The shell is made of metal, with a screw hole in the middle, which is connected to the ground. What are the benefits of connecting a 1M resistor in parallel with 33 1nF capacitors to the ground of the circuit board?
If the shell ground is unstable or has static electricity, if it is directly connected to the circuit board ground, it will damage the circuit board chip. Adding capacitors can isolate low-frequency high voltage, static electricity, and protect the circuit board. High frequency interference in circuits and the like can be directly connected to the casing by capacitors, playing a role in isolating direct current and alternating current.
Why add another 1M resistor? This is because without this resistor, when there is static electricity inside the circuit board, the 0.1uF capacitor connected to the ground blocks the connection to the shell ground, which is suspended. When these charges accumulate to a certain extent, problems will arise and they must be connected to the ground, so the resistor here is used for discharge.
With a resistance of 1M, if there is static electricity or high voltage outside, it can effectively reduce the current without causing damage to the chips inside the circuit.