Datasheet

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LAYOUT
DAC8554
SLAS431B – JUNE 2005 – REVISED OCTOBER 2006
As with the GND connection, AV
DD
should be
connected to a positive power-supply plane or trace
A precision analog component requires careful
that is separate from the connection for digital logic
layout, adequate bypassing, and clean,
until they are connected at the power-entry point. In
well-regulated power supplies.
addition, a 1 µ F to 10 µ F capacitor in parallel with a
0.1 µ F bypass capacitor is strongly recommended. In
The DAC8554 offers single-supply operation, and it
some situations, additional bypassing may be
will often be used in close proximity with digital logic,
required, such as a 100 µ F electrolytic capacitor or
microcontrollers, microprocessors, and digital signal
even a Pi filter made up of inductors and
processors. The more digital logic present in the
capacitors—all designed to essentially low-pass filter
design and the higher the switching speed, the more
the supply, removing the high-frequency noise.
difficult it is to keep digital noise from appearing at
the output.
Up to four DAC8554 devices can be used on a single
SPI bus without any glue logic to create a high
Due to the single ground pin of the DAC8554, all
channel count solution. Special attention is required
return currents, including digital and analog return
to avoid digital signal integrity problems when using
currents for the DAC, must flow through a single
multiple DAC8554s on the same SPI bus. Signal
point. Ideally, GND would be connected directly to an
integrity of SYNC, SCLK, and D
IN
lines will not be an
analog ground plane. This plane would be separate
issue as long as the rise times of these digital signals
from the ground connection for the digital
are longer than six times the propagation delay
components until they were connected at the
between any two DAC8554 devices. Propagation
power-entry point of the system.
speed is approximately six inches/ns on standard
The power applied to AV
DD
should be well-regulated
printed circuit boards (PCBs). Therefore, if the digital
and low-noise. Switching power supplies and dc/dc
signal rise time is 1ns, the distance between any two
converters often have high-frequency glitches or
DAC8554s has to be further apart on the PCB, and
spikes riding on the output voltage. In addition, digital
the signal rise times should be reduced by placing
components can create similar high-frequency
series resistors at the drivers for SYNC, SCLK, and
spikes. This noise can easily couple into the DAC
D
IN
lines. If the largest distance between any two
output voltage through various paths between the
DAC8554s must be six inches, the rise time should
power connections and analog output.
be reduced to 6ns with an RC network formed by the
series resistor at the digital driver and the total trace
and input capacitance on the PCB.
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