External ADC reading zero while input is zero Using ADS1115 (or any other external ADC), how its possible to read zero instead of random floating values while no any signal at the ADC's input pin?
The ADS1115 is driven by an op-amp buffer as follows.

The op-amp's + input is sometimes connected to a pH sensor, sometimes left unconnected.
The behaviour you are seeing is what happens when you have high-impedance inputs.  Basically, if you don't drive them, you get random values.

Unfortunately in your case, where the sensor you are using has no output current to speak of, I don't think there's anything you can do about this electronically.

You'll have to solve the problem in some other way if you really need to be able to disconnect the sensor.  A mechanical solution would be a connector which shorts when disconnected.  Switched headphone connectors do this, for example - when you pull out the jack, they make a connection so that the speakers can be switched on.