Friday 5 July 2013

Minimal ECG using an Arduino and Xoscillo

First off, the picture :)

To display the wave I am using XOSCILLO, a very cool and open source tool (which I wrote :P) that converts your Arduino into an oscilloscope.

Circuit diagram

Here is the scheme, I am using the typical instrumental amplifier(ins amp) and then another op amp to get some more gain. An ins-amp is like an op-amps but with a huge CMRR, I am using a AD8221 which comes in a tiny small package, it's lead pitch is only 0.65mm, if you want to know what I did to create a simple breakout board please follow this link.

I'm using a single supply for the amps that comes from the arduino 5V pin, and I am creating a buffered virtual ground, this is really important.

Here is, a bit dark, a pic of the setup in the breadboard.

Tricks of the trade:

This is what makes the difference between a working ECG and a non working one :)

The main problem is that the device is so sensitive that it get interference from almost everywhere, specially the mains hum at 50hz). As you can see I got a clean signal, and I am not using any kind of serious filtering, note that xoscillo has a low pass filter and I am not even using it, so what are the tricks?

  1. Run it from a laptop and unplug the charger!
  2. Don't use a second monitor, the HDMI port will cause all sorts of high frequency interferences
  3. Get about 1 meter away from the laptop
  4. Buy some serious ECG electrodes, mines are "H124SG * Ø 24 mm", they come in a box of 50 and should cost about 11 euros. 

That's all, if you like it or build it or need help feel free to write a comment, it will encourage me to write more articles :)