frequency
noise signals does not help reduce EMI.
GND
10 MILS
10 MILS
DXP
MINIMUM
10 MILS
DXN
10 MILS
GND
MAX1617A
Figure 2. Recommended DXP/DXN PC Traces
• Use guard traces flanking DXP and DXN and con-
necting to GND.
• Place the noise filter and the 0.1µF V
CC
bypass
capacitors close to the MAX1617A.
• Add a 200Ω resistor in series with V
CC
for best noise
filtering (see
Typical Operating Circuit).
Twisted Pair and Shielded Cables
For remote-sensor distances longer than 8 in., or in par-
ticularly noisy environments, a twisted pair is recom-
mended. Its practical length is 6 feet to 12 feet (typical)
before noise becomes a problem, as tested in a noisy
electronics laboratory. For longer distances, the best
solution is a shielded twisted pair like that used for audio
microphones. For example, the Belden 8451 works well
for distances up to 100 feet in a noisy environment.
Connect the twisted pair to DXP and DXN and the shield
to GND, and leave the shield’s remote end unterminated.
Excess capacitance at DX_ limits practical remote sen-
sor distances (see
Typical Operating Characteristics).
For very long cable runs, the cable’s parasitic capaci-
tance often provides noise filtering, so the 2200pF
capacitor can often be removed or reduced in value.
Cable resistance also affects remote-sensor accuracy;
1Ω series resistance introduces about +1/2°C error.
Low-Power Standby Mode
Standby mode disables the ADC and reduces the sup-
ply-current drain to less than 10µA. Enter standby
mode by forcing the
STBY
pin low or via the RUN/STOP
bit in the configuration byte register. Hardware and
software standby modes behave almost identically: all
data is retained in memory, and the SMB interface is
alive and listening for reads and writes. The only differ-
ence is that in hardware standby mode, the one-shot
command does not initiate a conversion.
Standby mode is not a shutdown mode. With activity on
the SMBus, extra supply current is drawn (see
Typical
Operating Characteristics).
In software standby mode,
9
PC Board Layout Checklist
•
•
•
•
•
Place the MAX1617A close to a remote diode.
Keep traces away from high voltages (+12V bus).
Keep traces away from fast data buses and CRTs.
Use recommended trace widths and spacings.
Place a ground plane under the traces.
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