@TomGeorge @johnerrington
Thanks for the replies, both of you!
I'll try to experiment tomorrow because of the time difference.
I'm a complete noob when it comes to "Star point". If you could give me a more detailed explanation (or share a link), that would be great.
When dealing with a mix of analog and digital circuits, noise is introduced from digital to analog through the ground, right?
If the noise gets on, the analog signal becomes a wrong signal and not a correct signal.
I would appreciate it if you could tell me how and why the grounding is appropriate when using an Arduino.
In fact, I once did a test by connecting decoupling caps (10μF) just after R1 (i.e., just behind 400kΩ). This reduced the noise at A0.