Getting rid of that spike was a little tricky. The upper pair of FETs function as an analogue switch, the lower pair are the inverter to drive it. They are connected to be on when the clock is low, after the spike is done.
I added an anti-aliasing filter, but I don't think it's really needed with the switch arrangement. Edit: At least the capacitor is needed, to retain charge when the switch is off. It also smooths a little spike resulting from the FETs' capacitance.
