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G781 Datasheet(PDF) 6 Page - List of Unclassifed Manufacturers |
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G781 Datasheet(HTML) 6 Page - List of Unclassifed Manufacturers |
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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 +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 10 MILS 10 MILS |
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