QI have bought AD9243 evaluation board and i would like to have some pieces of
information about grounding; i've seen there are
5 sets of pads and a jumper to connet analog ground and digital ground of PCB;
besides, i seem that the pads below ADC's IC are
connected. Now, am i supposed to connect the other set of pads to one another
as well? Last but not least, am i supposed to connect
the AGND and DGND outputs respectively to the analog and digital grounds of
the whole system, or need i connect AGND and DGND
together to the analog ground of the whole system?
AIt is a common mistake to assume that the DGND pin on an IC is an instruction
to connect this pin to the digital ground plane of your system. This is
DEFINITELY NOT the case.
The pin marked DGND on an IC is intended to let the system designer know that
this is where the return current for the digital circuitry on this IC comes
out. It's then up to the system designer to decide how to route this current
back to it's source. Some thing which you definitely do not want to do is
connect this pin (which is after all connected to the same substrate as you
precision converter) to a ground plane dedicated to a noisy digital ground
plane which is dedicated to the return of fast switching, high current digital
circuitry. PCB ground planes do not have zero impedance, and at high frequency
they start to look inductive, fast current pulses through an inductor causes
large potential differences. Most digital circuitry does not care about these
potential differences in the ground plane because of the inherent high noise
margin in digital circuits. But, your 12,14 or 16 bit precision converter
requires that the potential difference between it's AGND and DGND pins is
zero…. always and especially during a conversion.
I recommend that you connect AGND and DGND together directly at the ADC. Treat
this node as an analog return path and route it separately over a wide ground
plane back to the power supply. Do the same for any other ADCs you have in the
system, connect AGND and DGND together directly at the IC and route it
separately back to the power supply. This will mean that you have separate
ground return paths for each ADC.
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.