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/
Getting all the way down to the 1 Hz frequency range of EEG requires communication through a digital port; in the current era this would be a USB connector. A cheap and convenient device for recording and transmitting data through USB cables is the Arduino, which costs about $30 (less if you assemble your own). The Arduino performs reasonably fast analog--to-digital conversion and doubles as a platform for additional circuity. It's programmed as a serial port but automatically translates to USB. The software, including the Processing language, is open source, and large amounts of information are available on the internet for different platforms and applications. The techniques described here should work for Macs, PCs, and Linux. [However, CPU speed may be critical. The program timing works fine with my 2-year-old Mac Powerbook. It's too slow to be useful with my new Samsung netbook and Intel Atom.]
The downside of the Arduino is that it cannot resolve voltages as small as those handled by an audio or mike input, so a more complicated amplifier with greater gain is necessary. The upside is that when not recording EEG you will have a flexible device with vast potential for other interactions between your computer and the outside world.
The onboard power available from the Arduino is only 5 V. This leaves very little room for DC offsets from the electrodes, so the gain of the AD 620 is limited to about 20. A second stage using the ancient workhorse 3130 op amp provides additional gain plus signal filtering and DC biasing for the Arduino's analog input. Because that input is relatively noisy, it requires some low-pass filtering, which may as well do double duty for reducing 60-Hz noise. Another stage of filtering in the feedback loop needs only a single capacitor, so we will throw that in as well. The listed component values will therefore provide a second order low-pass filter. The combined filter is not well-characterized, but is good enough for our purpose. It helps ameliorate both the 60-Hz noise problem and muscle contraction artifacts, without unduly reducing normal EEG activity. (Because the Processing routine we have borrowed uses a rather low sampling rate, muscle noise might tend to produce aliasing, which looks like activity in the EEG frequencies and can be very difficult to decipher.)
You could use this circuit for EKG as well, but I don't recommend it. The gain is actually too high, and several circuit adjustments would be required just to switch back and forth between EEG and EKG. The only advantage would be a more accurate reflection of the ST segment and the T-wave. This is likely to be of importance only to doctors, who really should be using real EKG machines.
Preamp Parts List:
-Arduino Decimilla or Duemilanova , usb cable, protoboard/breadboard, hookup wire
- multimeter
-AD 620 instrumentation amplifier (consider buying a spare)
-3130 CMOS op amp (2)
(Again, make sure you get the DIP packages for the chips, not the surface mount)
-15k resistors (3)
-1M resistors (2)
-10K resistor (1)
-2.2K resistor (1)
-1M trim pot (1)
-10 uF electrolytic capacitor, preferably non-polarized (1)
- .0068 uF capacitor (1)
- .68 uF capacitor (1)
- .001 uF capacitor (2)
A schematic of the overall circuit and a closeup of the connections are shown on the next page. If at all possible use a breadboard that fits inside a protoboard, not sitting outside with long wire runs that can come loose or pick up noise.
By far the most important step for recording decent signals is to make proper electrical contact with the skin. If you don't take this statement seriously, you are likely to get nowhere. Especially for EEG. Just putting bare metal on the skin doesn’t make an adequate connection. The skin must be prepared, and an electrolyte—a conductive interface—is needed, along with a metal electrode. Electrolytes and metal electrodes in turn are the basic components of batteries—a property which can cause serious trouble. If the metal-electrolyte interface is in any way different at the two active electrodes, their "half cell" potentials will be different, resulting in a small DC offset. An offset of only 100 mV (which is puny by battery standards) when multiplied by a gain of 50 gives 5 volts. Since the AD 620 output cannot exceed the 2.5 - 4.5 volts positive or negative provided by the Arduino or the batteries (in reality, it won't even get that far), the amplifier will lock up, and the output will be a flat line. A bit of corrosion on the electrodes, or different electrolyte mixtures/different metals on different electrodes, can be enough to cause this.
Equally annoying is the 60-Hz electric field that permeates all of our bodies in electrified environments. This can easily exceed 0.1 volt. (Yes, that's about 2000 times larger than the EEG.) Instrumentation amplifiers like the AD 620 are optimized to reject this "common mode" artifact, which is about the same on different parts of the body. However, large and unequal electrode impedances will cause 60-Hz noise to sneak in and swamp the signal.
(It might be possible to get by with lousier skin connections using active instrumentation amplifiers with feedback to the neutral electrode. Such amplifiers are often labeled by their original usage in EKG as "driven right leg." However, they won't fix DC offsets, are more complicated, and in my experience are flakier than the simple passive amplifiers described here.)
