I am designing a PCB with 7 (seven!) AD7731 a/d
chips and am not sure which is the best point or
points to join analogue and digital ground planes.
a) one join at the 4th 7731 ?
b) 7 seperate joins at each 7731 ?
c) another way ?
The pin marked DGND on the AD7731 is the return path for the digital current
out of the AD7731. It does not necessarily mean that this pin should be
connected to the digital ground plane of your system.
Connect the AGND pin and the DGND pins of each individual IC together with a
broad, low inductance track. You can now treat each of these seven nodes as a
single analog ground connection to be routed onto your system analog ground
plane. You should provide a separate ground return path back to the power
supply for each converter in the system. If each AD7730 has 7mA of analog
current and 1mA digital, providing separate return paths means that there is
only ever 1mA digital current in each converters local ground plane. You can do
this by flooding the analog ground plane and sectioning it off with cuts to
provide a local ground return path for each converter. The digital current from
the AD7731 is relatively small in comparison to the analog current and provided
you use low inductance ground paths this small amount of digital current
should not adversely affect the noise floor.
You should ensure that AVDD is decoupled to AGND and DVDD is decoupled to DGND.
The system analog ground plane should be kept separate from the system digital
ground plane, which routes the return current for the "spikey", digital
components such as micro-controller, digital buffers, logic etc.
The connection between the system digital ground and system analog ground
should be made at a single point (commonly referred to as the "star point").
Where you place the star point will depend on your system but this is often
determined by the power supply you use. The single connection between analog
and digital ground is already made at the power supply. Your job as a designer
is to ensure that no digital current is allowed to flow in the analog ground
plane while ensuring that the potential of the analog and digital grounds in
your system remain equal.
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.