G781 datasheet(6/14 Pages) ETC | Â±1Â°C Remote and Local Temperature Sensor with SMBus Serial Interface

6 / 14 page

Ver: 1.0

Oct 02, 2002

TEL: 886-3-5788833

http://www.gmt.com.tw

Global Mixed-mode Technology Inc.

G781

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 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 crossunders 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

thermocouples. In general, PC board-induced ther-

mocouples 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 practi-

cal.

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

shield, and only ferrous materials such as steel work

will. Placing 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

Place the G781 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.

Place the noise filter and the 0.1µF VCC bypass

capacitors close to the G781.

Figure 2. Recommended DXP/DXN PC Traces

Twisted Pair and Shielded Cables

For remote-sensor distances longer than 8 in., or in

particularly noisy environments, a twisted pair is rec-

ommended. Its practical length is 6 feet to 12feet (typi

cal) 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 unterminated.

Excess capacitance at DX_limits practical remote sen-

sor distances (see Typical Operating Characteristics),

For very long cable runs, the cable’s parasitic capaci-

tance often provides noise filtering, so the 2200pF ca-

pacitor can often be removed or reduced in value. Ca-

ble resistance also affects remote-sensor accuracy; 1

Ω

series resistance introduces about + 0.6°C error.

Low-Power Standby Mode

Standby mode disables the ADC and reduces the

supply-current drain to about 10µA. Enter standby

mode by forcing high to the RUN /STOP bit in the con-

figuration byte register. Software standby mode be-

haves such that all data is retained in memory, and the

SMB interface is alive and listening for reads and

writes.

Software standby mode is not a shutdown mode. With

activity on the SMBus, extra supply current is drawn

(see Typical Operating Characteristics). In software

standby mode, the G781 can be forced to perform A/D

conversions via the one-shot command, despite the

RUN

/STOP bit being high.

GND

DXP

DXN

GND

10 MILS

MINIMUM

10 MILS

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