50hz noise removal from ECG power supply
(note: the question said the noise was in the signal. Seems to be the power supply; see my edit further down.)

A 50 or 60 Hz notch filter is typically done as a twin-T filter. A passive twin-T has a poor Q-factor however, meaning that neighboring frequencies also will be attenuated, which can cause the ECG profile to be distorted.  

An active notch filter may look like this:  

  

Don't forget to recalculate the resistor values for 50 Hz; R1 will be 11.8 M&Omega;. Any opamp will do. The difference with the passive filter is shown in this graph:



Maybe not so clear, but the active filter's graph is the vertical line at 60 Hz. Much better than the passive filter.

edit re your comment
If the 50 Hz is on the power supply like you say it needs decoupling. First the power supply itself. A good regulator won't have a 50 Hz ripple on its output, so maybe the smoothing capacitors on the inputs are too small. (On second thought that ripple should be 100 Hz. A defective diode in the rectifier bridge would explain both 50 Hz and a too high ripple.) If the input ripple is much too large it may go below the minimum input voltage of the regulator. Can you post a schematic of the power supply with component values? You can also place a 100 &micro;F capacitor at the output.

Also use a 100 &micro;F in parallel with a 1 &micro;F on the power supply inputs of ICs. If they're low power you can place a 10 &Omega; to 100 &Omega; resistor in series with the supply line, before the capacitors. So the capacitors are directly to the IC's pins. Note that the resistors will cause a voltage drop on the 5 V, so only use the 100 &Omega; if the supply current is less than 1 mA. Up to 10 mA you can use the 10 &Omega;. Higher is better, you'll have to see what you can afford. 10 mA through a 10 &Omega; resistor will cause a 100 mV drop, that's 2 %, which is probably acceptable.