Re: mosfet analog mux, RC timeconstants

> I thought they are 180 degrees out of phase.

Yes, you are right, my mistake.

You mentioned that the gate charge needs somewhere to go. I've been
thinking about it and I think you mean the charge on the gate-drain
parasitic capacitance:

Initially, the "on" mosfet has a charge on the driver side of its Cgs
and Cgd, and the "off" mosfet has a charge on the signal path side of
its Cgs and Cgd. Then when the control signals switch, the previously
on mosfet can discharge its capacitances via the (low) gate driver
source resistance, and the Cgs of the previously off mosfet can
discharge via the signal drivers low impedance. But the Cgd of the
previousl off mosfet (and Cload) must discharge via the 10M load in
parallel with the Rds of the other mosfet.

Is my understanding correct? If so, and assuming Cload ~ Cgd, and
assuming this discharging of Cgd+Cload is the dominant cause of my slow
edges, then if I remove Cload then the edge speed should approximately
double. I tried this in a sim, but the results shows significantly
faster edges - quite a bit more than doubling, which would suggest the
charging of Cload through the mosfet is a lot slower than it should be
(in addition to the number of other problems i appear to be having):

http://www.teamlinux.org.uk/steve/spice/no_load_cap.png

Thanks to everyone for their replys, they've been really helpful,

Steve

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