Rainbow Electronics MAX6628 - Manual (Page 7)

Filter high-frequency electromagnetic interference
(EMI) at DXP and DXN with an external 2200pF capaci-
tor connected between the two inputs. This capacitor
can be increased to about 3300pF (max), including
cable capacitance. A capacitance higher than 3300pF
introduces errors due to the rise time of the switched-
current source.
PC Board Layout
1) Place the MAX6627/MAX6628 as close as practical
to the remote diode. In a noisy environment, such
as a computer motherboard, this distance can be
4in to 8in, 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 parallel and close to
each other, away from any high-voltage traces such
as +12VDC. Avoid leakage currents from PC board
contamination. A 20M
Ω leakage path from DXP to
ground causes approximately +1°C error.
4) Connect guard traces to GND on either side of the
DXP/DXN traces (Figure 3). With guard traces in
place, routing near high-voltage traces is no longer
an issue.
5) Route as few vias and crossunders as possible 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
solder thermocouple exhibits 3µV/°C, and it takes
approximately 200µV of voltage error at DXP/DXN
to cause a +1°C measurement error, so most para-
sitic thermocouple errors are swamped out.
7) Use wide traces. Narrow traces are more inductive
and tend to pick up radiated noise. The 10mil
widths and spacings recommended in Figure 3 are
not absolutely necessary (as they offer only a minor
improvement in leakage and noise), but use them
where practical.
8) Placing an electrically clean copper ground plane
between the DXP/DXN traces and traces carrying
high-frequency noise signals helps reduce EMI.
Twisted Pair and Shielded Cables
For remote-sensor distances longer than 8in, or in par-
ticularly noisy environments, a twisted pair is recom-
mended. Its practical length is 6ft to 12ft (typ) before
noise becomes a problem, as tested in a noisy elec-
tronics laboratory. For longer distances, the best solu-
tion is a shielded twisted pair like that used for audio
microphones. For example, Belden #8451 works well
for distances up to 100ft in a noisy environment.
Connect the twisted pair to DXP and DXN and the
shield to ground, and leave the shield’s remote end
unterminated. Excess capacitance at DXN or DXP limits
practical remote-sensor distances (see Typical
Operating Characteristics).
For very long cable runs, the cable’s parasitic capaci-
tance often provides noise filtering, so the recommend-
ed 2200pF capacitor can often be removed or reduced
in value. Cable resistance also affects remote-sensor
accuracy. A 1
Ω series resistance introduces about
+1/2°C error.
MAX6627/MAX6628
Remote ±1°C Accurate Digital Temperature
Sensors with SPI-Compatible Serial Interface
_______________________________________________________________________________________ 7
MINIMUM
10mils
10mils
10mils
10mils
GND
DXN
DXP
GND
Figure 3. Recommended DXP/DXN PC Traces
SCK V CC
1
2
8
7
N.C.
SO DXN
DXP
GND
SOT23
TOP VIEW
3
4
6
5
MAX6627
MAX6628
CS
Pin Configuration