This page shows you with the help of YouTube clips how to build a bio-amplifier and how to use it to measure ECG, EEG, EMG and other bio signals.
Besides the hands-on approach in amplifier design you will learn how to to distinguish noise from actual biosignals and will see that often noise is “sold” as a biosignal, even in top level publications. With the help of these clips you can critically evaluate such results and make up your own mind.
The bio-amplifier is a two stage amplifier. The first stage is an instrumentation amplifier and the second stage is a standard non-inverting OP-amp. Between the first and second stage is a highpass filter which removes any DC generated by the electrodes. For that reason the gain of the instrumentation amplifier is kept low not to be driven into saturation. The highpass filter usually has a cutoff frequency in the region of 1 - 0.1Hz depending on the signals measured.
For those who are too scared of a soldering iron… We've made a fully tested and CE marked bioamp which is called "ATTYS". Vasso Georgiadou, our main presenter explains how it works (and why it's called Attys):
The Attys follows the same approach as shown on this page, namely two stage amplification but does the highpass filtering and 2nd stage amplification in software. Even that it's ready made you can still change all filter parameters in software. All software is open source so you can learn how it's done in digital and then hack the code to write your own applications. The Attys has software for general purpose data recording (AttysScope), ECG (AttysECG) and EEG (AttysEEG).
Here is the bill of materials:
|Qty||Value||Parts||Farnell order codes|
|4||1N4004||D1, D2, D3, D4||SC07335|
|5||100nF||C2, C3, C4, C5, C6||1006030|
|1||Pad per hole||PCB||2295747|
I show you step by step how to do it. Component by component. First, we assemble the 2nd stage amplifier which is a non inverting OP-amp at a gain of G=100. Then we add the 1st stage amplifier which is an instrumentation amplifier at a gain of G=5. The signal of the bio-amplifier is then fed into an USB-DUX sigma data acquisition board which also provides the electrically isolated power.
The prototyping PCB we use is a matrix board (not a vero/strip board) which has a solder pad per hole and the wires of the components will form the tracks. Martix boards allow optimal placement of the components so that tracks can be kept at minimum length.
After having assembled the amplifier watch this clip which explains in general how to measure any electrical bio-signal.
These sections show you that you can use the same amplifier to record ECG, EEG and EMG. Besides showing you how it's done we have a strong focus on the artefacts generated so that you can critically assess your own recordings and those of others. The section about non-biological signals gives you pointers to other biosignals and also shows in comparison with electrical signals that they are often much more reliable.
For offline data processing MATLAB and its free version OCTAVE are very easy to use. Here we show how to plot an ECG, its spectrum and how to remove 50 Hz mains noise from it. The same can also be done with EEG, EMG, etc:
When using a sigma delta converter with a high resolution (such as the USB-DUX sigma) you can do the highpass filtering and second stage amplification in software. This excellent application note describes how it can be done using a minimal amount of analogue components. Our ready made bio amp 'Attys' also uses this approach where the highpass filtering is done by a digital filter in the application software.