AD9430: Grounding strategy

Document created by analog-archivist Employee on Feb 23, 2016
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Should I use separate ground planes for digital- and analog ground in the PCB
for AD9430? If so, is it enough to have separate gnds on the component layer
gnd and let the inductance of the ground vias do the filtering or should all
gnd layers be separated?

 

All ground pins for the ad9430 should be connected together onto the same
ground plane and this ground plane should provide a low inductance path back to
the power supply. It is critical that the ground plane for the AD9430 is quiet
therefore ensure that any high frequency or large currents are routed back via
a separate ground connection. In most systems it is a bad idea to include
inductance between the AGND and DGND pins of a converter. The exception is if
you have a board which contains minimal support circuitry (i.e. the converter,
power supply connections and a buffer). A small inductance between the two
ground pins can give an extra 1dB in SNR performance. However as soon as you
add any significant digital circuitry to you board and route through the
digital ground plane, the inductance causes a voltage drop between analog and
digital ground plane and hence a voltage drop between the AGND and DGND pins of
the converter, increases common mode noise and if you are lucky significantly
reduces SNR performance and if you are unlucky actually prevents the converter
from working at all.

For an excellent discussion on different grounding strategies for high speed
converters refer to our latest seminar on Signal Processing. The final chapter
in all our seminar books is dedicated to hardware design techniques and deals
with such issues as grounding, decoupling, parasitic thermocouples and good PCB
design.

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