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Maxim > Design
support > App notes
> Audio
Circuits > APP 3660

Jul 04, 2006

Keywords: RF, Immunity, Susceptibility, RF
Immunity, RF Susceptibility, MAX9750, Layout, Audio,
Maxim
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parts

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APPLICATION NOTE 3660

PCB Layout Techniques to Achieve RF Immunity for
Audio Amplifiers

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.

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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.

MAX9750 Case Study: Engineering
evaluation has identified nine pins on the MAX9750 IC
that demonstrate susceptibility to RF: INL,
INR, BIAS, VOL, BEEP, OUTL_, and OUTR_.

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.

Figure
1A. Nonideal capacitor model.

Figure
1B. Nonideal capacitor model, impedance profile.

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.

MAX9750 Case Study: A 33pF capacitor
added at the BIAS pin improved RF immunity performance
on average by 3.6dB.

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.

MAX9750 Case Study: A 3x reduction
of input trace length and a ground plane flood around
the left, right, and PC-beep input pins further improved
the RF immunity performance of the MAX9750 IC (Figure
2).

Figure
2. RF immunity test results of the MAX9750C speaker amplifier,
Noise Floor = -94.4dBV.
Note: Figure 2 demonstrates the
typical RF immunity performance of the MAX9750 IC. External
factors such as antenna strength, cable
length, speaker type, etc., can also affect the RF immunity
performance.

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.

* At self-resonance, the
capacitive and inductive impedances cancel each other out
leaving only a resistive component. The selfresonance is given
by

Appendix
To obtain accurate and repeatable test
results, the device under test (DUT) must be exposed to a
known RF field strength. Maxim developed a test method for
quantifying repeatable RF-susceptibility results by use of an
RF anechoic test chamber, a signal generator, RF amplifiers,
and a field-strength sensor.

Figure
A. An RF immunity test circuit.

Figure A
above represents a typical test setup for an operational
amplifier (op-amp). The amplifier's non-inverting input is
shorted to GND, using a 1.5in wire loop to simulate a PCB
trace length. A standard 1.5in input trace length is selected
so that RF immunity performance can be compared across several
Maxim amplifiers (note that an input trace from the DUT to the
input source may effectively act as an antenna at the system's
sensitive frequency band). The amplifier's output is loaded
with an expected load. Next, the amplifier is placed in an
anechoic test chamber. The demodulated signal is monitored at
the amplifier's output as Maxim's RF Anechoic Test System
simulates an RF-rich environment.

Figure
B. Maxim's RF immunity test method.

Figure
B illustrates Maxim's RF Anechoic Test System, which
simulates the RF field environment necessary for RF immunity
testing. The test chamber is similar to a Faraday's Cage,
isolating the DUT from external electrical fields.

The
complete test set-up is comprised of the following equipment:

Signal generator: SML-03, 9 KHz to 3.3GHz (Rhode &
Schwarz)

RF power amplifier: 20MHz to 1000MHz, 20W (OPHIR 5124)

RF power amplifier: 1GHz to 3GHz, 50W (OPHIR 5173)

Power meter: 25MHz to 1GHz (Rhode & Schwarz)

Parallel wired cell (Anechoic chamber)

Electric field-strength sensor

Computer (PC)

Fluke DMM
(dBV meter)

The PC sets the range of frequencies,
modulation percentage, modulation type supplied by the signal
generator, and the power delivered by the RF power
amplifier. The modulated signal is fed to the appropriate
power amplifier (OPHIR 5124 for 20MHz to 1000MHz, 20W or OPHIR
5173 for 1GHz to 3GHz, 50W). The amplifier output is monitored
with a directional coupler and a power meter. The defined RF
field is uniformly radiated inside the test chamber.

At
Maxim, the DUT is placed in the center of the shielded
anechoic chamber. A field-strength sensor consistently
measures a uniform field strength of 50V/m at the DUT. An RF
sine wave whose frequency is varied between 100MHz and 3GHz is
100% amplitude modulated with an audio frequency of 1kHz.
Access ports on the side of the chamber provide power to the
DUT and a means to connect output monitors. A Fluke multimeter
(configured to report units in dBV) monitors the demodulated,
1kHz signal amplitude in real-time. As the RF sine-wave
frequency is varied between 100MHz and 3GHz in predetermined
steps, the Fluke multimeter reports are recorded. A sample
100MHz to 3GHz sweep is reported below in Figure
C.

Figure
C. MAX9750 RF immunity test results.

Related parts

MAX9750

2.6W Stereo Audio Power
Amplifiers and DirectDrive® Headphone
Amplifiers

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Download,
PDF format (81kB)
APP 3660: Jul 04, 2006

AN3660, AN 3660, APP3660,
Appnote3660, Appnote 3660

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