TS3A4751: disconnect pin voltage

Terry,

Solid state FET switches like the TS3A4751 will have some leakage current unlike a mechanical switch which physically moves a conductor to create a true open circuit.  A solid state switch that is turned off with create a high impedance path in the  mega ohm range but it is not an infinite impedance open circuit. 

You can calculate the series impedance when the switch is off using the off leakage parameter in the datasheet.  This post talks about the calculation in more detail.

Since you have 3.3 V pulled up on one side of the switch and the other side of the switch is floating, when you turn the switch off and create a Hi-Z signal path the voltage you measure on COM is floating and not really defined. It is reasonable that you see 0.45V on the multi meter because there could be parasitic board capacitance that getting weakly charged .  This is not an issue in actual systems because the voltage driving the COM through the Hi-Z path is so weak that almost any other signal connected to the COM node will be able to drive the node. 

If you place even a weak pull down resistor on the COM pin instead of floating it you will not expect to see the 0.45 V.

What type of circuit are you making using a signal switch with a floating COM pin? 

Let me know if this helps.

Thank you,

Adam

Hello Adam,

Many thanks your explaination so much ~