Re: Analog switch methods

Try a FST3125 or similar, if you don't need more than a volt or two of
> swing. =A0These are cheap, fast, NMOS only switches with built-in gate > drivers, and the substrate tied to a separate (negative supply rail) > pin. =A0(You'll have to provide a negative rail about equal to the > minimum of the signal swing, and a positive rail 5V or so above that.)

Plenty fast, but looks like TTL or HCMOS- 7V max. So I can pass a

+/-2V signal, which is fine, but I'd need a +/-2.5 or 3V supply to do that. I'd rather run from +/-5 or, better yet, +/-10V. And there is startlingly little data in either Fairchild or OnSemi datasheet - I mean, no isolation data? No V/I curve? The 4066 makes me feel more comfortable than this thing... too bad it's just not good enough.

No comments on using BJTs? Didn't one of the Johns here do something with those? John? Anyone? Bueller?...

Tim

Actually the FST3125 ron will get higher with swings close to the positive rail and it will turn off completely if both the source and drain of the NMOS switch reach within a threshold voltage of the positive rail. The threshold voltage under this condition is relatively high because the back-gate (substrate) is biased permanently to the negative rail AFAIK. Nevertheless, some amateur radio people have been making quite good mixers out of these cheap digital bus switches (google H-mode mixer). You would probably use

+/-1V swings, with a -1V and +4V power supply. I would not recommend using a digital bus switch in an analogue circuit for a commercial design unless you can make a lifetime buy of a qualified version of the switch so that you are immune to the manufacturer changing the process or design in a way that is only a problem for analogue circuits. Chris