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The Biosignal How-To

We have been featured on Hackaday: All About Biosignals


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 circuit diagram of the bio-amplifier

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.

Or do you want to buy an assembled bioamp instead to focus on the ECG/EEG/EMG experiments?

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).

How to get the components

Here is the bill of materials:

Qty Value Parts Farnell order codes
4 1N4004 D1, D2, D3, D4 SC07335
1 1K R2 9465170
1 1M R9 9465499
1 4.7uF C1 9752315
1 100K R1 9463895
2 100R R3, R4 9463909
5 100nF C2, C3, C4, C5, C6 1006030
1 INA126 IC4 1417402
1 TL071 IC1 1459696
1 Pad per hole PCB 2295747
  • Farnell have a really good product selection tree so that you find your components quickly. The order codes can directly be copy/pasted into their Product Search form at the top of the page.
  • RS will provide you also with all components but the web page is a bit more cumbersome.
  • MAPLIN is good for standard components such as opamps, capacitors, resistors, etc. and you can actually hold the components in your hand before you buy them.
  • Texas Instruments might send you a free sample of their instrumentation amplifier. Give it a go.

Assembling the bio-amplifier

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.

Non-inverting OP-amp (2nd stage amplifier)
Instrumentation amplifier (1st stage amplifier)

Using the bio-amplifier

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.

The oscilloscope program "comedirecord" was used for all recordings.


Non electrical biosignals

Processing the data with MATLAB or OCTAVE

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:

Advanced: Do you want to have less analogue circuitry and do more DSP?

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.


Further infos

start.txt · Last modified: 2017/11/20 20:34 by berndporr