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Maxim > Design Support > Technical Documents > Application Notes > Analog Switches and Multiplexers > APP 730
Maxim > Design Support > Technical Documents > Application Notes > Audio Circuits > APP 730
Keywords: analog switches, THD, audio filter, on-resistance distortion, flatness, linearity impedance
APPLICATION NOTE 730
Minimizing Total Harmonic Distortion Contributed
by Analog Switches in Audio Systems
Jan 25, 2002
Abstract: Analog switches are commonly used in audio systems to switch low-level inputs or adjust audio
filter characteristics. Selecting the proper analog switch can help optimize the total harmonic distortion
(THD) of a system within the designer's cost budget.
Analog switches are commonly used in audio systems to switch
low-level inputs or adjust audio filter characteristics. Selecting the
proper analog switch can help optimize the total harmonic distortion
(THD) of a system within the designer's cost budget.
THD specification plays a critical role in determining the quality or
fidelity of audio signals passing through or generated by audio
systems. Thus, when designing audio systems, importance must be
placed on selecting the proper components and board layout to
minimize THD.
When passing a signal through a switch, the switch must not degrade signal integrity or introduce any
new information onto the desired output signal. Any change in waveform is considered to be distortion
and is obviously undesirable and also impossible to avoid, and thus must be minimized.
Figure 1.
THD is defined as the square root of the ratio of the sum of all of the squared second, third, and higher
harmonics to the magnitude of the fundamental harmonic.
Selecting an analog switch with a minimum THD requires one with low on-resistance (R
ON
) and
consequently low on-resistance flatness (R
FLAT(ON)
). Some analog-switch data sheets neglect to specify
flatness, but it can be assumed to be approximately 10% of the on-resistance. Flatness is defined as the
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