I'm having a problem where the non activated lines from my sample and hold circuit is drifting up. Everything is fine as long as I am switching at 1 Hz or greater, but in some case I want to keep the hold activated to be able to take a reading. If I keep it held long enough the reading on the other channels will drift up to 6.4 V. This happens even if all the inputs are grounded.
So far I have tried:
Adding a buffer in place of R19
Added a pull down resistor to the output lines
Removed the zener diodes and directly connecting a +/- 7.5 V bench supply to +Vcc (pin 16) and -Vcc (pin 13)
grounded all unused input analog and S&H lines
grounded the unused output line
None of these seem to work. Any suggestions?