
Ver: 1.0 
Oct 02, 2002 
TEL: 886-3-5788833 
http://www.gmt.com.tw 
6 
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 +12V
DC
. Leakage currents from PC 
board contamination must be dealt with carefully, 
since a 10M
 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
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
10 MILS
10 MILS