Even after initial amplification, the EEG is so tiny that 60-Hz noise can be picked up from the preamp circuitry. The preamp gain of around 20 - 50 therefore represents a compromise between the need to get the signal up out of the noise as soon as possible while minimizing the perils of DC offset from the electrodes. The use of microphone cable or a twisted wire pair from the preamp to the audio jack helps to reduce additional 60 Hz pickup on the way to the computer.
Professional neurophysiologic recording is done with fancy electrodes made of gold or silver/silver chloride. The skin is often rubbed with fine suspensions of abrasives and EEG electrodes are glued on with specially-formulated sticky electrolytes. These items are generally unavailable to amateurs. However, if you happen to have a contact at a hospital or neurologist's office, I suggest you use it and get some commercial EEG paste. Usable alternatives are brass, very fine sandpaper, rubbing alcohol, and K-Y jelly thickened with flour. (Copper is widely available in the form of US pennies, but is very vulnerable to corrosion from electrolytes.) Prepare the skin by rubbing gently with the sandpaper, and then wipe it with alcohol and allow it to dry. Mix a cc of K-Y jelly with flour until it is the consistency of Play-Doh, and then scoop up a thick dab of it on each electrode.
Electrodes on the hands and arms can be taped in place for additional security, and you can get by with K-Y jelly alone. EKG and scalp electrodes are a problem, making the combination of K-Y jelly and flour more useful. The electrode and wire lead should be light and thin so that their weight and stiffness doesn't pull them off the head. I used thin, 1/2 in wide brass strips from Ace hardware, cut into octagons 1/2 in across. The leads were 26-gauge multistrand wire peeled from a connector strip and soldered to the back of the brass electrodes. (You DO NOT need thick electrodes, thick wire, shielded wire, or heavy insulation. Magnet wire would do fine except for the tendency to break with repeated use.) The active face of the electrode should be washed and polished gently before each use, to remove corrosion and polarization. The solder joint should not be part of the electrode-scalp interface. I cover it with epoxy.
When you are done for the day, soak the electrodes and clean off all the electrolyte before you put them away.
1. SETTING UP THE SOFTWARE
Using the Behringer U-Control:
Plug in the Behringer U-Control usb connector, wait until the new hardware is recognized on the PC, and start VA or MacScope II. Connect the AD 620 amplifier to the Behringer U-Control. In Audacity set the sound input device to audio codec. VA should recognize the codec automatically; otherwise set it manually. MacScope ii usually requires that the input be set manually.
For a PC, Zelscope is the simplest way to display data segments fro 50 mS to 10 seconds. VA will run robustly as an oscilloscope, frequency analyzer, and several other things, although it won't show more than about 100 msec chunks of data in real time. Thus it's most useful for EMG. The Capture Scope button on VA can grab long duration samples, and the Zoom Out option will show them, but it has automatic scaling that interferes with optimal display. Despite the name, MacScope II has versions for both Mac and PC, and will show up to 250 msec chunks in real time. All of these programs recognize the Behringer U-control. (Unfortunately, the PC version of MSII crashes on my Samsung netbook. It works fine on my old IBM tablet.) Set the display timebase to about 10 msec for VA, or 250 msec for MacScope II. You will probably need to set the display zoom between 50 and 256 on VA.
After verifying that you have a decent signal with VA or MacScope II, You can also record, save, and review long segments of EMG and EKG with Audacity. But on a PC or a Mac using the U-Control, the Behringer gain is too low to see the signal during live recording. You will have to record a segment blindly and then boost it using the Amplify control, whereas you can sort of monitor the signal while recording in VA and MacScope II.
Using the Macintosh audio line input:
You can use Audacity or MacScope II as above, or download CB. For CB, cut and paste one of the programs below. Open the Sound section of System Preferences, Select Input, and then Line In. Adjust Input volume to about 50-75%. Then go back to CB and enter 'run'. On my Powerbook CB gives an annoying flash every time it cycles, but it provides a decent display that you can modify however you want. Stop CB by typing the Apple key together with the "." key. If the data is either too large to fit on the display or too small, you can adjust it with the Sound input control or by changing the variable "sc" in line 20 of the CB program. CB seems to limit the graphics window to 1024 pixels wide, which corresponds to 93 milliseconds of data.
