scorpio

Hi, I am building my amplifier circuit for ecg and examining it by using an oscilloscope. The circuit consists of instrumentation amplifier(gain = 10), high pass filter(0.5Hz, gain = 3)and low pass filter(150 Hz, gain = 35). The voltage is regulated at 5V. Without feeding any input to the amplifier, I get the square wave(or attenuated sine wave) with frequency 50Hz and peak-peak voltage = 8V on oscilloscope. Is it really due to the power line interference and why the noise has such a large amplitude?? Will it affect my ecg signal?? and how to eliminate it? Thank you for reading my post and hope that you can clear my doubt.