CMOS switch clicking

[smashinator]

Hey folks,

I've got a painfully simple circuit, which of course means it's giving me tons of trouble.  Here goes:

I have a CMOS switch (4066, I believe), and I've got it set up as a shunt switch.  Eg, the input and output are connected to the same pin (pin 1), and the corresponding output pin (pin 2) goes to ground.  Right now it adds a click to the signal.  I've been able to reduce the clicking by putting a cap between pin 1 and 2, but it doesn't eliminate the clicks.

Does anyone have any suggestions for things I can do to reduce the clicking?

Thanks!

[R.G.]

CMOS switches inject a bit of charge from the gates of the devices that do the actual switching to the channel whenver they switch. CMOS switch datasheets often tell you this number in terms of picocoulombs of injected charge.

What's not always appreciated about CMOS switches is that each switch is composed of an N-channel and a P-channel in parallel, and that these are both turned on and off at the same time so the switch can conduct signal near the power supply and ground equally well. If the signal voltage happens to be near the middle of the power supply, where both N and P type devices turn on and off at the same time, they wind up injecting almost equal and opposite signed amounts of charge- that means the click nearly cancels.

So tie the signal ins and outs to half the CMOS power supply voltage with biggish (100K to 1M) resistors, and capacitor couple your signal to/from the switch.

They can get downright quiet if you do this well.

This technique is used in the article at GEO on bypassing with the CD4053.

[smashinator]

Ah-ha!  I'm going to go try that right now.

Thanks R.G.!  I'll let you know what happens.