The Biosignal How-To
This page shows you with the help of YouTube clips how to measure heart activity (ECG), brain waves (EEG), muscle activity (EMG) and other bio-signals.
You will also learn how to to distinguish noise from actual biosignals and will see that often noise is “sold” as a biosignal. These clips will help you to critically evaluate these claims and make up your own mind.
Check out our YouTube channel for additional videos and playlists.
The clips below take a homourous approach how to interpret biosignals and show that there is no easy route from mental states to, for example, blood pressure and that noise can be mistaken as EEG.
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.
| 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 |
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.
When using a sigma delta converter with a high resolution you can do the highpass filtering and second stage amplification in software. This excellent application note describes it in detail. Our ready made bio amp 'Attys' also uses this approach.
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).
