Analog Circuits on PCBs

Before the digital era began, analog circuits were the norm, and they had specific design rules. With the advent of digital circuits, many of the older design rules were no longer applicable. However, as many advanced applications are now decisively analog, or use mixed signals, Rush PCB Inc recommends revisiting the older guidelines for analog PCBs. Although all analog systems are different, following some best practices for their design can ensure they undergo successful EMI and noise testing.

Designers of analog PCBs must consider and implement these guidelines in a high-level approach. However, it is necessary to understand the reasons for implementing these concepts, as they also apply in many other types of boards.

As it is popular to have multiple analog signals at different frequencies on a board, or analog circuits along with digital, the guidelines will help routing and placement for minimizing interference.

Consistent Ground Potential

When designing analog PCBs, it is essential to have a uniform ground potential in the entire system. Typically, this means tying all the ground nets throughout the system such that a voltage measurement in any region of the board will be identical.

Designers achieve this by creating a solid ground plane for both analog and digital signals when interfacing them. Contrary to popular belief, splitting the ground plane into separate sections for analog and digital can create large return paths. This can easily result in EMI generation from different situations, such as:

• A floating ground region can start to radiate strongly, causing emission test failures.
• Routing signal paths over split ground regions can generate radiation.
• A ground offset can couple across ground splits to induce wrong voltages in signals.

Component Placement and Return Paths

Component placement in analog circuits is important. Just like in digital circuits, it is necessary to place components in analog systems above a ground plane. Mutual coupling can allow signals to interfere with one another, and this is another essential consideration for proper placement of components.

To prevent interference between signals, or their return current paths, designers must place analog and digital circuits in separate regions of the PCB. This separation is also necessary for boards using mixed signals. If a suitable separation is not possible, the designer must try to route them perpendicularly. A suitable separation is necessary for boards that use multiple analog interfaces operating at different frequencies.

Placing and Routing ADCs/DACs

Rather than splitting the ground plane under digital and analog domains, it is necessary to understand the role of ADCs and DACs in the system. ADCs and DACs are special, as these are components that deal with both analog and digital signals. Therefore, this situation demands obeying important power and signal requirements in ADC and DAC circuits:

• It is not necessary to separate the analog ground from the digital. A single ground plane must bridge the AGND and DGND pins.
• Isolating the digital and analog power supply with ferrite beads is not necessary, unless experiments or simulations validate the requirement.
• If ferrite beads do not help, it may be necessary to use two separate power supplies for the analog and digital sections.
• Digital IO signals are notorious for causing interference. It is necessary to route them away from digital signals, and also away from the area of the board containing the analog section.
• It is advisable to apply an RC filter for charge compensation at the inputs of analog sections. This will prevent conduction noise from entering the ADC/DAC.
• It is necessary to select and place voltage references at appropriate points that can handle noise, temperature drift, and voltage drops.
• Amplify the input analog signal to an ADC, such that it occupies most of the dynamic range of the ADC.

Impedance Matching and Power Transfer

Inputs in digital circuits typically exhibit high impedance. Therefore, high-speed buffers usually terminate a transmission line to match the real input impedance at the receiver. This prevents reflections at the receiver end.

Analog signals exhibit wave propagation as they travel along an interconnecting trace on a PCB. However, unlike digital systems, reception of analog signals may not involve a high-impedance input. Sometimes, analog signals may have to drive a moderate or low impedance input with a reactance. Different scenarios may be necessary in analog signals, some requiring to transfer maximum voltage, maximum current, or maximum power at some specific frequency.

The above situation in analog circuits calls for impedance matching both in circuit design and in PCB layout. The goal is to match the conjugate impedance. For instance, this involves the use of an impedance transformer at the load end, or wave impedance voltage matching circuit at the receiver end.

Reducing reflections and transferring maximum power require two different impedance matching methods. This depends on the type of interaction between signals necessary for the application.

Shielding Requirement

An analog system may be susceptible to noise, to mitigate which the designer may want to add shielding material. The designer has a few choices here, varying from using stitching vias with ground planes, to using shielding compounds, or using custom shielding cans, shielded enclosures, or shielded gaskets.

Layer Stackup

Typically, for designers, the layer stackup is the first stop when designing circuits. Although the analog layer stackup follows the same guidelines that digital PCB stackups do, some recommended good practices are:

Power and Ground Design

Preferably, use plenty of ground regions around traces carrying critical signals in the PCB board. Likewise, plan a proper routing for the power rail. Although it is customary to think in terms of routing important analog interconnections in the shortest possible paths, using ground regions judiciously around critical signal paths is more important.

Power at High Frequencies

If the analog circuit board requires transferring high power at high frequencies, it needs a very stable power supply capable of supplying high currents. Rather than using rails, a power plane on an interim layer is a better choice. Placing a ground plane on an adjacent layer is an additional help.

Material Selection

Typically, designers prefer using low-loss PTFE-based lamination at each layer in their analog boards. However, these are expensive materials, and there use is not always justifiable. As long as the operating frequency is well below multiple GHz, and the trace length required is small, a standard FR4 laminate will suffice very well. Special material is necessary only for long interconnections carrying high frequencies. In special cases, it is also possible to use hybrid stackups for PCBs.

Unused Op-Amps

Op-amps are common in analog circuits. When using ICs with many op-amps, some may remain unused. Unless terminated properly, unused leads of op-amps can produce noise that propagates into the operating ICs, thereby degrading their signal integrity.

Designers overcome the above issue by shorting the output back to the inverting input, provided the circuit is operating from a single power supply rail. This creates negative feedback, ensuring the output properly follows the input. Next, they connect a voltage divider to the non-inverting input between the supply voltage and the ground rail. This sets the input at the midpoint of the supply voltage. For circuits using a split rail, designers simply short the output to the inverting input while grounding the non-inverting input.

Conclusion

An analog PCB layout has a lot to think about. Rush PCB Inc recommends using the right design tools and rules-driven design software to implement the analog PCB design guidelines to keep the circuit noise-free and ensure power/signal integrity.