DAC1221 datasheet(14/17 Pages) TI2

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DAC1221

LAYOUT

POWER SUPPLIES

The DAC1221 requires the digital supply (DVDD) to be no

greater than the analog supply (AVDD) +0.3V. In the majority

of systems, this means that the analog supply must come up

first, followed by the digital supply and VREF. Failure to

observe this condition could cause permanent damage to the

DAC1221.

Inputs to the DAC1221, such as SDIO or VREF, 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 AVDD of the DAC1221 from one +3V supply, and the

digital section (and DVDD) from a separate +3V supply. The

analog supply should come up first. This will ensure that

SCLK, SDIO, CS and VREF do not exceed AVDD, that the

digital inputs are present only after AVDD has been estab-

lished, and that they do not exceed DVDD.

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 DVDD 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

AVDD supply should be used to power DVDD. This connec-

tion can be made via a 10

Ω resistor which, along with the

decoupling capacitors, will provide some filtering between

DVDD and AVDD. In some systems, a direct connection can

be made. Experimentation may be the best way to determine

the appropriate connection between AVDD and DVDD.

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 0.1

µF ceramic capacitors, should

be placed as close as possible to the pin being decoupled. A

µF to 10µF capacitor, in parallel with a 0.1µF ceramic

capacitor, should be used to decouple AVDD to AGND. At a

minimum, a 0.1

µF ceramic capacitor should be used to

decouple DVDD to DGND, as well as for the digital supply

on each digital component.

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