'CB PROGRAM 1: 93 MILLISECOND DISPLAY
10 ' THIS PROGRAM GRABS AND DISPLAYS 93 MS
11 ' BLOCKS OF EMG FROM THE AUDIO INPUT
12 ' AND DISPLAYS THEM. SOUND INPUT PREFS
13 ' MUST BE SET TO LINE IN, WITH INPUT
14 ' VOLUME AT 50%-100%. CHANGE THE SCALE
15 ' BY CHANGING sc IN LINE 20.
16 ' BECAUSE AUDIO SAMPLING RATE IS 11025,
17 ' 93 MSEC IS 1024 POINTS.
18 ' STOP IT USING [APPLE][PERIOD].
20 n = 1023 : v = 200 : sc = 52
21 ' (# OF SAMPLES: VERT OFFSET: VERT SCALE)
30 dim a(n)' WHERE THE SAMPLES ARE STORED
40 graphics 0 ' SET UP GRAPHICS WINDOW
50 graphics window 200,300,n,400
60 macfunction("record",a(0),n)' GRAB AUDIO
1000 graphics color 0,0,0
1008 graphics fillrect 0,0,n,400,1
1010 graphics color 0,0,100
1020 graphics moveto 220,20
1030 graphics drawtext "93 MSEC PER SCREEN"
1080 graphics color 100,0,0 ' DRAW EEG IN RED
2000 moveto 0,v-a(0)*sc
2010 for i = 0 to n
2020 lineto i,v-a(i)*sc
2030 next i
2035 x = macfunction("wait",1)
2040 goto 60
'CB PROGRAM 2: 5 SECOND DISPLAY FOR
'EEG, EMG, OR EKG
10 ' THIS PROGRAM GRABS AND DISPLAYS 5-SEC
11 ' BLOCKS OF DATA FROM THE AUDIO INPUT
12 ' AND DISPLAYS THEM. SOUND INPUT PREFS
13 ' MUST BE SET TO LINE IN, WITH INPUT
14 ' VOLUME AT 50-100%. CHANGE THE SCALE
15 ' BY CHANGING sc IN LINE 20.
16 ' BECAUSE AUDIO SAMPLING RATE IS 11025,
17 ' WE USE ONLY 1 SAMPLE IN EVERY 55.
18 ' STOP IT USING [APPLE][PERIOD].
20 n = 55225 : v = 200 : sc = 52
21 ' (# OF SAMPLES: VERT OFFSET: VERT SCALE)
30 dim a(n)' WHERE THE SAMPLES ARE STORED
40 graphics 0 ' SET UP GRAPHICS WINDOW
50 graphics window 200,300,1005,400
60 macfunction("record",a(0),n)' GRAB AUDIO
1000 cls ' CLEAR SCREEN, THEN GRAPH NEXT DATA
1010 graphics color 0,0,100
1020 graphics moveto 420,20
1030 graphics drawtext "5 SECONDS PER SCREEN"
1040 ' DRAW BLUE LINES FOR SECONDS
1050 for j = 200 to 800 step 200
1060 moveto j,50 : lineto j,350
1070 next j
1080 graphics color 100,0,0 ' DRAW DATA IN RED
2000 moveto 0,v-a(0)*sc
2010 for i = 0 to n step 55
2020 lineto i/55,v-a(i)*sc
2030 next i
2040 goto 60
To use Audacity on a Mac with line input, start with Input volume at 100%. See if you can also listen to the EMG by checking the playthrough boxes. You should now be able to hear and see the muscle signal. (If you hear/see 60-Hz noise, go back and check your circuit and your leads.) To see the EMG in full detail, you will have to zoom in on the recorded signal and display only 50-100 msec at a time.
Using a PC Microphone input (EMG only):
The preamp output must have the signal from pin 6 connected to the tip of a mini stereo plug, and the neutral reference to the base of the plug. Leave the middle part of the plug unconnected. Set up VA or MacScope II as above. Because mike input gains vary greatly, you may have to play with this. On both my PC laptops, there seems to be some sort of AGC, so if you contract continuously the signal fades away after several seconds. Intermittent contraction works fine.
2. PUTTING ON THE EMG ELECTRODES
If you spread your fingers apart, you should see a small muscle contracting on the back of your hand, between thumb and index finger. This is the first dorsal interosseous (FDI), and a good place to put one active electrode. After wiping the skin over it briskly with a small amount of alcohol and letting it dry, cover the face of an active electrode with a 1-mm layer of electrode paste and stick it on. Prepare the skin and put the other active electrode on the side of the knuckle of the index finger. In the same way, put the neutral electrode on the back of the hand. Hold the electrodes down with whatever tape you consider friendly to your skin. Connect the batteries.
