adc1204802 National Semiconductor Corporation, adc1204802 Datasheet - Page 29

• Manufacturer Part Number: adc1204802
• Description: 12-bit Plus Sign 216khz 8-channel Sampling Analog-to-digital Converter
• Manufacturer: National Semiconductor Corporation

Analog Application Information
10 µF tantalum electrolytic capacitor in parallel with a 0.1 µF
monolithic ceramic capacitor is recommended for bypassing
each power supply. The key consideration for these capaci-
tors is to have low series resistance and inductance. The
capacitors should be placed as close as physically possible
to the supply and ground pins with the smaller capacitor
closer to the device. The capacitors also should have the
shortest possible leads in order to minimize series lead
inductance. Surface mount chip capacitors are optimal in
this respect and should be used when possible.
When the power supply regulator is not local on the board,
adequate bypassing (a high value electrolytic capacitor)
should be placed at the power entry point. The value of the
capacitor depends on the total supply current of the circuits
on the PC board. All supply currents should be supplied by
the capacitor instead of being drawn from the external sup-
ply lines, while the external supply charges the capacitor at a
steady rate.
The ADC has two V
that each of these V
DGND with a 0.1 µF plus a 10 µF capacitor. The layout
diagram of Figure 21 shows the recommended placement
for the supply bypass capacitors.
PC BOARD LAYOUT AND GROUNDING
CONSIDERATlONS
To get the best possible performance from the ADC12048,
the printed circuit boards should have separate analog and
digital ground planes. The reason for using two ground
planes is to prevent digital and analog ground currents from
sharing the same path until they reach a very low impedance
power supply point. This will prevent noisy digital switching
currents from being injected into the analog ground.
Figure 21 illustrates a favorable layout for ground planes,
power supply and reference input bypass capacitors. It
(Continued)
D
+ and DGND pins. It is recommended
D
+ pins be separately bypassed to
29
shows a layout using a 44-pin PLCC socket and
through-hole assembly. A similar approach should be used
for the PQFP package.
The analog ground plane should encompass the area under
the analog pins and any other analog components such as
the reference circuit, input amplifiers, signal conditioning
circuits, and analog signal traces.
The digital ground plane should encompass the area under
the digital circuits and the digital input/output pins of the
ADC12048. Having a continuous digital ground plane under
the data and clock traces is very important. This reduces the
overshoot/undershoot and high frequency ringing on these
lines that can be capacitively coupled to analog circuitry
sections through stray capacitances.
The AGND and DGND in the ADC12048 are not internally
connected together. They should be connected together on
the PC board right at the chip. This will provide the shortest
return path for the signals being exchanged between the
internal analog and digital sections of the ADC.
It is also a good design practice to have power plane layers
in the PC board. This will improve the supply bypassing (an
effective distributed capacitance between power and ground
plane layers) and voltage drops on the supply lines. How-
ever, power planes are not as essential as ground planes are
for satisfactory performance. If power planes are used, they
should be separated into two planes and the area and
connections should follow the same guidelines as mentioned
for the ground planes. Each power plane should be laid out
over its associated ground planes, avoiding any overlap
between power and ground planes of different types. When
the power planes are not used, it is recommended to use
separate supply traces for the V
impedance supply point (the regulator output or the power
entry point to the PC board). This will help ensure that the
noisy digital supply does not corrupt the analog supply.
A
+ and V
D
+ pins from a low
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