adc12d800rfrb National Semiconductor Corporation, adc12d800rfrb Datasheet - Page 47

• Manufacturer Part Number: adc12d800rfrb
• Description: 12-bit, 1.6/1.0 Gsps Rf Sampling Adc
• Manufacturer: National Semiconductor Corporation

17.5 SUPPLY/GROUNDING, LAYOUT AND THERMAL
RECOMMENDATIONS
17.5.1 Power Planes
All supply buses for the ADC should be sourced from a com-
mon linear voltage regulator. This ensures that all power
buses to the ADC are turned on and off simultaneously. This
single source will be split into individual sections of the power
plane, with individual decoupling and connection to the dif-
ferent power supply buses of the ADC. Due to the low voltage
but relatively high supply current requirement, the optimal so-
lution may be to use a switching regulator to provide an
intermediate low voltage, which is then regulated down to the
final ADC supply voltage by a linear regulator. Please refer to
the documentation provided for the ADC12D800RFRB for
additional details on specific regulators that are recommend-
ed for this configuration.
Power for the ADC should be provided through a broad plane
which is located on one layer adjacent to the ground plane(s).
Placing the power and ground planes on adjacent layers will
provide low impedance decoupling of the ADC supplies, es-
pecially at higher frequencies. The output of a linear regulator
should feed into the power plane through a low impedance
multi-via connection. The power plane should be split into in-
dividual power peninsulas near the ADC. Each peninsula
should feed a particular power bus on the ADC, with decou-
pling for that power bus connecting the peninsula to the
ground plane near each power/ground pin pair. Using this
technique can be difficult on many printed circuit CAD tools.
To work around this, zero ohm resistors can be used to con-
47
nect the power source net to the individual nets for the differ-
ent ADC power buses. As a final step, the zero ohm resistors
can be removed and the plane and peninsulas can be con-
nected manually after all other error checking is completed.
17.5.2 Bypass Capacitors
The general recommendation is to have one 100nF capacitor
for each power/ground pin pair. The capacitors should be
surface mount multi-layer ceramic chip capacitors similar to
Panasonic part number ECJ-0EB1A104K.
17.5.3 Ground Planes
Grounding should be done using continuous full ground
planes to minimize the impedance for all ground return paths,
and provide the shortest possible image/return path for all
signal traces.
17.5.4 Power System Example
The ADC12D800RFRB uses continuous ground planes (ex-
cept where clear areas are needed to provide appropriate
impedance management for specific signals), see
Power is provided on one plane, with the 1.9V ADC supply
being split into multiple zones or peninsulas for the specific
power buses of the ADC. Decoupling capacitors are connect-
ed between these power bus peninsulas and the adjacent
ground planes using vias. The capacitors are located as close
to the individual power/ground pin pairs of the ADC as possi-
ble. In most cases, this means the capacitors are located on
the opposite side of the PCB to the ADC.
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Figure
20.