Audio circuitry rarely gets a board to itself. In a cellular phone, the audio chain sits on the same PCB as high-speed digital logic and an RF transceiver, and every placement decision made during layout determines how much of that neighboring activity ends up as audible noise at the speaker or headphone jack. Component placement is therefore not a cosmetic step performed after the schematic is finished. It sets the length of the audio signal path, the distance between sensitive inputs and aggressive switching nodes, the amount of board area available for ground partitioning, and the difficulty of routing every other subsystem on the board.
This article covers the placement strategy for audio circuits in mixed-signal products, the way the board should be partitioned into analog, digital, and RF regions, the practical consequences of a poor layout, and the routing rules that matter for the signal path itself, including supply routing for speaker amplifiers and the use of differential signaling. The goal is a repeatable set of layout decisions that reduce coupled noise, limit RF interference, and keep EMI radiation from switching audio amplifiers within the limits regulators test for.
Highlights:
- Partitioning the PCB into analog, digital, and RF regions keeps switching noise and RF carriers away from the sensitive audio front end, where they would otherwise be demodulated into audible noise.
- Place audio amplifiers as close as possible to the headphone jack and speaker, and keep analog audio inputs close to the amplifier input, to shorten antenna-like input leads and reduce Class-D EMI radiation.
- Route speaker amplifier supplies short and wide, and route differential pairs with matched length and tight coupling so common-mode noise is rejected rather than converted into output noise.
Component Layout
The first step of RF PCB design is choosing where each component will sit on the board. Careful component placement reduces signal interconnect length, simplifies ground partitioning, lowers noise coupling, and reduces the board area the circuit occupies. For audio designs these benefits compound, because placement determines both how much noise is picked up and how easily the resulting problems can be fixed later.
Partitioning the Board into Analog, Digital, and RF Regions
A cellular phone contains both digital and analog circuits. To prevent digital noise from interfering with sensitive analog circuits, the two must be kept apart. Dividing the PCB into a digital zone and an analog zone improves routing performance for this class of design and gives the layout a clear rule for where signals and components belong.
Although the RF section of a cellular phone is usually treated as analog circuitry, one concern appears in many designs regardless of how the blocks are labeled: RF noise. RF energy must not couple into the audio circuit, because once it reaches an amplifier it is demodulated and becomes audible noise at the output. To address this, the RF circuits and the audio portion of the analog circuitry should be separated as far as the board allows. Treating RF as a third region, separate from both the digital zone and the audio portion of the analog zone, makes that separation an explicit part of the placement plan.
Placing the Analog Components
Once the PCB has been divided into analog, digital, and RF regions, the next step is deciding how the components of the analog section are arranged. Placement should be driven by one objective: making the audio signal path as short as possible.
Audio amplifiers should be placed as close as possible to the headphone jack and the speaker. This keeps the EMI radiation of a Class-D audio amplifier to a minimum and minimizes noise coupled onto the headphone signal. The analog audio signal should likewise reach the amplifier input over the shortest possible distance, which minimizes noise coupled into that input.
There is a second reason to shorten these connections. Every input lead behaves as an antenna for RF signals, so reducing lead length reduces the antenna radiation effect in the corresponding frequency band. Shorter leads capture less RF energy and deliver less of it into the amplifier, where it would be demodulated.
What a Poor Layout Produces
Figure 1 shows an unreasonable audio component layout. The most serious problem is that the audio amplifier is placed too far from the audio signal input. Because the leads run near noisy digital circuits and switching circuits, the probability of noise coupling rises. The longer leads also strengthen the RF antenna effect described above.
In a GSM cellular phone, these antenna-like leads can pick up the GSM transmit signal and feed it into the audio amplifier. Nearly all amplifiers can demodulate the 217Hz envelope to some degree, which produces noise at the output. In the worst case, that noise can completely drown out the audio signal. Shortening the input leads is an effective way to reduce the noise coupled into the audio amplifier.
The layout has a second problem: the operational amplifier sits too far from the speaker and the headphone jack. If the audio amplifier is a Class-D amplifier, longer headphone leads increase that amplifier's EMI radiation. Such radiation can prevent a product from passing the test standards established by various governments. Longer headphone and microphone leads also increase lead impedance, which reduces the power that the load can actually obtain.
Finally, because the components are spread out so widely, the connections between them are forced to cross other subsystems. This makes the audio section harder to route, and it increases the routing difficulty of those other subsystems as well. A placement error in the audio block therefore propagates into the rest of the board.
A Better Layout
In this version the rearranged components use the available space more efficiently and shorten the lead lengths. Note that all audio circuits are allocated near the headphone jack and the speaker, so the audio input and output leads are much shorter than in the previous arrangement, and no audio circuits are placed in other areas of the PCB.
This design reduces system noise across the board, reduces RF interference, and keeps routing simple for both the audio block and its neighbors.
Signal Path Design
The influence of the signal path on audio output noise and distortion is quite limited. In other words, the compromises that must be made to guarantee performance are also limited, which makes this part of the layout comparatively straightforward once the placement rules are followed.
Supply Routing for Speaker Amplifiers
Speaker amplifiers are typically powered directly from the battery and draw considerable current. If long, thin power leads are used, the supply ripple increases. Compared with short, wide leads, long and thin leads present a higher impedance. The current variation flowing through that impedance is converted into a voltage variation, which is then fed back into the device itself. The supply lead therefore acts as a path for noise to re-enter the amplifier, and its geometry determines how much of that noise appears at the output.
Using Differential Signaling
Differential signals should be used wherever possible. A differential input offers high noise rejection, because the differential receiver is able to suppress the common-mode noise that appears on both the positive and the negative signal lines.
To take full advantage of a differential amplifier, it is important to keep the two lines of a differential pair the same length so that they present the same impedance, and to keep them as close to each other as possible so that they pick up the same coupled noise. The differential input of the amplifier is very effective at suppressing noise coming from the system's digital circuitry, which makes matched, tightly coupled pairs the preferred routing method for audio signal paths that share a board with digital logic.
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