SS8018TR SSC [Silicon Standard Corp.], SS8018TR Datasheet - Page 6

• Manufacturer Part Number: SS8018TR
• Manufacturer: SSC [Silicon Standard Corp.]

Rev.2.01 6/06/2003
ADC Noise Filtering
The ADC is an integrating type with inherently good
noise rejection. Micro-power operation places con-
straints on high-frequency noise rejection; therefore,
careful PC board layout and proper external noise filter-
ing are required for high-accuracy remote measure-
ments in electrically noisy environments.
High-frequency EMI is best filtered at DXP and DXN
with an external 2200pF capacitor. This value can be
increased to about 3300pF (max), including cable ca-
pacitance. Higher capacitance than 3300pF introduces
errors due to the rise time of the switched current
source.
Nearly all noise sources tested cause the ADC meas-
urements to be higher than the actual temperature,
typically by +1°C to 10°C, depending on the frequency
and amplitude.
PC Board Layout
Place the SS8018 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.
Do not route the DXP-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.
Route the DXP and DXN traces in parallel and in close
proximity to each other, away from any high-voltage
traces such as +12V
contamination must be dealt with carefully, since a
10M
+1°C error.
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.
Route through as few vias and cross-unders as
possible to minimize copper/solder thermocouple ef-
fects.
When introducing a thermocouple, make sure that both
the DXP and the DXN paths have matching thermocou-
ples. In general, PC board-induced thermocouples are
not a serious problem, A copper-solder thermocouple
exhibits 3µV/°C, and it takes about 240µV of voltage
error at DXP-DXN to cause a +1°C measurement error.
So, most parasitic thermocouple errors are swamped
out.
Use wide traces. Narrow ones are more inductive and
leakage path from DXP to ground causes about
DC
. Leakage currents from PC board
www.SiliconStandard.com
tend to pick up radiated noise. The 10 mil widths and
spacing recommended on Figure 2 aren’t absolutely
necessary (as they offer only a minor improvement in
leakage and noise), but try to use them where practical.
Keep in mind that copper can’t be used as an EMI shield,
and only ferrous materials such as steel work will. Plac-
ing a copper ground plane between the DXP-DXN
traces and traces carrying high-frequency noise signals
does not help reduce EMI.
PC Board Layout Checklist
n
n
n
n
n
n
n
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 12feet (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. Connect the twisted pair to DXP and DXN
and the shield to GND, and leave the shield’s remote
end un-terminated.
Excess capacitance at DX limits practical remote sensor
distances (see Typical Operating Characteristics). 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; 1
tance introduces about + 0.6°C error.
necting to GND.
Place the SS8018 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 spacing.
Place a ground plane under the traces
Use guard traces flanking DXP and DXN and con-
Place the noise filter and the 0.1µF VCC bypass ca-
pacitors close to the SS8018.
10 MILS
Figure 2. Recommended DXP/DXN PC Traces
10 MILS
GND
GND
DXP
DXN
series resis-
SS8018
10 MILS
10 MILS
MINIMUM
6 of 14