AN1561.0 Datasheet, PDF (1/6 Pages) Intersil Corporation – Radiated Interference in Audio Circuits

Application Note 1561
Author: Don LaFontaine
Radiated Interference in Audio Circuits
Abstract
The proliferation of wireless transceivers in portable
applications has created a need for increased attention to
an electronic circuits' ability to operate in the vicinity of
high frequency radio transmitters. This Application Note
will investigate an application using an op amp for the
voice band audio in close proximity to a high frequency
source. In gigahertz radio systems, the close proximity of
the radio antenna to low frequency amplifier
sub-assemblies, can result in the demodulation of the
radio signal causing a disruptive interference in the
receiving circuit. This Application note presents a simple
method of producing a controllable test platform suitable
for testing and characterizing Radio Frequency
Interference (RFI) in op amp audio circuits. A comparison
between Intersil's ISL28291 (bipolar input) and EL5220
(MOS input) dual precision amplifiers susceptibility to RF
interference is investigated using this test platform.
Research findings related to this topic will also be
discussed with new information suggesting, in today's
wireless hand held products, that radiated interference
as well as conductive interference needs to be
considered.
Introduction
Several studies, experiments and calculations have
shown the propensity for operational amplifiers to
demodulate RF signals principally at the emitter-base
junction of the input differential pair[1, 2, 3] [8].
Demodulation occurs even though the amplifiers
bandwidth is much lower than the RF's out-of-band
signal. As long as the RF voltage at the junction is not
significantly greater than 26mV, the rectified offset
voltage follows the square-law relationship and is
proportional to the absorbed power. Under these
conditions, the audio interference will follow the envelope
of the RF signal.
Our study of this phenomenon began when a customer
reported their Bluetooth signal was being demodulated in
the amplifier and showing up as audio noise on the
output audio amplifier. The customer evaluated three
opamps in their system and reported the ISL28291 is the
worst of the three, National's LMV722 was in the middle
and the EL5220 was the best. Even through the EL5220
was rated the best, all three parts exhibited the
demodulated signal to some degree at their outputs.
Although the noise caused by the Bluetooth transmitter
was readily observed at the amplifier output of the
ISL28291, (EL5220 was clean) the frequency hopping
and complex encoded modulation of the Bluetooth signal
made it impossible to get consistent results. A test
platform to quantitatively measure a circuit's
susceptibility to RF signals was constructed using
standard equipment found in most high frequency analog
labs. This platform is capable of generating a swept
carrier frequency for 100kHz to 6GHz with an external
modulated signal. The platform uses an HP8753D
network analyzer and a simple function generator to
modulate the signal [4]. The swept modulated carrier
frequency was connected to a simple antenna as shown
in Figure 1. The output power of the carrier frequency
was adjusted to 0dbm to match the standard Bluetooth
signal. Consistent readings and, even more importantly,
the ability to sweep the carrier frequency from 100kHz to
6GHz was now possible. This allowed the ability to
investigate the sensitivity of the circuit as a function of
carrier frequency and to focus on a specific frequency to
study the demodulated signal.
This Application Note will:
1. Define the difference between conductive and
radiated interference for this discussion.
2. Present new data suggesting that radiated
interference needs to be considered for portable
wireless applications where the antenna is in close
proximity to audio circuits.
3. Present lab results comparing ISL28291 and
EL5220.
4. Draw conclusions based on this study and other
papers to give suggestions for applications and
design engineers for operating in the vicinity of high
frequency transmitters.
FIGURE 1. MODIFIED EVALUATION BOARD TO
SIMULATE CUSTOMERS CIRCUIT
Conductive And Radiated
Interference
Conductive interference is defined as the modulated RF
signal collected by cables, PCB traces and external
components and fed directly into the input pins of the
audio amplifier. Radiated interference is defined as the
combination of conducted interference plus the
April 26, 2010
AN1561.0
1
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