Getting rid of noises after RC low pass filter stage
Radiated or Conducted?
True error or measurement error?     

Ground shift ? probing error**?  induction loop current , capacitive coupling?  

Also avoid long grounds on scope probe gnd loop near switched inductive sources.

For a 10:1 probe** ground lead , 10cm is too long.  Best Test Eng practise is  by removing gnd clip and probe tip and use twin test points to tip pin and barrel. If not avail make them with resistor wire leads by cut and solder as test points. 

Or get the accessory for the probe or make one.


Use twisted pair for all sources of interference and use low ESR caps on Supply.    Use braided or heavy ground conductors.
Keep wiring short and neat.   

Show schematics and photo of possible source and of interference.