DAC900-Q1

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SBAS505 – JUNE 2010

Grounding, Decoupling, and Layout Information

Proper grounding and bypassing, short lead length, and the use of ground planes are particularly important for

high frequency designs. Multilayer pc-boards are recommended for best performance since they offer distinct

advantages such as minimization of ground impedance, separation of signal layers by ground layers, etc.

The DAC900 uses separate pins for its analog and digital supply and ground connections. The placement of the

decoupling 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 0.1uF ceramic chip capacitors at each

supply pin are adequate 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.

Further supply decoupling with surface mount tantalum capacitors (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 DAC900. It is recommended to use a

multilayer pcboard utilizing separate power and ground planes. Mixed signal designs require particular attention

to the routing of the 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 reference 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" (3mm).

The power to the DAC900 should be provided through the use of wide pcb runs or planes. Wide runs will present

a lower trace impedance, further optimizing the supply decoupling. 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 DAC900.

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.

Copyright © 2010, Texas Instruments Incorporated

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Product Folder Link(s): DAC900-Q1