A Closer Look at Analog PCB Design – Comprehensive Guide

In the past, analog PCBs were commonplace, but with the emergence of digital systems, traditional design principles were challenged. Today, many advanced appliances rely on analog or mixed-signal designs. It’s imperative for designers to reassess their approach to analog printed circuit board design and overcome outdated guidelines. While every analog system is unique, adhering to a few key design principles can help achieve low noise & effective EMI testing. 

This article outlines the key principles for designing analog PCBs & provides practical tips for implementing them. While these guidelines offer a general approach to analog PCB design, it’s essential to grasp their underlying concepts. Understanding these concepts is vital, as they apply to various types of PCBs beyond analog circuits. 

Differences Between Digital and Analog PCB Design 

• Digital circuits are less prone to noise distortion than analog circuits. The binary nature of digital signals allows them to tolerate a broad data range as legitimate input, resulting in reliable performance. In contrast, even minor noise interference can significantly affect the operation of analog circuits. Moreover, analog circuits encompass a series of functions such as modulation, reception, transmission, amplification, and demodulation, among others. Noise distortion can arise at these stages, making it challenging to maintain the necessary noise level ratio and minimize crosstalk. This complexity adds to the design and implementation of analog circuits. 
• Achieving efficiency & accuracy in analog circuit layout is complex and costly. In contrast, digital circuit design is relatively straightforward, with a wide range of automated equipment. As a result, digital PCBs are not only cost-effective but also precise.
• Analog circuits interface with this physical world directly, without requiring conversion, thus avoiding any information loss. On the other hand, digital circuits rely on (ADC) and DAC circuits. It can lead to data loss & degradation of information. Therefore, precise guidelines must be followed for the routing and layout of the signals to minimize these effects. 
• Analog signals in the mixed-signal circuitry are particularly susceptible to interference from digital signals due to their more harmonic frequency. Therefore, great care should be taken when designing High-frequency clock signals. SMPS circuits in analog-digital PCBs to prevent noise interference that could disrupt the operation of the analog circuits. 
• To ensure optimal performance, frequency terminations must be carefully selected for sensitive analog circuits. To mitigate the effects of space radiation, it’s recommended to separate the analog and digital circuits physically. The shield box effectively shields against electromagnetic distortion. 
• In analog circuits, the ground plane design is of utmost importance since any interference can disrupt the return path of signals. However, this is not a significant concern with circuits as they can tolerate noise in signal reference planes.
• Parasitic components, such as capacitance and inductance, can create issues in PCB design when pads are placed near. If a significant voltage change occurs in one trace, it can negatively impact the voltage capacity of neighboring traces, resulting in errors. Digital circuits frequently experience high inrush currents, leading to scenarios where parasitic components can cause issues. To minimize electromagnetic noise, it is advisable to decrease the inductive impedance of these digital circuit traces (pads). Also, limit capacitive coupling in analog circuits. 

While there are commonalities in designing digital and analog circuits, it’s crucial to appreciate the differences to develop an efficient design strategy in PCB design. With advanced EDA tools, designers can benefit from in-built support to assess potential noise problems in the layout. This capability is particularly advantageous when creating mixed-signal printed circuit boards, helping to minimize circuit re-spins.

When designing PCBs, it is crucial to consider various factors that apply to both analog circuits & speedy digital circuits. Fortunately, numerous contract manufacturers possess the necessary expertise to support and fabricate both types of PCBs. Additionally, they are equipped to manage electromagnetic interference (EMI) problems and ensure regulatory compliance. It is essential for successful printed circuit board prototyping/assembly. Teaming up with proficient CMs is highly beneficial. They possess knowledge of the distinctive approaches needed for digital and analog PCB design(layout).

Analog PCB Design Guidelines: Layer Stackup

After creating your circuit designs, the next step in the design process is to establish the layer stack. Typically, constructing the analog layer stack follows similar principles to those utilized in constructing a digitally printed circuit board stack-up. It’s important to keep in mind the following guidelines. 

1. Power and ground: When laying out your PCB, it’s crucial to incorporate an ample number of ground traces that surround crucial signals and plan the power distribution routing accordingly. Although newer designers may initially focus on significant routing analog interconnects. It’s advantageous to prioritize this task early on to enable effective planning of your signal and power routing.
2. Power at high frequencies: In the case of an analog printed circuit board requiring high power & high frequencies, it’s essential to provide highly stable power, which may entail high currents. It is recommended to use a layer inside a circuit board for a power layer. Also, place a ground layer on layer next to it.
3. Material selection: In my opinion, most designers would favor using low-loss Teflon laminates for all the layers in the analog circuit boards. However, it’s not necessary to use these expensive materials. If you are not working at high frequencies and your routes are short, an FR4 laminate will likely suffice when your interconnects aren’t too long. If you require a low-loss laminate, you can contact your fabricator. Also, inquire about using a hybrid printed circuit board stack-up. 

