MAX1668_00 Datasheet, PDF (9/16 Pages) Maxim Integrated Products – Multichannel Remote/Local Temperature Sensor

Multichannel Remote/Local
Temperature Sensor
as a computer motherboard, this distance can be
4 inches to 8 inches (typical) or more as long as the
worst noise sources (such as CRTs, clock genera-
tors, memory buses, and ISA/PCI buses) are avoid-
ed.
2) Do not route the DXP_ to DXN_ lines next to the
deflection coils of a CRT. Also, do not route the
traces across a fast memory bus, which can easily
introduce +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 +12VDC. 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_ to 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_ to DXN_ to
cause a +1°C measurement error. So, most para-
sitic thermocouple errors are swamped out.
7) Use wide traces. Narrow ones are more inductive
and tend to pick up radiated noise. The 10mil
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) Copper can’t be used as an EMI shield, and only fer-
rous materials such as steel work well. Placing a cop-
per ground plane between the DXP_ to DXN_ traces
and traces carrying high-frequency noise signals
does not help reduce EMI.
PC Board Layout Checklist
• Place the MAX1668/MAX1805 as close as possible
to the remote diodes.
• 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.
10mils
10mils
GND
DXP_
DXN_
GND
10mils
MINIMUM
10mils
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 MAX1668/MAX1805.
• 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 inches, or in
particularly 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, 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 12µA. 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.
Activate hardware standby mode by forcing the STBY
pin low. In a notebook computer, this line may be con-
nected to the system SUSTAT# suspend-state signal.
The STBY pin low state overrides any software conversion
command. If a hardware or software standby command
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