Currently I am trying to build a portable ECG device where by the ECG signal will then be sent to Arduino UNO where the data will be transmitted via Bluetooth and the waveform will be displayed on Android Phone. I've already built a circuit and have a good waveform. I also managed to successfully transmit the data via Bluetooth and gotten a signal displayed on the phone. I just found out (unfortunately) that Arduino UNO only reads from 0 - 5V. However, my signal range from -220mV to + 550mV (Seen on oscilloscope with AC coupling). That being said, the signal that is being shown on the phone are clipped or only halved are shown and the negative waveform disappears. I have tried non-inverting summing amplifier and it manages to shift the voltage level. I did so by generating an artificial 2.5 volts pseudo ground into a new op-amp.

Here is the problem. When I set the oscilloscope to DC coupling, DC saturation "noise" is still present. The signal waveform is still half. I have attached a schematics of my ECG circuit for references(but it does not include the new non-inverting summing amplifier). I am supplying my circuit with 1x 9V battery where the power is split into +4.5V/-4.5V. I also have successfully built an analogue 50Hz notch filter. How do I remove the DC saturation "noise" If anyone knows how to solve this, please share.

This signal is in DC coupling. Take note this is before the level shifting. After shifting it is a few div above the 0 axis.

This signal is in AC coupling.

Take note, I have 2 signals that I am sending to the UNO. A0 Pin is receiving the filtered signal, and A1 Pin is receiving the unfiltered signal.I am from a Mechanical background, so don't be too hard on me.

 

Use a difference amplifier and subtract a DC voltage from your signal.

Using the scope on AC coupling is distorting the waveform because the frequency response on AC coupling does not go low enough in frequency.

Avoid using any kind of high pass filter.

 

Use a difference amplifier and subtract a DC voltage from your signal.

Using the scope on AC coupling is distorting the waveform because the frequency response on AC coupling does not go low enough in frequency.

Avoid using any kind of high pass filter.

Do you mind elaborating on the difference amplifier. I am quite new to this.

What do you mean by subtract a DC voltage from my signal ? Is it the +2.5V that I added to shift the signal ?

Which high pass filter are you referring to ?

 

A summing amplifier is just as you have shown.
You can inject a DC voltage (or current) into either inputs of the op-amp (inverting or non-inverting).
Whether you get a sum or difference depends on the polarity of the DC voltage and which input it is connected to.