G768B View Datasheet(PDF) - Global Mixed-mode Technology Inc
Part Name
Description
Manufacturer
G768B
Global Mixed-mode Technology Inc
G768B Datasheet PDF : 15 Pages
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Global Mixed-mode Technology Inc.
G768B
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.
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.
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 20MΩ leakage path from DXP to ground
causes about +1°C error.
GND
DXP1
DXN
DXN
DXP2
DXP1
DXN
G768B
DXP2
GND
Chip Boundary
Fig 2(a) Connect the common DXN as close as
possible to the DXN pin on IC.
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
GND
(Figure 2a).
Connect guard traces to GND on either side of the
DXP-DXN traces (Figure 2b). With guard traces in
10 MILS
DXP
place, routing near high-voltage traces is no longer an
issue.
10 MILS
DXN
10 MILS
MINIMUM
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
GND
10 MILS
Fig 2 (b) Recommended DXP/DXN PC
Ver 1.3
Oct 28, 2002
TEL: 886-3-5788833
http://www.gmt.com.tw
7
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