3. PUTTING ON THE EKG ELECTRODES
EKG can be recorded almost anywhere on the body. For fastidious subjects who wish to keep all their clothes on, you can just put one of the active electrodes on each wrist and the neutral lead over the crease on the inside of either elbow. But you will get a larger signal if one electrode is over the heart. Instead of an active electrode on the left wrist, put that one just below the middle of the left pectoralis muscle, over the highest spot where rib is not covered by muscle. Prepare the skin and electrodes as described just above, and then connect the batteries.
4. RECORDING EMG AND EKG
UNPLUG THE LAPTOP POWER SUPPLY. If you have a voltmeter handy, check that the voltage coming from the preamp at the output plug is some value smaller than the voltage of the batteries. Stick the amplifier output plug into the audio input jack. (On a Mac this is the one with the picture of a microphone or tiny triangles, not the one with the earphones). Open the Sound Input Panel and set it to Line input or Audio Codec, depending on what you are using. For Mac line in, turn input volume to between 50 and 75% to start with. Run the CB EMG program, MacScope ii, or Audacity. If you see prominent 60-cycle noise, turn off the power, pull out the audio plug, and check your connections. DO NOT TINKER WITH THE CIRCUIT WHILE THE POWER IS ON. Push down on the electrodes to be sure they are making contact. Turn off and if possible unplug all nearby electrical gadgets, including overhead lights. Pictures of 60-cycle noise and good signals are shown below. If the signal is too big, adjust the input volume on the laptop. (Click on figures to enlarge.)
Good EMG with CB
60-Cycle Noise
Spread and relax your fingers. You should see EMG signals come and go. Try different finger movements and see which ones maximally activate the FDI. With VA, you can easily see the frequency spectrum of the EMG in real time. It moves toward both higher amplitude and higher frequencies as you contract harder. If you contract the FDI long enough to make it fatigued, this will also shift the frequency spectrum.
EMG Bursts Recorded with Audacity
EMG Spread Out with Audacity
EMG for 100 Msec with PC Mike Input and VA
With either a PC or a Mac, you can use Audacity, VA, and MacScope II to do a frequency analysis of the recorded data. If you have a Mac and are truly ambitious, CB also includes fast Fourier analysis routines, with which you can write your own frequency analysis programs.
EKG Recorded With Low-pass Filter and CB
EKG Without Filter on Audacity
EKG With Filter on Audacity
5 Seconds of EKG without Filter Using Zelscope and U-Control InputTo display long-duration signals with Zelscope, set the sampling rate to 11025 and the buffer length to 5000 mS.
5 Seconds of EKG with Filter on MacScope II
The different sizes of the tall narrow QRS complexes are due to undersampling on the display.
For convenience we are going to put two electrodes side by side, about 2 inches apart, on the forehead--just below the hairline. One of these should be the neutral lead. The third electrode, which should NOT be the neutral one, goes on the back of the head for maximum EEG signal. There is a bump at the back of the skull where it sticks out farthest, just above the neck. Go an inch above that bump. Prepare the scalp, mix up some electrode paste, clean and prepare the electrodes as described in that section.
Homemade electrode paste may not be enough by itself to hold the electrodes on. Some skin-friendly tape should take care of the two on the forehead. For the third electrode I took a 26-inch piece of double-sided velcro, an inch wide, and wrapped it around the head from the posterior (occipital) electrode to just above the eyebrows. This should be just snug enough to keep the occipital electrode secure.
After connecting the Arduino amplifier, start the Processing program as described under "Installing the Software for Arduino." (Once it's installed, the Arduino will run the acquisition program indefinitely every time it is plugged in, until you do something to change it.)
Or, using the simple preamplifier and the Mac audio input, hook up the leads, plug the jack into the audio line in plug, start the appropriate CB program, and turn on the preamp power.
On PCs without an Arduino, the easiest combination is the Behringer U-Control and Zelscope, although the maximum signal height will be relatively small. Using the Behringer U-Control and MSII for EEG allows a larger display but is less direct, because MSII only displays 250 mSec of data in real time. However, it has the advantage of showing you very clearly whether you have excess 60-Hz noise (or muscle contraction artifact). I have found this approach to be more difficult for EEG. If you try it, first plug in the U-Control and let Windows recognize it. Then start MSII; it should also recognize the U-Control as input. Then plug in and turn on the simple preamplifier. You will have to play with this one.
