AD8021 datasheet(24/32 Pages) AD

24 / 32 page

AD5429/AD5439/AD5449

Rev. 0 | Page 24 of 32

PCB LAYOUT AND POWER SUPPLY DECOUPLING

In any circuit where accuracy is important, careful consider-

ation of the power supply and ground return layout helps to

ensure the rated performance. The printed circuit board on

which the DAC is mounted should be designed so that the

analog and digital sections are separated and confined to

certain areas of the board. If the DAC is in a system in which

multiple devices require an AGND to DGND connection, the

connection should be made at one point only. The star ground

point should be established as close as possible to the device.

These DACs should have ample supply bypassing of 10 µF

in parallel with 0.1 µF on the supply located as close to the

package as possible, ideally right up against the device. The

0.1 µF capacitor should have low effective series resistance

(ESR) and effective series inductance (ESI), like the common

ceramic types that provide a low impedance path to ground at

high frequencies, to handle transient currents due to internal

logic switching. Low ESR 1 µF to 10 µF tantalum or electrolytic

capacitors should also be applied at the supplies to minimize

transient disturbance and filter out low frequency ripple.

Fast switching signals such as clocks should be shielded with

digital ground to avoid radiating noise to other parts of the

board, and should never be run near the reference inputs.

Avoid crossover of digital and analog signals. Traces on opposite

sides of the board should run at right angles to each other. This

reduces the effects of feedthrough on the board. A microstrip

technique is by far the best, but not always possible with a

double-sided board. In this technique, the component side of

the board is dedicated to the ground plane, while signal traces

are placed on the soldered side.

It is good practice to employ compact, minimum lead-length

PCB layout design. Leads to the input should be as short as

possible to minimize IR drops and stray inductance.

The PCB metal traces between VREF and RFB should also be

matched to minimize gain error. To maximize high frequency

performance, the I-to-V amplifier should be located as close to

the device as possible.

POWER SUPPLIES FOR THE EVALUATION BOARD

The board requires ±12 V and +5 V supplies. The 12 V VDD

and VSS are used to power the output amplifier, while the 5 V

is used to power the DAC (VDD1) and transceivers (VCC).

Both supplies are decoupled to their respective ground plane

with 10 µF tantalum and 0.1 µF ceramic capacitors.

EVALUATION BOARD FOR THE DACS

The evaluation board includes a DAC from the AD5429/

AD5439/AD5449 family and a current-to-voltage amplifier,

AD8065. On the evaluation board is a 10 V reference, ADR01.

An external reference can also be applied via an SMB input.

The evaluation kit consists of a CD-ROM with self-installing

PC software to control the DAC. The software allows the user to

write a code to the device.

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