The first time I built a simple DC board for powering a small group of sensors, I was astounded at the level of noise in my voltage measurements. Surely, the $5,000 meter I was using couldn’t be faulty, and the output fluctuations from the power supply were lower than the noise being measured, so how could my signals be experiencing such noise problems?
Little did I know, some poor design decisions on my simple DC board created the opportunity for noise to build up throughout my board. Working with the right design and simulation software, as well as some smarter design decisions, would have saved weeks worth of time and headaches.
Power and Ground Planes in Power Integrity Analysis
Your power and ground planes are critical for completing a circuit throughout your PCB. The resistance of these planes should be made as small as possible, which can be difficult given their span and thinness. This increases their resistance, which can lead to two effects.
First, power is lost via IR drop in the power plane and traces that deliver power to components. This causes a slight drop in the voltage seen by downstream components. Likewise, ground loops can flow through various components in your board if the resistance across your ground plane is too large.
Ground loops arise due to potential differences between two grounding points in a power plane. This causes current to flow backwards through the circuit, which can interfere with signals that are referenced to the ground plane. Natural fluctuations in the ground plane potential due to Johnson noise, 1/f noise, any noise induced due to EMI, ripple, or any other spontaneous variations in the output voltage and ground reference also cause fluctuations in your ground loop currents. These fluctuations then back-couple inductively into the remainder of the board.
Identifying Power Integrity Problems
Because a ground plane has nonzero resistance, there will inevitably be some variation in the potential across a ground plane and a power plane. As a designer, and for manufacturers, your goal is to identify these potential differences and minimize them through proper design techniques.
You can identify IR loss and possible ground loops using a single tool that simulates the power distribution network throughout your board. The power distribution network includes the ground plane, allowing you to identify excessive IR drop and potential differences in your ground plane simultaneously. A good power delivery network analyzer will show you the potential and current throughout your power and planes, as well as your power rails and any voltage dropped by vias.
This type of output from a power delivery network is normally superimposed on your layout using a color map. This makes it very easy to visually identify any pairs of points that may exhibit high potential differences, possibly leading to ground loops. Pay attention to vias that are used for signal returns to your power plane as these can drop more voltage than the traces connected to them.
As a rule of thumb, potential differences in your ground plane should reach sub-mV levels. A difference in ground plane potential as small as 1 mV is enough to cause ground loops that interfere with sensitive DC measurements, although this will not be large enough to cause involuntary switching in digital circuits. Noise induced by thermal or power supply-dependent fluctuations in the ground loop potential can be reduced by using a large capacitor across the power supply outputs or across the power and ground plane connections to the power supply.
Design Guidelines to Ensure Power Integrity
Going further to reduce noise created by grounding problems and ensuring power integrity throughout your device depends in part on the type of power supply you use. For example, a switching power supply generates its own RF noise due to the switching of ICs. Unregulated power supplies output significant ripple in addition to their intended power output, as do regulated power supplies to a lesser extent.
These variations cannot be solved completely at the PCB level, although a designer can make an attempt to filter out noise at the ripple or RF switching frequencies. You should always use best design practices to protect critical signal traces and power rails from EMI, regardless of whether this EMI originates from other components on a board, from a power supply, or externally.
Ground loops, as well as a similar phenomenon called ground bounce, can arise for similar reasons in a PCB due to routing signals through a ground plane. As was mentioned earlier, vias can drop a large amount of voltage when they are added as a current return path to a ground plane.
Vias have some natural inductance (usually at the nH level), causing their impedance to vary with signal frequency. Vias providing a return path to ground can create impedance discontinuities, especially with digital and high frequency analog signals. This causes signal reflection at various vias throughout the board.
Finally, when a large number of signals in a net or across components switch simultaneously, mutual inductance among vias causes a signal to be induced back through the circuit. This counters the current flowing to ground. According to Ohm’s law, this requires that the ground potential increase near the via, which could lead to involuntary switching in upstream digital circuits. This is the essence of ground bounce, and the ground potential difference that arises due to ground bounce can be identified with a power delivery analysis tool.
Identifying power integrity problems, their influence on signal integrity, and determining the best options for correcting them takes PCB design software with the best simulation and analysis tools on the market. Altium Designer contains all of these features and many more in a single interface. The signal integrity tools come standard with Altium Designer, and a powerful PDN analyzer is available as an add-on that is conveniently available within the program.
Now you can download a free trial of Altium Designer if you’re interested in learning more about its signal and power integrity analysis tools. You’ll also have access to the industry’s best layout, routing, and data management features. Talk to an Altium expert today to learn more.