Thank you Lefty for your comment.
Sorry for the mess. I'm very new to electronics. The upper ADC is not from an Arduino, but part of a different system which evaluates the NTC and I can't alter it. But it seems I got it wrong. The L,R,C parts should somehow be organized in order to filter the signal because the NTC is connected via a long wire
I hope it is getting clearer why I want to use an optocoupler and a relay. It's because I can only connect to the system directly at the NTC and I want a galvanic separation of the arduino and the other system.