AD5378ABC datasheet(26/28 Pages) AD

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AD5378

Rev. PrA | Page 26 of 28

POWER SUPPLY DECOUPLING

In any circuit where accuracy is important, careful considera-

tion of the power supply and ground return layout helps to

ensure the rated performance. The printed circuit board on

which the AD5378 is mounted should be designed so that the

analog and digital sections are separated and confined to

certain areas of the board. If the AD5378 is in a system where

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.

For supplies with multiple pins (VSS, VDD, VCC), it is recom-

mended to tie these pins together and to decouple each

supply once.

The AD5378 should have ample supply decoupling of 10 µF in

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

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

10 µF capacitors are the tantalum bead type. The 0.1 µF capaci-

tor should have low effective series resistance (ESR) and

effective series inductance (ESI), such as the common ceramic

types that provide a low impedance path to ground at high

frequencies, to handle transient currents due to internal

logic switching.

Digital lines running under the device should be avoided,

because these couple noise onto the device. The analog ground

plane should be allowed to run under the AD5378 to avoid

noise coupling. The power supply lines of the AD5378 should

use as large a trace as possible to provide low impedance paths

and reduce the effects of glitches on the power supply line. Fast

switching digital signals 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. It is essential to mini-

mize noise on all VREF(+) and VREF(−) lines. The VBIAS pin should

be decoupled with a 10 nF capacitor to AGND.

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 through 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 ground plane,

while signal traces are placed on the solder side.

As for all thin packages, care must be taken to avoid flexing the

CSPBGA package and to avoid a point load on the surface of

this package during the assembly process.

POWER-ON

An on-chip power supply monitor makes the AD5378 robust to

power sequencing. The supply monitor powers up the analog

section after (VDD − VSS) is greater than 7 V (typical). The

output buffers power up in CLR mode forced to the DUTGND

potential, even if VCC remains at 0 V. After VSS is applied, the

analog circuitry powers up and the buffered DAC output level

settles linearly within the supply range.

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