analog switch in EEG front-end circuitry ?

You may need to worry about stray capacitances - the data sheet lists a bunch of channel capacitances from 68pF to 220pF - and you may want to isolate the positive and negative supply pins from the power rails with LCR filter networks to keep supply ripple out of the signal inputs.

Reed relays are nicer in this respect, and if you can control the orieintation of the board carrying the switches, mercury wetted reeds last longer than dry reeds (10^8 operations as opposed to 10^7) and offer slightly lower and more stable on-resistance).

-- Bill Sloman, Nijmegen

Sure. A cmos switch has zero offset and just Johnson noise. As Bill says, keep the power supplies clean.

Yup, thermocouples and nanovolt signals and stuff.

Small telecom relays are even better; milliohms on, teraohms off, GHz bandwidth, kilovolt isolation. Use latching relays if thermal offsets are an issue.

John

Any particular reason why you don't simply use a nice small reed relay? A solution with far fewer gotcha's.

Dave.

I did take a look at many reed relays, but the problem is their size (specially their height). All the parts used should be as flat as possible.

And no, I cannot control the orientation of the switches, since this will be a portable device.

Thanks.

Yes, that sounds critical, too. Supplies should be clean, although Fig. 21 in the ADG1636 datasheet suggests good power supply rejection (by the way, I wonder what happens below 50 Hz).

I'm not too worried about the 220 pF, since the input RC network has 1 nF and 10 nF capacitors.

Thanks.

How does 3.4mm sound?

Dave.

Doesn't sound bad. A little pricey, though (and I would need four of these, because they are 1xSPST and I need 1xDPDT). I'll think about it.

Thank you.

Which part numbers of CMOS switch have you used? Could you recommend a manufacturer?

Do you buffer the switch with an op amp? Is the buffer placed before or after the switch?

Just an HC4051 type is good. Agilent uses them in their 6.5 digit DVMs. There are lots of fancier/more expensive parts around too.

Depends on what you're driving. One caution is that most CMOS switches kick out a bunch of charge when they switch, and that can mess up some opamps for a bunch of microseconds.

John

Thanks, John!

It is very neat to know that such parts are used by Agilent.

Perhaps the best way to deal with this would be to simply ignore measurements taken immediately after each switch? I can see that this wouldn't be much of a problem for calibration, which is occasionally done.

I would imagine that good CMOS switches would have low ON resistance and low capacitance.