Free Trials

Download a free trial to find out which Altium software best suits your needs

How to Buy

Contact your local sales office to get started on improving your design environment


Download the latest in PCB design and EDA software

  • Altium Designer

    Complete Environment for Schematic + Layout

  • CircuitStudio

    Entry Level, Professional PCB Design Tool

  • CircuitMaker

    Community Based PCB Design Tool


    Agile PCB Design For Teams

  • Altium 365

    Connecting PCB Design to the Manufacturing Floor

  • Altium Concord Pro

    Complete Solution for Library Management

  • Octopart

    Extensive, Easy-to-Use Component Database

  • PDN Analyzer

    Natural and Effortless Power Distribution Network Analysis

  • See All Extensions

    World-Renowned Technology for Embedded Systems Development

  • Live Courses

    Learn best practices with instructional training available worldwide

  • On-Demand Courses

    Gain comprehensive knowledge without leaving your home or office

  • Altium 365 Viewer

    View & Share electronic designs in your browser

  • Altium Designer 20

    The most powerful, modern and easy-to-use PCB design tool for professional use


    Annual PCB Design Summit

    • Forum

      Where Altium users and enthusiasts can interact with each other

    • Blog

      Our blog about things that interest us and hopefully you too

    • Ideas

      Submit ideas and vote for new features you want in Altium tools

    • Bug Crunch

      Help make the software better by submitting bugs and voting on what's important

    • Wall

      A stream of events on AltiumLive you follow by participating in or subscribing to

    • Beta Program

      Information about participating in our Beta program and getting early access to Altium tools

    All Resources

    Explore the latest content from blog posts to social media and technical white papers gathered together for your convenience


    Take a look at what download options are available to best suit your needs

    How to Buy

    Contact your local sales office to get started improving your design environment

    • Documentation

      The documentation area is where you can find extensive, versioned information about our software online, for free.

    • Training & Events

      View the schedule and register for training events all around the world and online

    • Design Content

      Browse our vast library of free design content including components, templates and reference designs

    • Webinars

      Attend a live webinar online or get instant access to our on demand series of webinars

    • Support

      Get your questions answered with our variety of direct support and self-service options

    • Technical Papers

      Stay up to date with the latest technology and industry trends with our complete collection of technical white papers.

    • Video Library

      Quick and to-the-point video tutorials to get you started with Altium Designer

    PCB Design Rules for Analog Circuits

    Zachariah Peterson
    |  March 18, 2019

    Glowing vacuum tube on a blue background

    Some people just don’t understand how electronics engineers see the world. Sometimes, it’s like we’re Neo at the end of The Matrix, looking at everything in digital data. Analog design is like a retreat back to the real world.

    Even though building analog systems might seem like going back in time to the days of vacuum tubes, analog signalling is not going away any time soon. Purely analog boards and mixed signal boards are still important in many products, and new designers should take note of some important PCB design rules for analog circuits.

    Routing Guidelines for Analog PCBs

    Traces in your analog PCB should be made as short as possible for several reasons. First, traces carrying analog signals can exhibit transmission line behavior if the trace is too long, just like a trace carrying a digital signal. This transition is sometimes said to occur if the propagation delay is larger than one-quarter of the oscillation period of the analog signal, although the real transition is not so specific. When determining whether a trace will exhibit transmission line behavior, you need to compare the trace's characteristic impedance with the circuit's input impedance and examine any deviation from the ideal input impedance value seen by your driver. Take a look at this article to see how you can determine the transmission line critical length. In essence, if the trace is short enough, the trace's impedance won't matter, and the (line + receiver) input impedance will appear to simply be the receiver's impedance.

    Keeping traces short, especially in higher frequency circuits, will help prevent transmission line effects. Upon transition to transmission line behavior, analog signals can reflect from an impedance mismatch, potentially leading to analog signal resonance in the trace. This forms a standing wave on the transmission line that generates strong crosstalk (EMI) in nearby components.

    The use of vias on critical traces should also be minimized. All vias have inherent self inductance thanks to their geometry. While signal resonance for vias are typically in the GHz range, inductance adds when multiple vias are placed in series, lowering the resonant frequency for the trace in question. Another problem with routing analog traces between layers is that the vias require placing holes in ground planes, creating a high impedance discontinuity that can cause ground loops. Instead, try to place all analog signal traces on a single layer.

