Building the Amplifier


All of these techniques use an AD 620 amplifier chip, connected to the hand or head through 3 wires.   For the line in or microphone input of the laptop, the output is a mini stereo plug.  For the Behringer U-Control, it's an audio connector, most conveniently attached to a cut off length of audio cable.   Except for EEG on an Arduino (described on separate pages), one chip, a mini stereo plug, and a power supply (batteries) are almost all the hardware you need.  However, it's a good idea to add an on-off switch and a LED that reminds you to turn the power off.  Otherwise, like all other mortals, you will forget to disconnect the batteries and run them down.  The AD 620 amplifier chips have reasonable internal protection against static electricity and other excess voltages on the inputs;  in 10 years I have never blown one out.  But use common sense, and discharge any static electricity from the user and subject before touching the chip or any other part of the circuit.  You can mount the parts on a breadboard or solder them to a DIP socket.  (Do any soldering before you put the chip(s) in the socket.)

It would be difficult, although not impossible, to blow out your laptop's audio input using a battery power supply. To avoid this, I recommend using two 4.5 V supplies rather than two 9 V batteries.  3-cell AA and AAA battery holders are available online or at Radio Shack.  4.5V is safely inside the protection limits of the Mac or U-Control audio input—unless perhaps you go to the very great extreme of putting the terminals of the combined batteries directly onto the audio input jack.  Alternatively, a few more parts can be used to split a single 9 V battery supply with another op amp, as shown in the second schematic.

Almost certainly you will have to mail-order the AD 620.  Newark Electronics has lots in stock.  Make sure you get the DIP and NOT the surface mount (soic) version.  Since shipping will probably cost more than the chip, you might as well get an extra one and make sure you include anything else you need like breadboard, sockets, battery clips, switches, LEDs, wiring, etc.  However, most or all of the other stuff can also be found at Radio Shack.  

A very basic schematic is shown below.  The upper left shows the amplifier circuit without the battery connections.  The lower right shows the battery connections, including the DPDT switch needed to turn on and off two batteries, plus an LED and resistor to indicate whether it's on or off.  The battery connections may look more complicated than the basic amplifier circuit itself, but this is misleading.  The "floating ground" connections should all come together at pin 5.  The LED resistor can be anything from 1K to 10K, but smaller values will just drain the batteries faster.  The 1000 ohm resistor sets the gain of the AD 620 to about 50.  (Click on images to enlarge.)



Below is a photo of the simplest possible circuit with the battery leads connected directly to the chip. The leads 
marked "B" connect the batteries. The 3 leads connect to the subject and the output connects to the laptop via the
mini phone plug. Make sure that the tip of the mini phone plug connects to pin 6, and the bottom plug section to pin 5.
 The output wires should either be twisted for most of their length, or made from a piece of thin coax, like microphone
or stereo cable. Make sure that you get the battery connections right, and that all connections to the breadboard are
tight. This may require tinning the ends of braided wires with solder so that they jam in firmly, or even soldering them
to thicker pieces of solid wire.

                                                                            
                                                                            


A slightly more complicated version of the circuit includes an op amp to split the power supply from a single 9V battery.  Almost any op amp will do;  I have used a 741 and a 3130. (The latter will need a small compensation capacitor.)  An extra op amp is much cheaper than an extra battery. The + and - power supply connections to the chips are still at pins 7 and 4.  The paired ~15K resistors could be anything near that value as long as they are the same.  The neutral connections are now green wires.

For EEG recording, which involves much smaller signals, it can be useful to add a low-pass filter section and thereby reduce the 60 Hz noise.  This filter is formed by the 1.8 K resistor and 4.5 uF non-polarized electrolytic capacitor at the output.  The filter reduces 60 Hz noise by roughly 2/3, without much affecting the EEG.  You could use it with some benefit for EKG as well, but it would wipe out most of the muscle signal and should not be used for EMG.


                                                                              


Even commercial EEG systems and professional electrodes can be overwhelmed by 60 Hz noise in highly electrified environments, like intensive care units.  They occasionally need a 60 Hz "notch" filter to seriously clean up the signal.  If you just can't get rid of 60 Hz noise—despite proper skin preparation—and have some experience at breadboarding, you can use one half of a dual FET op amp, like a TL082, to split the power supply and the other half to build a one-chip notch filter as described here:   http://freecircuitdiagram.com/2008/11/27/notch-filter-the-circuits-diagram-and-the-design-formula/ 
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