I'm doing an ECG project where I've built most of the front in circuit and once finished I want to sample with an ADC and then either plot or calculate heartbeat from. Set up This is the set up right now. Its running off two 9 volt batteries in series for positive and negative voltage. The diff amp's gain is set to 125 and the non inverting op amp is set to 19.5. This gives me a signal around -1 to 3.5 V's. ECG wave This photo is when the gain was a little higher for testing but shows the general wave. The problems, I'm having are that the signal "moves"; its not always centered around 0. This is an example of when it drifted down Then I only have the peaks being read by the ADC. The second problem is the negative part of the wave. I've tried to add in a DC offset by a voltage divider on the output but just lose my signal. Any idea why the signal moves besides noise and only being a 3 lead electrode and more importantly can someone tell me what I need to do to add some DC offset to this configuration. I'll either be using the arduino's or the mbed's ADC to sample the signal. Any help is greatly appreciated!