help needed: switching caps in most basic low pass circuit

[Estebandito]

Hi,
I have a very basic low pass filter circuit in which I use a 4051 multiplexer to switch capacitors. This works quite nicely apart from audible clicks whenever I switch from one to the next. I have tried polarised as well as ceramic caps but the issue remains. Anybody have any ideas on how to eliminate them?
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[Steveg]

You could put another low pass filter downstream from that one to filter the clicks.
You could also try setting the capacitors up so that the lowest value is always in circuit in parallel with the 4051 and its previous MUX position is left unconnected. If exact values are important you will need to reduce the other capacitor values by the value of the fixed capacitor.
it might also help to have a single capacitor value and use the 4051 to switch the resistors.

[blue hell]

It probably helps to sync the switching to some signal level. That way the capacitors will always have more or less the same voltage when a switch occurs.

To do this you will need to make a voltage detector, the voltage detector should control a latch then to only pass the changed ABC control signals when a certain voltage is being detected.

This should better things up a lot, but will probably not completely get rid of the clicks, as the analog switches do have some leakage current which will alter the voltage on the caps after some time.

This would depend on how long the filter is left in a fixed state (i.e for rapid switching the clicks should be less prominent).

As an asside, for better working you better take the output from after the resistor (instead of before it as shown in your drawing).

Oh, and resistor switching (as Steveg mentioned) probably works better too.

[Estebandito]

Thanks for your answers, I will try out those suggestions - I have a feeling switching resistors might be the best solution.

[dk]

Just out of curiosity, would it make a difference if the caps were on the other side of the mux? ie., the resistor would always be attached to all the caps, the mux would select which one connects to ground. I'd assume this would have an effect on the frequency of the filter (at the very least at the time of switching), but would it eliminate the clicking?

[blue hell]

[dk]

Gotcha, thanks for answering! For some reason, I had thought that maybe the caps being unable to discharge would help smooth said spikes out, as opposed to the caps being left tied to ground when switched out. I guess it sounded nice, despite being totally unrealistic