Reducing signal noise arduino to simulink analog read
You can reduce the amount of noise generated by the MCU itself by telling it to enter ADC Noise Reduction sleep mode. This will turn off the CPU, I/O, and synchronous timers (which will disrupt delay() et alia) while still allowing the ADC and other critical functions to run. A no-op ADC interrupt will allow the device to wake from sleep once the conversion has completed.

See the "Power Management" section, "ADC Noise Reduction Mode" subsection and the "Analog-to-Digital Converter" section, "ADC Noise Canceler" subsection of the MCU used in your specific Arduino for more details.