The DAC904 uses separate pins for its analog and digital
supply and ground connections. The placement of the decou-
pling capacitor should be such that the analog supply (+VA)
is bypassed to the analog ground (AGND), and the digital
supply bypassed to the digital ground (DGND). In most cases
0.1µF ceramic chip capacitors at each supply pin are ad-
equate to provide a low impedance decoupling path. Keep in
mind that their effectiveness largely depends on the proximity
to the individual supply and ground pins. Therefore, they
should be located as close as physically possible to those
device leads. Whenever possible, the capacitors should be
located immediately under each pair of supply/ground pins
on the reverse side of the pc-board. This layout approach will
minimize the parasitic inductance of component leads and
pcb runs.
different supply currents and signal traces. Generally, analog
supply and ground planes should only extend into analog
signal areas, such as the DAC output signal and the refer-
ence signal. Digital supply and ground planes must be
confined to areas covering digital circuitry, including the
digital input lines connecting to the converter, as well as the
clock signal. The analog and digital ground planes should be
joined together at one point underneath the DAC. This can be
realized with a short track of approximately 1/8 inch (3mm).
The power to the DAC904 should be provided through the
use of wide pcb runs or planes. Wide runs will present a
lower trace impedance, further optimizing the supply decou-
pling. The analog and digital supplies for the converter
should only be connected together at the supply connector of
the pc-board. In the case of only one supply voltage being
available to power the DAC, ferrite beads along with bypass
capacitors may be used to create an LC filter. This will
generate a low-noise analog supply voltage that can then be
connected to the +VA supply pin of the DAC904.
Further supply decoupling with surface mount tantalum ca-
pacitors (1µF to 4.7µF) may be added as needed in proximity
of the converter.
Low noise is required for all supply and ground connections
to the DAC904. It is recommended to use a multilayer pc-
board utilizing separate power and ground planes. Mixed
signal designs require particular attention to the routing of the
While designing the layout, it is important to keep the analog
signal traces separate from any digital line, in order to
prevent noise coupling onto the analog signal path.
DAC904
SBAS095C
15
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