G768 datasheet(7/15 Pages) ETC

7 / 15 page

Ver 1.3

Oct 28, 2002

TEL: 886-3-5788833

http://www.gmt.com.tw

G768B

Global Mixed-mode Technology Inc.

ADC Noise Filtering

The ADC is an integrating type with inherently good

noise rejection, especially of low-frequency signals

such as 60Hz/120Hz power-supply hum. Micro-power

operation places constraints on high-frequency noise

rejection; therefore, careful PC board layout and

proper external noise filtering are required for

high-accuracy remote measurements 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 (see Typical Operating Characteristics).

PC Board Layout

Place the G768B 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 +12VDC. Leakage currents from PC

board contamination must be dealt with carefully,

since a 20M

Ω leakage path from DXP to ground

causes about +1°C error.

Route the 2 pairs of DXP1-DXN and DXP2-DXN

traces independently (Figure 2a). Connect the com-

mon DXN as close as possible to the DXN pin on IC

(Figure 2a).

Connect guard traces to GND on either side of the

DXP-DXN traces (Figure 2b). With guard traces in

place, routing near high-voltage traces is no longer an

issue.

Route through as few vias and crossunders as possi-

ble to minimize copper/solder thermocouple effects.

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 about

200µV 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 practi-

cal.

Keep in mind that copper can't be used as an EMI

shield, and only ferrous materials such as steelwork

will. Placing a copper ground plane between the

DXP-DXN traces and traces carrying high-frequency

noise signals do not help reduce EMI.

PC Board Layout Checklist

Place the G768B 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-

necting to GND.

Route two DXPx-DXN pairs independently

Connect the common DXN as close as possible to

the DXN pin on IC.

Place the noise filter and the 0.1F Vcc bypass

capacitors close to the G768B.

Fig 2(a) Connect the common DXN as close as

possible to the DXN pin on IC.

Fig 2 (b) Recommended DXP/DXN PC

DXP1

DXN

DXP2

DXP1

DXN

G768B

DXP2

GND

Chip Boundary

GND

DXP1

DXN

DXP2

DXP1

DXN

G768B

DXP2

GND

Chip Boundary

GND

DXP

DXN

GND

10 MILS

MINIMUM

10 MILS

GND

DXP

DXN

GND

10 MILS

MINIMUM

10 MILS

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