Nice Wires
EEG signals at the scalp surface are very weak. We need high gain and low noise
amplification.
Source impedance tends to be high. Good electrode contact to scalp becomes critical.
Above conditions make 50Hz (or 60Hz) power line interference a headache.
This amplifier is based in the AD620, a very good instrumentation op-amp. It provides
differential input and high common mode rejection.
A 50Hz notch filter follows. The rest of the circuit adds amplification factor and
adequate overall frequency response.
It is then followed by a simple PWM analog to digital converter and an optical isolator
for safety. The digital output will be read through the CD serial port line.
I use two 9v batteries to provide +9v/-9v power supply to the amplifier and +9v to the
PWM. The isolator takes its power from the RTS line.
Please, be always very careful when operating electrical devices in contact with
any part of your body. Be sure to be effectively isolated from ac power lines and, better,
also from ground.
The amplifier schematic:
The frequency response:
The amplifier pcb:
The converter and isolator schematic:
The converter and isolator pcb:
