power supply noise remove

I am trying with Mega2560 and ADS1232 A/D chip and all connected to to my laptop. when the power supply adapter is connect, signals (in serial plotter) are noisy & biased! when i touche the GND of board or GND of USB cable with my hand, noises & biases are removed!

*** GND of board & A/D chip & power supply are connected too.

please help me!!!!!!

thanks

Try connecting the Gnds to an actual mains earth.

Laptop supplies give off significant AC component from capacitive coupling with the mains, so unless properly grounded a laptop will tend to float at perhaps a few dozen volts AC, which may be interfering with your measurements. If your whole circuit is screened to its own ground, this wouldn't be a problem, but you are no doubt capcitively coupling the signals you are measuring to a true ground, hence inducing noise.

When you touch the laptop, you are effectively grounding it. [Or perhaps it is grounding you and you were the source of noise in the first place]