PCB Layout Techniques to Achieve RF Immunity for Audio Amplifiers

label: PCB layout,Printed-circuit-board

Abstract: PCB layout techniques are discussed as a method to optimize the RF immunity performance of an audio amplifier IC. Maxim's MAX9750 IC is highlighted as a case study.

Introduction
RF immunity, or RF susceptibility, is quickly becoming as important a design consideration as PSRR, THD+N, and SNR in the audio world of cellular phones, MP3 players, and notebook computers. Bluetooth is proliferating as a wireless-serial-cable replacement for headsets and microphones in mobile applications. Wireless LAN (WLAN), using the IEEE 802.11b/g protocol, is practically standard in PC and laptop computers. The TDMA multiplexing scheme found in GSM, PCS, and DECT technologies remains a considerable RF nuisance. Today's dense RF environment raises concerns regarding an electronic circuit's susceptibility to RF and RF's impact on the integrity of the overall system. The audio amplifier is a system block susceptible to RF.

An audio amplifier can demodulate the RF carrier and reproduce the modulated signal and its harmonic components at its output. Some of the frequencies fall into the audio baseband, producing unwanted audible 'buzz' at the system's speaker output. To avoid this problem, a system designer must fully understand the limitations of the selected amplifier IC and its respective PCB layout. This article will guide designers to optimize the RF immunity performance of an audio amplifier at the board level.

Finding the Source of RF Noise
The key to a successful layout (i.e., high immunity to RF) is first to identify the source of RF noise coupling. If an evaluation kit is available for the selected audio amplifier, use it to identify the pins susceptible to RF. Pick a frequency of interest in the system, 2.4GHz, for example, in a WLAN application. Antenna theory states that a trace length of ~1.2in (a quarter wavelength of the 2.4GHz RF signal) is a highly efficient antenna at 2.4GHz.

l = c/(4*f)
where l = length, c = 3X108, f = frequency.

Cut a 1.2in wire and solder it directly to a pin on the IC. Measure (see Appendix below) the IC's RF-immunity performance at the frequency of interest (2.4GHz ±10%). Remove the 1.2in wire and solder it to a different pin on the amplifier. Repeat the RF measurement. It is important to ensure that the test setup is identical for each test run. Continue in this fashion until the 1.2in wire has been placed at every amplifier pin and an RF measurement recorded at the frequency of interest. Finally, measure the IC's RF immunity performance without an antenna connected at the pins.

The last test run provides a baseline for the amplifier's performance. Compare this result to the previous series of test runs. The comparison will identify the amplifier pins most susceptible to RF demodulation. Given this data, a PCB can be optimized to reduce the amount of RF noise coupled into the amplifier pins.

The Role of the Capacitor
Take, for example, the BIAS pin of the selected IC. Assume that the BIAS pin exhibits poor RF immunity at the frequency of interest. The first, and most obvious PCB consideration would be to limit the trace length from the BIAS pin to the decoupling capacitor. If the trace length is optimized and RF demodulation is still a concern, consider adding a small bypass cap (on the order of 10pF to 100pF) to GND at the amplifier pin. The capacitor's impedance profile can create a notch filter at the system's most sensitive frequency (in this case, 2.4GHz). Refer to the impedance profile of a capacitor model (C1) in Figure 1A below.

If C1 were an ideal capacitor, the impedance profile would decrease as the frequency increased (XC = 1/[2π x f x C]). Ideal capacitors do not, however, exist in a practical environment. The impedance of a nonideal capacitor model (Figure 1B) takes a dip at self-resonance* and then begins to increase with frequency. The inductive component takes over (XL = 2π x f x L) at frequencies greater than fo. This behavior may produce disappointing results if the capacitor is selected for a filter operating at frequencies near or above its self-resonance. However, if the capacitor is selected to shunt high-frequency components to GND, the capacitor's self-resonance soon becomes more useful.

Controlling Noise at the Input Pins
An audio amplifier's input pins will always be a source of RF noise coupling. Ensure that the input trace lengths are shorter than 1/4 the wavelength of the system's RF signal. A quiet ground plane can also be implemented to reduce the amount of RF noise coupled into the input pins. Flood the system's quiet ground plane around each input trace of the IC. This ground plane will aid in isolating any high-RF signals from the input pins of the selected audio amplifier.

Expensive methods such as LC filters on RF-susceptible amplifier pins or low-ESR capacitors added to the application diagram can also be implemented at board level. These methods are very effective, but costly. If the source of RF noise is identified and understood, expensive solutions can be avoided.

Summary
Poor RF immunity performance of an audio amplifier will impact the integrity of the overall system design. If the problem is identified and understood, corrective action can be taken to ensure that audible RF demodulation is avoided. In general, keep the input, output, bias, and power supply traces shorter than 1/4 the wavelength of the system's RF signal. If a higher level of RF immunity is required, connect a small-value capacitor to ground directly at the pin of the IC (even when a larger value capacitor is already connected at the pin). Utilize ground plane floods near highly susceptible amplifier pins. Finally, physically place high-RF energy system blocks away from highly susceptible audio amplifier pins. Equipped with this general understanding, the unwanted audible demodulation 'buzz' can be eliminated.