Page 19 - EngineerIT October 2022

ELECTRONICS

In general, high performance data collecting systems
should have four or more layers. The top layer is often used
for digital/analogue signals, while the bottom layer is used
for auxiliary signals. The second layer (ground layer) serves
as a reference plane for impedance-controlled signals and is
used for reducing IR drops and shielding the digital signals in
the top layer. Finally, the power plane is on the third layer.
Power and ground planes must be adjacent to each other
as they provide additional interplane capacitance, which
helps with the high frequency decoupling of the power
supply.
For the ground layer, advice has changed over the years Figure 4: Return current for system with a solid ground plane.
for the mixed-signal design. Over the years, splitting the
ground plane between analog and digital has made sense the ground reference points of the devices. However, for
but for modern mixed-signal devices, a new approach is higher frequency signals, a certain portion of the return
recommended. Proper floor planning and separating signals current will try to follow the signal path back. This is
should prevent any issues with noisy signals. because the impedance is lower following this path, since
it minimises the size of the loop formed between the
Ground plane: To split or not to split? outgoing and returning current.
Grounding is an essential process in the layout design of a
mixed-signal PCB. A typical 4-layer PCB must have at least Analogue and digital ground separation
one layer dedicated to the ground plane to ensure a low For complex systems where a solid ground scheme can
impedance path for return signals. All integrated circuit be challenging to achieve, a split ground may be more
ground pins should be routed and connected correctly to appropriate. A split ground plane is another popular
the low impedance ground plane to minimise the series approach where the ground plane is split in two: analogue
inductance and resistance. and digital ground planes. This is applicable for more complex
It has become a standard grounding approach for mixed- systems with multiple mixed-signal devices consuming high
signal systems to separate analogue and digital ground. digital current. Figure 5 shows an example of a system with a
However, mixed-signal devices with low digital current can split ground plane.
be best managed with a single ground. Moving forward, a For systems with a split ground plane, the simplest
designer must consider which grounding practice suits best, solution to achieve a cohesive ground is to remove the break
depending on the mixed-signal current requirement. There in the ground planes and allow the return current to take a
are two grounding practices a designer must consider. more direct route through a star ground junction. The star
ground is the junction where the analogue and digital ground
Single ground plane planes are joined together on a mixed-signal layout design.
For mixed-signal systems with a single ADC or DAC with low In common systems, the star ground can be associated
digital current, a solid single ground plane would be the best with a simple, narrow, continued junction in between
approach. To understand the importance of a single ground
layer, we need to recall return current. Return current is
the current that flows while returning to ground and traces
between devices to complete a loop. To prevent mixed-signal
interference, each return path must be tracked throughout
the PCB layout.
The simple circuit in Figure 4 shows the advantage of a
single solid ground plane over a split ground plane. The signal
current has an equal but opposite flow of return current. This
return current flows in the ground plane back to the source
and it will follow the path of least impedance.
For low frequency signals, the return current will take
the path of least resistance, usually a straight line between Figure 5: Return current for systems with a split ground plane.

EngineerIT | October 2022 | 17