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Maxim > Design
support > App notes
> Analog
Switches and Multiplexers > APP 730
Maxim > Design support >
App notes
> Audio
Circuits > APP 730

Jan 25, 2002

Keywords: analog switches, THD, audio filter,
on-resistance distortion, flatness, linearity impedance

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APPLICATION NOTE 730

Minimizing Total Harmonic Distortion Contributed by
Analog Switches in Audio Systems

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.

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Analog
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 (RON) and consequently low
on-resistance flatness (RFLAT(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 difference between the maximum and
minimum values of on-resistance as measured over the specified
analog-signal range.

Figure
2. RFLAT(ON).

The on-resistance curves
are determined by the length L, width W, electron and hole
mobility (µn and µp, respectively),
oxide capacitance COX, threshold voltage
VT, and signal voltage VGS
(VIN) of both MOSFETs.

As
illustrated above, the on-resistance for the PMOS and the NMOS
are in parallel,
resulting in resistance
R, which is a nonlinear function. The source
of distortion arises from parallel PMOS and NMOS in series
with constant load, causing a nonlinear attenuation of the
signal passing through the switch. With multiple switches,
THDs will add together. The following schematic shows this
resistor-divider relationship of the switch and the
load.

Figure
3.

In practical terms, maximum THD is determined by
the following equation:

As
observed in Figure 2, parallel RON, and therefore
RFLAT(ON), can vary depending on the analog input
signal, and maximum THD will occur at the greatest difference
between the maximum and minimum values of the parallel
RON. Therefore, an analog input signal operating
about a point below the maximum value will experience a lower
THD.

Three different switches are shown below along
with their on-resistance and corresponding measured THD. A
standard test load of 10 kilohms was used to obtain the
plot.

Figure
4. THD vs. frequency.

In audio systems, impedance
loads of 1200, 600, 36, 32, 16, 8, 4, and 3 kilohms will be
encountered, and, especially with very low impedances,
on-resistance flatness (RFLAT(ON)) must be taken
into consideration to minimize THD. Equation 4 will provide a
starting point in selecting an analog switch if THD is the
highest priority.

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Download,
PDF format (50kB)
APP 730: Jan 25, 2002

AN730, AN 730, APP730,
Appnote730, Appnote 730

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