AD5379
Rev. B | 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 AD5379 is mounted should be designed so that the
analog and digital sections are separated and confined to
certain areas of the board. If the AD5379 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 (V
SS
, V
DD
, V
CC
), it is recom-
mended to tie these pins together and to decouple each
supply once.
The AD5379 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 AD5379 to avoid
noise coupling. The power supply lines of the AD5379 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 V
REF
(+) and V
REF
(−) lines. The V
BIAS
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 is the case 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 AD5379 robust to
power sequencing. The supply monitor powers up the analog
section after (V
DD
− V
SS
) is greater than 7 V (typical). The
output buffers power-up in
CLR
mode forced to the DUTGND
potential, even if V
CC
remains at 0 V. After V
SS
is applied, the
analog circuitry powers up, and the buffered DAC output level
settles linearly within the supply range.
