Rainbow Electronics MAX6622 - Manual (Page 16)

Slave Address
The slave address of the MAX6622 is 9Ah or 1001 101.
PCB Layout
Follow these guidelines to reduce the measurement
error when measuring remote temperature:
1) Place the MAX6622 as close as is practical to the
remote diode. In noisy environments, such as a com-
puter motherboard, this distance can be 4in to 8in
(typ). This length can be increased if the worst noise
sources are avoided. Noise sources include CRTs,
clock generators, memory buses, and PCI buses.
2) Do not route the DXP-DXN lines next to the deflec-
tion coils of a CRT. Also, do not route the traces
across fast digital signals, which can easily intro-
duce +30°C error, even with good filtering.
3) Route the DXP and DXN traces in parallel and in
close proximity to each other. Each parallel pair of
traces should go to a remote diode. Route these
traces away from any higher voltage traces, such as
+12VDC. Leakage currents from PCB contamination
must be dealt with carefully since a 20MΩ leakage
path from DXP to ground causes about +1°C error. If
high-voltage traces are unavoidable, connect guard
traces to GND on either side of the DXP-DXN traces
(Figure 5).
4) Route through as few vias and crossunders as possi-
ble to minimize copper/solder thermocouple effects.
5) Use wide traces when practical. 5-mil to 10-mil
traces are typical. Be aware of the effect of trace
resistance on temperature readings when using
long, narrow traces.
6) When the power supply is noisy, add a resistor (up
to 47Ω) in series with V CC .
Twisted-Pair and Shielded Cables
Use a twisted-pair cable to connect the remote sensor
for remote-sensor distances longer than 8in or in very
noisy environments. Twisted-pair cable lengths can be
between 6ft and 12ft before noise introduces excessive
errors. For longer distances, the best solution is a
shielded twisted pair like that used for audio micro-
phones. For example, Belden 8451 works well for dis-
tances up to 100ft in a noisy environment. At the
device, connect the twisted pair to DXP and DXN and
the shield to GND. Leave the shield unconnected at the
remote sensor. For very long cable runs, the cable’s
parasitic capacitance often provides noise filtering, so
the 2200pF capacitor can often be removed or reduced
in value. Cable resistance also affects remote-sensor
accuracy. For every 1Ω of series resistance, the error is
approximately +1/2°C.
MAX6622
5-Channel Precision Temperature Monitor
16 ______________________________________________________________________________________
5 MILS TO
10 MILS
5 MILS TO
10 MILS
5 MILS TO
10 MILS
MINIMUM
5 MILS TO
10 MILS
GND
DXP
DXN
GND
Figure 5. Recommended DXP-DXN PCB Traces. The two outer
guard traces are recommended if high-voltage traces will be
near the DXN and DXP traces.