[sdiy] Question about CMOS switches

[Magnus Danielson]

On 01/16/2011 11:39 AM, ASSI wrote:
> 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.

Agree. You need to look at the spec of the set of switches you use to 
see what the safe region is for them and adapt accordingly.

>> 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.

Agree.

Alternatively just use a buffering transistor NPN and let the input 
resistor(s) be adapted to the source and the output resistor suitably 
adapted to the local rails.

Regardless, resolving the issue is cheap, easy and proven.

Cheers,
Magnus