Mixed-signal Grounding

If your board includes a digital portion, the placement of components becomes more complicated. When working with digital circuits, it’s best to use only one ground plane instead of separating them physically since digital circuits work faster and could cause interference. 

It is recommended to carefully plan the layout to ensure that return tracks from the analog and digital blocks are separated. So, Using analog & digital planes separately is still advocated by many design guidelines. Because it can be challenging to achieve natural separation of return paths at low frequencies.

Mixed-signal Power

In dual-mode power, it is common to split the power/energy plane into separate analog and digital circuit sections, just as you would with a digital plane working at various supply voltages. Try to put these sections in the same place on the circuit board and connect them to the nearby ground plane. 

Dealing with Unused Op-amps

An operational amplifier is an essential component in analog circuit boards and is likely present. However, some op-amps within an op-amp circuit may not be useful. If left unconnected, these floating leads can generate noise that may spread to the active op-amps, ultimately leading to a deterioration in the signal quality.

When employing a power supply source, it’s important to first connect the output to the minus input. It establishes negative feedback & guarantees that the output will accurately track the input signal. Then, attach a voltage splitter with matching resistors to both the non-reversed input & ground pin to set the Inlet potential to the middle of a linear range. 

Routing Guidelines for Analog PCB Design

Routing in analog PCBs is with preserving the integrity of the analog signals through interconnects to the receiver end. Unlike digital PCBs, analog PCBs typically have fewer nets. It allows for early experimentation with different layouts until a feasible floor plan is available. To aid in this process, consider the following routing guidelines: 

• Trace length: When designing analog PCB, it’s crucial to keep the traces as short & direct as possible, especially as signal frequency increases. Additionally, it’s essential to pay close attention to the lengths of the signals and minimize any potential losses. 
• Enforce impedance matching: Even when dealing with very short trace lengths, it’s advisable to enforce capacitance(impedance) matching to ensure optimal power transfer between circuits without any reflections. It may involve designing capacitance-matching networks for critical circuits/components to achieve this goal.
• Consider coplanar routing: Printed circuit board routing can allow you to achieve high isolation while maintaining impedance control. 
• Minimize use of vias: To minimize the loss in an S-matrix of interconnect, it’s advisable to reduce the number of vias and avoid unnecessary layer transitions. Additionally, the remaining vias can act as antennas and generate strong radiation. You can find more information about this topic in our article.

When working with a particular frequency range on your board, It’s a good idea to think about routing through a layer inside the circuit board between the plane layers. It is important to ensure that the vias have the proper size & space from the planes to accommodate your working frequency. However, this task is challenging and cannot be easily calculated. Therefore, we advise examining this point through measurements, such as S-parameters. When a signal goes through the via into the interior layers of the circuit board, the impedance of the via is affectable by nearby planes and other conductors.

Designing an analog printed circuit board layout requires careful consideration. Still, by utilizing appropriate design tools & rules-driven software, you can implement the design principles for all digital circuits to maintain a noise-free analog circuit system and ensure signal and power quality. 

Altium Designer® offers the best features for PCB layout. Whether for analog, mixed-signal, or digital PCBs, all within a single layout environment. It includes powerful routing equipment that can enhance your productivity. 

 5 Fundamental Analog PCB Design Guidelines

The objective of the guidelines presented here is to prevent interference in analog boards that either comprise a digital segment or multiple analog signals with various frequencies by facilitating efficient routing & placement. 

1. Keep Consistent Ground Potential

Achieving a consistent ground potential across the entire system is the primary rule for creating analog PCBs. Typically, this involves connecting ground nets in the system, ensuring that any voltage steps taken at one area of the printed circuit board will yield the same result as that taken at any other area. 

When dealing with digital and analog signal interfaces, we recommend employing a continuous ground layer for both kinds of signals. It is advisable not to divide the ground plane into separate sections and attempt to route different signals between them, as this could result in large/non-existent return paths. Such a setup could lead to an EMI (Electromagnetic Interference) issue through the given mechanisms:

• · A floating ground area could cause significant radiation, leading to a failure in emissions testing.
• The person creating the design can put the path of the circuit over gaps in the ground parts, and this might create waves that go out.
• Ground offsets across signal couplings trailing ground splits may cause incorrect voltage readings.