    Power and Ground Planes

    The placement of power and ground planes on your multilayer board is critical to ensuring signal integrity. The ground and power planes typically overlap and are decoupled with a capacitor in a digital board. In contrast, the analog power and ground planes should not overlap.

    In mixed signal boards, you’ll need to adopt a combination of digital and analog rules. The digital block and the analog block should be separated by splitting the ground plane. This ensures that all signals are referenced to the same ground. The join between the digital and analog ground planes should be placed as close as possible to the ground connection for the power supply.

    The digital and analog power planes should also not overlap. If these two planes do overlap, the two planes will have some capacitance between the overlapping areas. This is likely to cause RF emissions that propagate between these two planes.

    If you are using a PLL for frequency synthesis with an analog signal to form a reference oscillator or a clock pulse train, you’ll need to very carefully route the clock output back into the digital section. This can be very tricky and depends on whether you are using a PLL IC or if you intend to design your own from separate components.

    In the digital section of the board, it may be a good idea to define one or more signal layers between the digital power and ground planes. This gives you a place to route some signals that need to be protected from EMI; the two large sections of conductor will block EMI. Be sure to avoid excessive use of vias to pass between these inner signal layers as vias can easily couple noise back into the power plane, and vice versa.

    Once digital traces are routed over the digital ground plane, either on the surface or the interior, this helps ensure that your return signal is only induced in the ground plane and travels to the ground connection on the power supply. The ground plane will suppress any signal from being induced in the power plane. For sensitive signal traces routed between the power and ground planes, the decoupling capacitor can compensate any digital signal induced in the power plane.

    Computer motherboard with digital and analog connections

    Through-hole vs. Surface Mount Components

    Through-hole components are normally used on boards where space is not an issue. If real estate on your board is limited, it is better to use surface mount components. Through-hole components can also have higher power ratings than surface mount components.

    If you must use through-hole mounted passive components, avoid mounting them vertically if possible, especially in high speed boards. If you can use shorter leads and stretch them horizontally, the area enclosed by your circuit will be smaller, minimizing self inductance. This will also help minimize EMI into other components, especially in high frequency circuits.

    A cleaner alternative, both from a signal integrity standpoint as well as aesthetically, is to use surface mounted passive components. Although through-hole components are cheaper, fabricating a board to support through-hole components can be more expensive due to the extra drilling steps required.

    Dealing with Unused Op-amps

    One component that is bound to appear in an analog board is an operational amplifier. In many op-amp ICs, some of the op-amps will be left unused. Any unused leads on the IC should be terminated properly. Unterminated leads on op-amps in an IC can produce noise that propagates into the operating ICs, degrading signal integrity.

    If you are using a single power supply rail, you should first short the output back to the inverting input. This creates negative feedback and ensures that the output will properly follow the input. Next, connect a voltage divider with equal resistors to the non-inverting input and the ground pin. This will set the input potential to the midpoint of the linear range. If you are using a split rail, you can simply short the output to the inverting input and ground the non-inverting input.

    Surface mount IC in tweezers over a blue PCB

    Building devices that act as wireless beacons, are used to control and gather data from sensor networks, or that interface with external analog systems require design software with signal integrity tools, a powerful PDN Analyzer™, and an intuitive layer stackup manager. Altium Designer®​ contains all of these features and many more in a unified, rules-driven design platform.

    If you’re interested in learning more about Altium , you can download a free trial and get access to the industry’s best layout, routing, and simulation features. Talk to an Altium expert today to learn more.

    About Author

    About Author

    Zachariah Peterson has an extensive technical background in academia and industry. Prior to working in the PCB industry, he taught at Portland State University. He conducted his Physics M.S. research on chemisorptive gas sensors and his Applied Physics Ph.D. research on random laser theory and stability.His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental systems, and financial analytics. His work has been published in several peer-reviewed journals and conference proceedings, and he has written hundreds of technical blogs on PCB design for a number of companies. Zachariah works with other companies in the PCB industry providing design and research services. He is a member of IEEE Photonics Society and the American Physical Society.

    most recent articles

    Back to Home