A circuit design can only be deemed “successful” after it’s demonstrated on its printed circuit board (PCB) assembly. There are several important guidelines needed to produce a good PCB that minimizes noise and avoids oscillations. Decoupling capacitors and their electrical connections provide that function.
The importance of decoupling capacitors
As a full-time professor of electrical engineering and electronics engineering technology, I was known as “Professor” to my colleagues at Sundstrand, where I worked part-time as an electronics engineer. They weren’t shy about coming to me for help.
My new cubemate, Dave, had just graduated from an excellent engineering school. He was bright and capable. We were both working in Ground Support Equipment (GSE) and Dave had just finished his first design.
Dave approached me sheepishly. “I need your help, Professor.”
I looked up from my work and responded, “What’s up, Dave?”
“My first design just shipped. I just realized I did not use any decoupling capacitors.”
Startled, I replied, “I thought our manager, Barry, looked over your design and approved it.”
Dave lamented, “Yes, he did, but decoupling capacitors are usually shown as a table on the last page of the schematic. He assumed I did not provide it because it’s so routine.” I joked, “Well, Dave, fewer components mean the reliability is increased.” Dave was in no mood for humor.
Dave and I chatted some more. His unit was going to be used in an electrically quiet lab environment and the unit had passed its Acceptance Test Procedure (ATP). I assured Dave everything would most likely be fine and suggested that he inform Barry after we went over some design basics.
How to use decoupling capacitors
Dave pulled up a chair and I began a brief tutorial. Frustrated, he began. “I did well in my engineering classes, but decoupling capacitors were never mentioned.” I replied, “Even good engineering programs may not necessarily emphasize functioning circuits.”
I explained the notion of a decoupling capacitor is based on two fundamentals: circuit elements in parallel experience the same voltage, and capacitors oppose a change in voltage. Decoupling capacitors are connected across circuits—typically integrated circuits—to prevent any noise that disturbs the DC supply voltage from reaching the powered circuits. Also, if a given circuit produces noise, the decoupling capacitor prevents that noise from affecting other circuits which share the same DC power supply. Since the circuits share a common DC power supply, it is possible stages will interact to produce oscillations. The decoupling capacitors behave like localized energy reserves.
In addition to its capacitance (C) the decoupling capacitor sports an equivalent series resistance (ESR) and an equivalent series inductance (ESL). The equivalent circuit is shown in the image below. Basically, we have a series-resonant circuit with the frequency response also shown below.
The decoupling capacitor provides the idealized response at low frequencies up to resonance. Beyond resonance, the capacitor’s impedance increases. If the frequencies above resonance are a concern, smaller value capacitors are placed in parallel.
This is the shown in the below image. The electrolytic capacitor handles the low-frequency decoupling. Since it becomes inductive above its resonant frequency, small disc ceramic capacitors are placed in parallel. They shunt the high-frequency noise as the electrolytic capacitor (inductive) impedance increases.
Power and ground planes improve decoupling and more
Dave told me he had transferred his schematic diagram to the PCB designer and was asked about PCB layers. Dave confessed, “I had a basic idea about layers and planes. To play it safe I told the designer to ‘do the usual.’” I suggested that Dave and I run through the basics.
A power plane is simply a flat plane of copper connected to a power supply (VCC). A ground plane is connected to the ground connection of the power supply. Whenever a component needs to draw power, a trace is run to a via that connects to the power plane. The same is true for a ground connection. When both VCC and ground are connected, the power circuit is complete. Dave’s eyes fluttered a bit like a deer caught in a car’s headlights. I continued, “A via consists of two pads in corresponding positions on different copper layers of the board.” Eye flutter removed.
You will usually only see power planes in PCBs with four or more layers. This is because the best practice for multilayer PCBs is to use an even number of layers. Odd layer multilayer PCBs don’t bring any cost savings, and the resulting asymmetry can lead to problems like warping, twisting and other structural defects. The commercially popular two-layer board will generally benefit more from a ground plane rather than a power plane and relying on traces to deliver power from a source.
Narrow traces offer increased resistance and long traces create increased inductance. Both effects tend to reduce the effectiveness of a decoupling capacitor. Power planes offer advantages over traces when they are used in a PCB design, such as improved decoupling between circuits. Further, the surface of a power plane can create a parallel plate decoupling capacitor between the insulating layer and the ground plane. This prevents noise from propagating through the power supply from one circuit to another. Ferrite beads are sometimes used to increase inductance to combat high-frequency noise.
Using vias to connect to the power and ground planes produces shorter (low resistance and low inductance) paths. Further, the shorter paths promote better electromagnetic control (EMC) performance. Planes can handle more current than traces. The lower resistance of power and ground planes means less power dissipation. The PCB assembly operating temperature will be lower.
Signal layers are usually placed to occupy the top and bottom layers of a multilayer PCB. This makes them easier to probe. Although hopefully not required, it also makes it easier to place cuts and jumps to modify the circuit.
PCB grounds
Dave had been taking copious amounts of notes. I assured him that I would be around to provide answers to any follow-up questions and suggested that we expand on the ground plane by chatting about grounds. I started with a quote from our boss. Barry once proclaimed, “If you can’t trust ground, what can you trust?”
“He was right,” I told Dave. “Ground is our point of reference and the reference for the devices used in a design. Ground is sometimes characterized as 0V. Nothing magic; a voltage point taken as a reference to itself is zero. Remember, voltage is always defined as being between two points.”
A proper grounding system must be engineered as well, or better, than any other part of the system. I told Dave about a new, but experienced, engineer that was assigned a mixed-mode (analog and digital) project. He grounded everything as if ground were generic. When he tested his PCB, it was totally messed up. The only way he could test the board was to have a technician use a band saw to cut it in two to separate the analog system from the digital system. On redesign, he separated the analog and digital grounds and tied them together at a single-point ground (SPG). The new design worked great.
“So, what happened to him?” Dave asked. Smiling I reported, “They made him a manager.”
“They rewarded him?!”
With a chuckle I told him, “Sometimes they promote you to management to keep you away from hardware design!” Dave grinned and left to report to Barry.