[sdiy] Question about CMOS switches

[ASSI]

On Sunday 16 January 2011, David G. Dixon wrote:
> Is there any problem driving analog switches in the DG series with
> switching signals that swing to (or near) the rails (provided, of
> course, that the switches are powered from the rails)?

Which DG series, there are so many?  IIRC, some would have a problem and 
some not and this also depends on what the analog and (if present) digital 
supply voltages are set to.  So, you either select the switch that does what 
you want (likely a bit more expensive) or you use the garden-variety switch 
and work on the input side.

> I.e., if I
> wanted to control such a switch with an opamp comparator, does this
> create any kind of problem?  The datasheet seems to indicate not, but I
> would like a second opinion before I commit to a design.

It is usually not the best idea to use large-swing and high-speed signals 
near audio, unrelated to any issues with the input range of some IC.  The 
tried method of completely avoiding the problem is to use a comparator that 
has an open collector output and feed that into a pullup-resistor that is 
hooked to some lowish voltage depending on what's the trip point for the 
input in question is.  A proper comparator will also not have issues with 
saturation and phase reversal that a "true" opamp might present.


Achim.
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