Re: Analog Switch question

MRW wrote:
Hello

I have an analog switch question. For this example, I would like to
use Texas Instrument's TS5A3159A SPDT Analog Switch.

For the digital input (triggers the switch ON or OFF), it has the
following electrical characteristics:

Input Logic High - 2.4 to 5.5 V
Input Logic Low - 0 to 0.8 V

I have the following questions:

1. What happens to the switch at 0.8 < Voltage at digital input < 2.4?
Does the switch either turns ON or OFF?

Possibly. Or possibly leaks a bit of signal through a high resistance, and sucks power supply current. It is an ill defined operating point, so generally you try to get through it pretty quickly.

2. What happens to the switch if I leave the digital input open?

The input leakage current spec is a couple nanoamperes at room temperature, and in either direction, which implies a very high input impedance. If left floating, the voltage on the input is indeterminate and may float to anywhere between the supply rails, and if any static electricity is nearby, it may get driven a bit beyond that. This is not a good idea. If it is possible that the input may be disconnected from its logic signal, even briefly, with the power on, I would connect a pull up or pull down resistor that is a low enough resistance to make sure the input voltage stays on one side or the other of the .8 t o2.4 volts no man's land. For instance, if you want it to default to the low voltage state (less than .8 volts with the absolute maximum leakage current of 100 nA trying to pull it positive) I would connect a .8V/100nA=8 meg resistor from input to ground. Actually, I would probably use a 1 meg resistor, since that would add an insignificant load to the logic driver.

3. What other ways can I use to control the digital input
electronically? As of now, I can only think of using a comparator.

Logic gates and inverters can also be used.


4. Would it be okay to leave the NC or NO terminal floating until a
signal source is connected to it?

Yes.
.