Piezoelectric sensor in the disc brake balancing machine shows 50 hz wave when idle. How to remove this noise, I want to get only piezo waveform?
You need to use special semi-rigid shielded twisted pair to protect high impedance inputs. 

Shield, Earth and amplifier ground must be chosen carefully to prevent sharing any ground current with machines or other equipment line noise filters. Normally Earth grounded at instrument but shield terminated only at sensor end to earth ground under sensor.

The reason for stiffness is that any cable vibration will look like a change in C or charge just like the Piezo.

Piezo devices have high resistance making the wires good antenna to power lines which also make good antenna and the crosstalk is due to this high input impedance needed to prevent loss of charge.

Another possible solution is to add large Clamshell Ferrite CM choke around cable to raise the CM impedance and thus allow 50Hz to cancel by coupling to each wire equally, but I doubt this will work unless design is carefully analyzed for impedance.  

Simplified differential Charge Amp.


showing shielded pair cable.




Here T= RC=540e6 *240e-12= 0.13  with f-3dB(hpf)=0.35/T = ~ 3Hz