Datasheet

Data Sheet AD5535B
Rev. A | Page 15 of 16
POWER SUPPLY DECOUPLING RECOMMENDATIONS
On the AD5535B, it is recommended to tie all grounds together
as close to the device as possible. If the number of supplies must be
reduced, bring all supplies back separately and make a provision
on the board via a link option to drive the AV
CC
and V
+
pins
from the same supply. Decouple all power supplies adequately
with 10 µF tantalum capacitors and 0.1 µF ceramic capacitors.
GUIDELINES FOR PCB LAYOUT
Design printed circuit boards such that the analog and digital
sections are separated and confined to the designated analog
and digital sections of the board. This facilitates the use of ground
planes that can be separated easily. A minimum etch technique
is generally the best for ground planes because it optimizes
shielding of sensitive signal lines. Join digital and analog ground
planes in one place only, at the AGND and DGND pins of the
high resolution converter. To isolate the high frequency bus of
the processor from the bus of the high resolution converters, buffer
or latch data and address buses on the board. These act as a
Faraday shield and increase the signal-to-noise performance of
the converters by reducing the amount of high frequency digital
coupling. Avoid running digital lines under the device because
they couple noise onto the die. Allow the ground plane to run
under the IC to avoid noise coupling.
Use as large a trace as possible for the supply lines of the device
to provide low impedance paths and reduce the effects of glitches
on the power supply line. Shield components, such as clocks with
fast-switching signals, with digital ground to avoid radiating noise
to other sections of the board. Never run clock signals near the
analog inputs of the device. Avoid crossovers of digital and analog
signals. Keep traces for analog inputs as wide and short as possible
and shield with analog ground if possible. Run traces on opposite
sides of the 2-layer PCB at right angles to each other to reduce the
effects of feedthrough through the board.
A microstrip technique is by far the best, but it is not always
possible to use with a double-sided board. In this technique,
the component side of the board is dedicated to ground planes,
and signals are placed on the solder side. Multilayer printed
circuit boards with dedicated ground, power, and tracking
layers offer the optimum solution in terms of obtaining analog
performance, but at increased manufacturing costs.
Good decoupling is vitally important when using high resolu-
tion converters. Decouple all analog supplies with 10 µF tantalum
capacitors in parallel with 0.1 µF ceramic capacitors to analog
ground. To achieve the best results from the decoupling components,
place them as close to the device as possible, ideally right up
against the IC or the IC socket. The main aim of a bypassing
element is to maximize the charge stored in the bypass loop
while simultaneously minimizing the inductance of this loop.
Inductance in the loop acts as an impedance to high frequency
transients and results in power supply spiking. By keeping the
decoupling as close to the device as possible, the loop area is
kept as small as possible, thereby reducing the possibility of
power supply spikes. Decouple digital supplies of high resolution
converters with 10 µF tantalum capacitors and 0.1 µF ceramic
capacitors to the digital ground plane. Decouple the V
+
supply
with a 10 µF tantalum capacitor and a 0.1 µF ceramic capacitor
to AGND.
Decouple all logic chips with 0.1 µF ceramic capacitors to digital
ground to decouple high frequency effects associated with
digital circuitry.