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參數(shù)資料
| 型號： | G766 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | Remote Temperature Sensor with SMBus Serial Interface |
| 中文描述： | 遠(yuǎn)程溫度傳感器，帶有SMBus串行接口 |
| 文件頁數(shù)： | 6/13頁 |
| 文件大小： | 248K |
| 代理商： | G766 |
Ver 1.0
Dec 11, 2001
TEL: 886-3-5788833
http://www.gmt.com.tw
6
Global Mixed-mode Technology Inc.
G766
Table 1. Remote-Sensor Transistor Manufacturers
MANUFACTURER
Philips
Motorola(USA)
National Semiconductor(USA)
Note:Transistors must be diode-connected (base
shorted to collector).
MODEL NUMBER
PMBS3904
MMBT3904
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. Micropower
operation places constraints on high-frequency noise
rejection; therefore, careful PC board layout and prop-
er 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 G766 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 +12V
DC
. Leakage currents from PC
board contamination must be dealt with carefully,
since a 20M
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
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 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 G766 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 G766.
Add a 200
resistor in series with Vcc for best
noise filtering (see Typical Operating Circuit).
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