DAC1221 Datasheet, PDF (14/14 Pages) Burr-Brown (TI) – 16-Bit Low Power DIGITAL-TO-ANALOG CONVERTER

LAYOUT
POWER SUPPLIES
The DAC1221 requires the digital supply (DV_DD) to be no
greater than the analog supply (AV_DD) +0.3V. In the majority
of systems, this means that the analog supply must come up
first, followed by the digital supply and V_REF. Failure to
observe this condition could cause permanent damage to the
DAC1221.
Inputs to the DAC1221, such as SDIO or V_REF, should not
be present before the analog and digital supplies are on.
Violating this condition could cause latch-up. If these sig-
nals are present before the supplies are on, series resistors
should be used to limit the input current.
The best scheme is to power the analog section of the design
and AV_DD of the DAC1221 from one +3V supply, and the
digital section (and DV_DD) from a separate +3V supply. The
analog supply should come up first. This will ensure that
SCLK, SDIO, CS and V_REF do not exceed AV_DD, that the
digital inputs are present only after AV_DD has been estab-
lished, and that they do not exceed DV_DD.
The analog supply should be well regulated and low noise.
For designs requiring very high resolution from the DAC1221,
power supply rejection will be a concern. See the “PSRR vs
Frequency” curve in the Typical Performance Curves sec-
tion of this data sheet for more information.
The requirements for the digital supply are not as strict.
However, high frequency noise on DV_DD can capacitively
couple into the analog portion of the DAC1221. This noise
can originate from switching power supplies, very fast
microprocessors, or digital signal processors.
If one supply must be used to power the DAC1221, the
AV_DD supply should be used to power DV_DD. This connec-
tion can be made via a 10Ω resistor which, along with the
decoupling capacitors, will provide some filtering between
DV_DD and AV_DD. In some systems, a direct connection can
be made. Experimentation may be the best way to determine
the appropriate connection between AV_DD and DV_DD.
GROUNDING
The analog and digital sections of the design should be
carefully and cleanly partitioned. Each section should have
its own ground plane with no overlap between them. AGND
should be connected to the analog ground plane, as well as
all other analog grounds. DGND should be connected to the
digital ground plane, and all digital signals referenced to this
plane.
The DAC1221 pinout is such that the converter is cleanly
separated into an analog and digital portion. This should
allow simple layout of the analog and digital sections of the
design.
For a single converter system, AGND and DGND of the
DAC1221 should be connected together, underneath the
converter. Do not join the ground planes. Instead, connect
the two with a moderate signal trace. For multiple convert-
ers, connect the two ground planes at one location, as central
to all of the converters as possible. In some cases, experi-
mentation may be required to find the best point to connect
the two planes together. The printed circuit board can be
designed to provide different analog/digital ground connec-
tions via short jumpers. The initial prototype can be used to
establish which connection works best.
DECOUPLING
Good decoupling practices should be used for the DAC1221
and for all components in the design. All decoupling capaci-
tors, and specifically the 0.1µF ceramic capacitors, should
be placed as close as possible to the pin being decoupled. A
1µF to 10µF capacitor, in parallel with a 0.1µF ceramic
capacitor, should be used to decouple AV_DD to AGND. At a
minimum, a 0.1µF ceramic capacitor should be used to
decouple DV_DD to DGND, as well as for the digital supply
on each digital component.
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DAC1221
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