SN74LVC2G66-Q1: question about isolation between associated switch section

The I/O pins have clamping diodes to V_CC, so you should not apply voltages in the power-down state.

Hello junsong,

To add on to what Clemens said I think if you want the device to isolate a signal when it is powered off you will need a device with power off protection.

A good replacement for this application would be to use 2x TS5A3166-Q1(which has power off protection) which would give you the same functionality as the device you mentioned.

Here is a comparison of these 2 devices 

Please let me know if you have any further questions

Hello Junsong, 

So  to first answer your question this device is  disconnected but  it will have some resistance value when vcc=0 but it will not be an infinite resistance. It will have a value but it should be in very high like in the kohm range. 

But to further help you understand this device signal paths are basically mosfet switches. So when the power supply voltage(vcc) is zero  and you put a high enough voltage on the io pins it will send the switch into a mode called the breakdown region where it basically becomes a short circuit and allows current to increase through the switch exponentially. But if you follow the recommended operating conditions of this device you should never see this short circuit phenomenon. Also when vcc=o your device will be in the cutoff region where its Ron will between the source and drain will be high like you have observed

Please let me know if you have any further questions! I