The famous alpha rhythm comes out with the subject awake, relaxed, and eyes closed. It varies greatly in amplitude among normal subjects. Below are two examples of my EEG, which is not particularly high amplitude, using the Arduino. (Click on the figures to enlarge them.)
Five seconds of EEG sampled at 180 Hz, which is the fastest the processing program runs on my laptop, and the best result. Arrow 1 points to the "artifact" from the moving lids as I close my eyes. Arrow 2 points to the "artifact" as I open them. In between is a train of alpha waves around 10-11 Hz that is maximum for a few seconds after eye closure.
Ten seconds of EEG sampled at 120 Hz. On the left is a series of eye blinks ending in eye closure (arrow 1). On the right is a train of alpha. Arrow 2 shows eye opening.
Below is a sample of my wife's EEG using the simplest amplifier and CB on a Macbook Pro. There is an eye closure on the left,
with alpha most prominent just afterwards. Note the high frequency hash throughout. This is 60 Hz noise with an unfiltered amplifier.
Once you have completed the Arduino preamp circuit, download the Arduino and processing software appropriate for your laptop. Set them up and test them according to the tutorials for your model and platform. Then connect the Arduino using the USB cable, and using the Arduino software copy, paste, and upload this program:
// THE ARDUINO CODE FOR EEG
#define ANALOG_IN 0
void setup() {
Serial.begin(19200);
analogReference(INTERNAL);
}
void loop() {
int val = analogRead(ANALOG_IN);
Serial.print( 0xff, BYTE);
Serial.print( (val >> 8) & 0xff, BYTE);
Serial.print( val & 0xff, BYTE);
}
Then disconnect the Arduino and in processing-- not in the Arduino software-- copy and paste the program below. Note
that both these programs are adapted from one by Sofian Audry
// SET FOR 5 SECONDS EEG, 180 Hz. A LONGER DISPLAY MAY RUN SLOW,
// DEPENDING ON THE SPEED OF YOUR CPU.
import processing.serial.*;
Serial port; // Create object from Serial class
int val; // Data received from the serial port
int cnt;
int hht;
int wm1;
int[] values;
void setup()
{
size(900, 400); //currently set to 5 sec
// Open the port that the board is connected to and use the same speed (19200 bps)
port = new Serial(this, Serial.list()[0], 19200);
values = new int[width];
hht = 750; // Sets display DC offset; must adjust if gain is changed#
wm1= width-1;
cnt = 1;
frameRate(180); //read/draw 180 samples/sec
for (int s=0;s<width;s++) { //set initial values to midrange
values[s] = 550;
}
}
void draw()
{
background(0);
while (port.available() >= 3) { //read the latest value
if (port.read() == 0xff) {
val = (port.read() << 8) | (port.read());
}
}
values[cnt] = val; //put it in the array#
cnt++; //increment the count
stroke(60);
for (int d = 0; d < width-1; d = d + 180) { //**draw lines for seconds
line(d,0,d,400);
}
stroke(255,255,0);
line(cnt,100,cnt,300); //draw the leading edge line
stroke(255,0,0);
for (int x=2; x<wm1; x++) {
line (x-1, hht-values[x-1], x, hht-values[x]); //increment the data line
}
if (cnt > wm1) { //back to beginning
cnt = 1;
}
}
When all the software is ready, disconnect the Arduino, connect the EEG leads as described in "Recording the EEG", reconnect the Arduino, and run the Processing program. If the signal noise/amplitude is poor, or you have a slow CPU, the display may run slow, because processing is not designed to collect and display samples at precisely timed intervals. If everything is working, you should see activity like that illustrated on the accompanying pages.
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Notch Filter: The Circuit’s Diagram and The Design Formula
Notch filter is known as band-cut filter or band-reject filter. The fuction of this filter is to remove some frequency portion of a signal. The schematic diagram of this filter circuit, a second order notch filter is depicted in the figure below.
The formula is actually complicated, but by taking the R3=R4 and C1=C2, the filter’s frequency response formula become very simple, as shown in the figure above. By selecting C1, C2 = 47 nF, R1, R2 = 10 k, R3, R4 = 68 k, you’ll get a frequency respon of 50Hz notch, similar to hum filter in our previous circuit ( hum filter using op-amps and hum filter using transistor).
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