Rainbow Electronics MAX6636 - Manual (Page 16)

ADC Noise Filtering
The integrating ADC has good noise rejection for low-
frequency signals, such as power-supply hum. In envi-
ronments with significant high-frequency EMI, connect
an external 2200pF capacitor between DXP_ and
DXN_. Larger capacitor values can be used for added
filtering, but do not exceed 3300pF because it can
introduce errors due to the rise time of the switched
current source. High-frequency noise reduction is
needed for high-accuracy remote measurements.
Noise can be reduced with careful PCB layout as dis-
cussed in the
PCB Layout section.
Slave Address
The 7-bit slave address of the MAX6636UP9A+ is
1001 101 (binary).
PCB Layout
Follow these guidelines to reduce the measurement
error when measuring remote temperature:
1) Place the MAX6636 as close as is practical to the
remote diode. In noisy environments, such as a
computer 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 pos-
sible to minimize copper/solder thermocouple
effects.
5) Use wide traces when practical. 5mil to 10mil 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 .
MAX6636
7-Channel Precision Temperature Monitor
16 ______________________________________________________________________________________
5–10 mils
5–10 mils
5–10 mils
MINIMUM
5–10 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.
MANUFACTURER MODEL NO.
Central Semiconductor (USA) CMPT3904
Rohm Semiconductor (USA) SST3904
Samsung (Korea) KST3904-TF
Siemens (Germany) SMBT3904
Zetex (England) FMMT3904CT-ND
Table 10. Remote-Sensors Transistor
Manufacturer
Note: Discrete transistors must be diode connected (base
shorted to collector).