Introduction: BME 305 EEG
An electroencephalogram (EEG) is a device that is used to measure the electrical brain activity of a subject. These tests can be very useful in diagnosing different brain disorders. When trying to make an EEG, there are different parameters that need to be kept in mind before creating a working circuit. One thing about trying to read brain activity from the scalp is that there is a very small voltage that can actually be read. A normal range for an adult brainwave is from about 10 uV to 100 uV. Due to such a small input voltage, there will need to be a large amplification at the total output of the circuit, preferably greater than 10,000 times of the input. Another thing that needs to be kept in mind while creating an EEG is that the typical waves that our outputted range from 1 Hz to 60 Hz. Knowing this, there will need to be different filters that will attenuate any unwanted frequency outside the bandwidth.
-LM741 operational amplifier (4)
-8.2 kOhm resistor (3)
-820 Ohm resistor (3)
-100 Ohm resistor (3)
-15 kOhm resistor (3)
-27 kOhm resistor (4)
-0.1 uF capacitor (3)
-100 uF capacitor (1)
-Arduino microcontroller (1)
-9V batteries (2)
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Step 1: Instrumentation Amplifier
The first step in creating an EEG is to create your own instrumentation amplifier (INA) that can be used to take in two different signals, and output an amplified signal. The inspiration for this INA came from the LT1101 which is a common instrumentation amplifier used to differentiate signals. Using 2 of your LM741 operational amplifiers, you can create the INA using the various ratios given in the circuit diagram above. You can use a variation of these ratios, however, and still get the same output if the ratio is similar. For this circuit, we suggest you use a 100 ohm resistor for R, 820 ohm resistor for 9R, and 8.2 kOhm resistor for 90R. Using your 9V batteries you will be able to power the operational amplifiers. By setting up one 9V battery to power the V+ pin, and the other 9V battery so that it inputs -9V into the V- pin. This instrumentation amplifier should give you a gain of 100.
Step 2: Filtering
When recording biological signals, it is important to keep in mind the range you are interested in and potential sources of noise. Filters can help solve this. For this circuit design, a band pass filter followed by an active notch filter are used to achieve this. The first part of this stage consists of a high pass filter and then a low pass filter. The values for this filter are for a frequency range from 0.1Hz to 55Hz, which contains the EEG signal frequency range of interest. This serves to filter out signals coming from outside the desire range. A voltage follower then sits after the band pass before the notch filter to ensure the output voltage to the notch filter has low impedance. The notch filter is set up to filter noise at 60Hz with at least a -20dB reduction in the signal due to large noise distortion at his frequency. Finally another voltage follower to complete this stage.
Step 3: Non-inverting Operational Amplifier
The final stage of the this circuit is made up of a non-inverting amplifier to increase the filtered signal to the 1-2V range with a gain of about 99. Due the very small input signal strength from the brain waves, this final stage is needed to yield an output waveform that is easy to display and understand compared to potential ambient noise. It should also be noted that a DC offset from non-inverting amplifiers is normal and should be taken into consideration when analyzing and displaying the final output.
Step 4: Analog to Digital Converstion
Once the whole circuit is finished, the analog signal that we amplified throughout the circuit needs to be digitized. Thankfully, if you use an arduino microcontroller, there is already a built in analog to digital converter (ADC). Being able to output your circuit to any of the six analog pins built into the arduino, you are able to code an oscilloscope onto the microcontroller. In the code shown above, we use the A0 analog pin to read the analog waveform and convert it into a digital output. Also, to make things easier to read, you should convert the voltage from a range of 0 - 1023, to a range of 0V to 5V.