Many years ago I did some of that .. As I recall, the "analog
switch IC's" are really mosfets, series or shunt. The
distortion spec was made under certain operating conditions.
Since you didn't say what you are using, or the circuit, I
can't judge how to make it better.
This type of switch works best at a virtual ground, so the
voltage is zero or constant. Since it is used as a series
element, the distortion can be minimized by using a larger
series resistor. This makes the nonlinear part of the
resistance a smaller part of the total. This may increase
noise, which may or may not be an issue. You need a pair of
switches, or a single-pole-double-throw, for each signal. You
need to consider what happens during switching. Make sure that
you never short the op-amp input to ground (which effectively
removes negative feedback) and you never let the voltage get
big. Either of these problems will result in a pop or click
during switching.
You also need to consider that distortion numbers usually
increase as the signal gets larger. Running at a lower signal
level will usually result in lower distortion. For the devices
I used, the distortion was almost all second order, which is
the least audible type.
Considering all that, by the time all optimization was done, the
performance impact of the switch IC was essentially zero. Its
distortion and noise were masked by other distortion and noise
in the system, and that was so low that the distortion was
difficult or impossible to measure, and noise was essentially
what has predicted by theory and dominated by other stages.
I don't remember what brand or device we used. I do remember
that it was mainstream and cheap.
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