Analog Switch for RS232 Level.

The Vee connection (named Vss by ADI by the way) of the ADG333 limits how low your voltage can go on any of its pins.

If you look at the absolute maximum ratings in the data sheet, you will see that this limit is Vss -2V or 20mA whichever occurs first. The device has protection features in it that start conducting if you exceed the supply voltage range. This is what you are observing. You will need to provide the switch with a negative supply.

So instead of hooking pin 5 to ground connect it to pin 6 of the MAX232 which is its negative supply voltage.

This will still limit your range for any incoming RS232 signals which might have larger voltage ranges.

A sneaky way of solving this would be to just connect a 10nF cap to ground on pin 5 of the ADG333.

This would charge it through the protection diodes to the lowest voltage seen on its inputs. You could add a zener to limit the maximum voltage. Still not really production ready, but Vss currents are specified at 5uA max so this will very likely work.

Klaus

The Vee connection (named Vss by ADI by the way) of the ADG333 limits how low your voltage can go on any of its pins.

If you look at the absolute maximum ratings in the data sheet, you will see that this limit is Vss -2V or 20mA whichever occurs first. The device has protection features in it that start conducting if you exceed the supply voltage range. This is what you are observing. You will need to provide the switch with a negative supply.

So instead of hooking pin 5 to ground connect it to pin 6 of the MAX232 which is its negative supply voltage.

This will still limit your range for any incoming RS232 signals which might have larger voltage ranges.

A sneaky way of solving this would be to just connect a 10nF cap to ground on pin 5 of the ADG333.

This would charge it through the protection diodes to the lowest voltage seen on its inputs. You could add a zener to limit the maximum voltage. Still not really production ready, but Vss currents are specified at 5uA max so this will very likely work.

Klaus