Analog PCBs used to be the norm, but digital systems came along and turned all the design rules on their heads. Now with many advanced applications being decisively analog, or at least mixed signal, designers need to revisit design rules and get past some of the very bad guidelines surrounding analog PCBs. While all analog systems are a bit different, there are a few top guidelines for analog PCB design that should be followed to ensure low noise and successful EMI testing.
In this article, we’ll look at some of the main guidelines for analog PCB design and how to implement them in a PCB. These guidelines should illustrate a high-level approach for most analog PCBs and systems, but what is more important is the concepts that justify their use. Make sure to understand why these are implemented as these concepts apply in many other types of PCBs.
Five Essential Analog PCB Design Guidelines
Because many analog boards contain a digital section, or possibly multiple analog signals at different frequencies, the guidelines shown here intend to aid routing and placement so that interference is prevented.
1. Maintain Consistent Ground Potential
The most important guideline for designing analog PCBs is to ensure a uniform ground potential throughout a system. In general, this means ensuring that ground nets are tied together in the system so that a voltage measurement taken in one region of the PCB will be the same if taken at all other regions of the PCB.
For the analog and digital signal interfaces, this means using a solid ground plane for both types of signals. Do not split the ground plane into two different sections and try to route different signals between sections because this creates large (or non-existent) return paths. The result is an EMI problem through one or more of the following mechanisms:
- One of the ground regions could end up floating and can radiate strongly, causing an emissions test failure
- The designer will route over splits in the ground regions, creating radiation
- Signals that are coupled correctly across ground splits could have the wrong voltage reading due to a ground offset
2. Understand Placement and Return Paths
The next important point to understand is placement of components. Components in analog systems should be placed above a ground plane, just as is the case with digital components. The location of components will also be a major factor determining how signals can interact with each other through mutual coupling.
To prevent signals from interfering and to ensure their return current paths do not commingle, place digital and analog signals in different regions of the PCB. If you are working with multiple analog signals, try to separate these as well, or route them in a perpendicular fashion. The analog-digital separation could also be used when the PCB has multiple analog interfaces and components operating at different frequencies.
3. Learn to Place and Route ADCs/DACs
Rather than drawing a ground plane split as a border between digital and analog domains, it’s important to note the role of ADCs and DACs in these systems. ADCs and DACs are the components where the digital and analog worlds meet. There are some important power and signal requirements in ADCs and DACs that should be obeyed:
- Do not use separate analog and digital grounds (see above); bridge the DGND and AGND pins together on a single plane
- It is recommended to not attempt to isolate the analog and digital power input pins with a ferrite bead unless the use of a bead can be validated with simulations or experiments
- In the above case, when a ferrite is not appropriate, then two different power supplies may be needed for the analog and digital power inputs
- Route the digital signals away from the digital I/O section and keep those signals in a different area of the board than the analog section
- Consider applying charge compensation (RC filter) at the analog input to prevent noise conduction into the ADC/DAC
- Understand how to select and place voltage references that can withstand power droop, temperature drift, and noise
- In an ADC, make sure to amplify the input analog signal so that it takes up most of the ADC input’s dynamic range
4. Understand Power Transfer and Impedance Matching
In digital systems, inputs on a digital circuit are high impedance (equivalent to a shunt capacitance), so they can reflect strongly. For this reason, high-speed buffers normally terminate a transmission line such that the input impedance at the receiver is real, and thus reflection is prevented at the receiver.
In analog systems, it is true that signals can reflect at a receiving component because all signals will exhibit wave propagation while traveling along an interconnect in a PCB. However, not all analog systems involve reception of a signal at a high impedance input. In some cases, analog signals are being used to drive a low or moderate impedance input, and that impedance could have some reactance. In these cases, the requirement may be to transfer maximum power, maximum voltage, or maximum current at some particular frequency.
The main point here is to understand impedance matching circuit design and impedance matching in the PCB layout. The goal would be to provide either conjugate impedance matching, such as an impedance transformer at the load end with stub lines, or voltage wave impedance matching circuit placed at the receiver input.
Reflection suppression and maximum power transfer refer to two different impedance matching methods, depending on the type of signal interaction you need in your components.
5. Identify When Shielding Is Needed
It is sometimes the case that an analog system has some noise problem, and a designer’s first instinct may be to add some shielding material to the problem nets. The designer’s instinct may then be to apply some shielding, either as stitching vias with ground pour, using a shielding compound, custom shielding can, shielded enclosure, or possibly using a shielded gasket. Make sure to read this article about shielding materials.
When you need to place and route analog PCBs, make sure you use OrCAD, the industry’s best PCB design and analysis software from Cadence. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.
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