2. Comprehend Placement & Return Paths

Another crucial aspect to consider is the placement of elements in analog circuits. Similar to digital components, components in analog circuits should be positioned above the ground layer. Additionally, the location of the components will play a significant role in determining how signals interact through coupling.

To avoid signal interference and ensure that the return path of the current does not mix, it is advisable to place analog and digital signals in separate areas of the printed circuit board. So, If working with many analog signals, attempt to separate them perpendicularly. Similarly, when the printed circuit board has many analog interfaces & components working at various frequencies, employing an analog-digital separation approach could be beneficial.

3. Know to Place & Route ADCs/DACs

It is crucial to recognize the significance of ADCs & DACs in systems that combine digital & analog domains instead of delineating a ground plane split as a boundary between them. ADCs & DACs are key elements where the analog and digital worlds converge. It is essential to adhere to specific power & signal needs in ADCs & DACs, such as: 

• We don’t recommend employing separate analog & digital grounds. Instead, connect the DGND & AGND pins on one plane. 
• It is also not advisable to use a ferrite bead for isolating the digital and analog input pins unless you don’t confirm its efficiency via experiments or simulations.
• If the ferrite isn’t deemed suitable in the scenario mentioned above, it may be necessary to employ two separate power supplies for the digital and analog power inputs. 
• It may be beneficial to implement charge compensation at an analog input to avoid transmitting noise into the ADC or DAC. 
• It is essential to comprehend how to choose and position voltage references that can endure power droop, noise, and temperature drift. 

4. Apprehend Power Transfer & Impedance Matching

Inputs on a digital circuit in digital systems have more impedance, equal to a shunt impedance, causing them to reflect strongly. Consequently, speedy buffers are typically used to finalize the signal to ensure that an input capacitance at a receiver is original, thereby preventing reflection at a receiver. 

In an analog system, signals can reflect at the receiving element as all signals demonstrate wave propagation by traversing an interconnect on a PCB. Nonetheless, one should note that not every analog system necessitates the reception of the signal at very high impedance(capacitance) input. In certain instances, you can use these signals to drive the moderate or low impedance input, which may possess some reactance. In such scenarios, the objective could be to provide power, voltage, or current at a certain frequency. 

The primary focus here is on comprehending the design of capacitance-matching circuits and incorporating capacitance matching in a printed circuit board layout. The objective is to achieve conjugate impedance matching, which involves using an impedance (capacitance) transformer with the stub lines.

5. Identify the Need for Shielding 

One common approach to address noise problems in analog systems is to add shielding to the affected nets. However, it’s important to carefully consider the best method for achieving effective shielding. It could involve stitching through holes, applying a shielding element, and utilizing a custom-shielded gasket. The choice of shielding method will depend on the specific requirements of the system and the type of noise being addressed. 

How to do Analog PCB Design?

Analog integrated circuit design generally follows the top-down layout and implementation procedure, followed by the bottom-up process. However, there are numerous variations in this approach. The following are the fundamental steps:

• Create a high-level design specification that outlines the intended functions of the design and sets targets for its performance, power consumption, and cost (i.e., area).
• Create a high-level design plan that can meet the desired specifications by utilizing macro functions like amplifiers, comparators, and filters. 
• The next step involves creating device-level descriptions that support a top-level layout using capacitors, resistors, and transistors. One can do it by drawing from the library of already defined functions.
• Verify the designer’s ability to meet all its requirements by performing simulations. So the tool used for this step usually involves modeling the board using non-linear and linear components optimized for the specific fabrication process. It is where one evaluates the device’s robustness in the face of the manufacturing process & operational variability. 
• Assemble pre-defined designs of all elements to implement a physical design layout. During this step, one can perform the optimization of the design density to minimize price. Adherence to placement rules is crucial to ensure the layout is designed for signal integrity and manufacturability. This step, known as physical verification, validates compliance with these rules. 
• The next step involves extracting the equivalent board from the physical layout. This process considers parasitic effects, including crosstalk & wiring resistance. They were not present in a circuit model useful for simulation. The extracted circuit is then re-simulated to verify the design meets its performance targets with the added parasitic effects. Additionally, the extracted design is compared to the original layout. It is to ensure that the correct applications were used & connected as intended. This process is LVS checking. 
• In this step, they add things to test the circuit. After this, the design is ready for a bigger digital design. When you add an analog design to a bigger digital design, it’s called AMS/analog signal design.