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SS8018

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

contamination must be dealt with carefully, since a

10M

Ω leakage path from DXP to ground causes about

+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

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.

Figure 2. Recommended DXP/DXN PC Traces

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

necting to GND.

pacitors close to the SS8018.

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

Ω series resis-

tance introduces about + 0.6°C error.

GND

DXP

DXN

GND

10 MILS

MINIMUM

10 MILS

10 MILS

10 MILS

Rev.2.01 6/06/2003