EEG/EMG/EEG preamp

Download

ecg_eeg-1.0.tar.gz which contains the eagle files and the BOM.

Description

The amplifier operates at a gain of G>=500. Thus, if you have an ECG of approx 1mV you'll get with a gain of G=500 0.5Vp-p at the A/D converter. For EEG recordings you can increase the gain to G=10000: An EEG has an amplitude of approx 100uV so that you'll get 1V at the input of the USB-DUX. You can further increase the gain. However, it's better to have a lower gain as eye movements and other muscle activity can generate much stronger signals. A simple low pass at 0.1Hz filters out electrode drifts.

Please note that the circuit has no anti-alias filter so far. The reason is that the USB-DUX operates at 1kHz which is usually far above the highest frequency of EEG or ECG signals (approx 100Hz). So, you can do the anti-alias filtering by software (sinc function, etc).

How to record?

Einthoven ECG: Close the jumpers called "Einthoven". Connect shielded cables (IEC convention) to the pins which are labled "red, yellow, green and black". To record use, for example, the program comedirecord. With the same setup a vector ECG can be recorded.
EMG: same as above.
EEG: Leave the Einthoven jumpers open. Connect the electrodes directly to SV1, SV2, SV3 and SV4. Set the gain to 10,000 by setting R13, R4, R15, R18 = 47R (instead of 470R)
Pressure: you can connect a pressure sensor or a microphone to channel 4. It provides regulated 3V for the sensor. If only changes are needed use 0.1uF capacitors between the sensor and differential input pins. Or you can configure it as an additional EEG or ECG channel.