adc12041 National Semiconductor Corporation, adc12041 Datasheet - Page 25

• Manufacturer Part Number: adc12041
• Description: 12-bit Plus Sign 216 Khz Sampling Analog-to-digital Converter
• Manufacturer: National Semiconductor Corporation

Analog Application Information
and 22 (DGND) the SSOP and PLCC package. The layout
diagram in 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 ADC12041,
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
shows a layout using a 28-pin PLCC socket and
through-hole assembly. A similar approach should be used
for the SSOP 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 cir-
cuits, and analog signal traces.
The digital ground plane should encompass the area under
the digital circuits and the digital input/output pins of the
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ADC12041. 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 sec-
tions through stray capacitances.
The AGND and DGND in the ADC12041 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 in-
ternal 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 con-
nections 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 sup-
ply traces for the V
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 impedance
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