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Design of Electromyogram (EMG)
1. Introduction
1.1Electromyograms
The biceps in the human body are actually made of a large number of parallel fibers
functionally arranged into individual motor units. When each motor unit is activated
by nerve commands, from the nervous system, electrical impulses propagate down
the length of the fibers that make up the unit. Two neighboring electrodes placed on
the skin’s surface near the muscle will pick up different voltages in the fringe regions
of these electrical impulses – a plot of these voltages is called an Electromyogram or
EMG (“myo” is a root meaning “muscle”). The strength of the voltage at the skin
surface is very small in the order of microvolts. Therefore, amplification is required
to record the EMG or displaying the EMG on an oscilloscope. A third electrode,
called reference electrode, is placed away from the muscle so that it doesn’t pick up
any noise from nearby electrical equipment. Fig. 1 illustrates the system for
measuring an EMG.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram
1.2Electronics
1.
Quantities

Voltage: It is the difference of electrical potential between two points of
an electronic circuit. The units are expressed in “volts”.

Current: It is the flow of electric charge. The units are measured in
“Amperes”.

Power: It is defined as the rate at which electrical energy is transferred by
an electrical circuit. The units are measured in ”Watts”.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram
2. Devices

Resistor: A resistor is a two-terminal electronic component that opposes
an electric current by producing a voltage drop between its terminals in
proportion to the current.

Capacitor: It is an electronic device that can store energy in the electric
field between a pair of conductors. The process of storing energy in the
capacitor in called charging. A capacitor is much like a battery.

Inductor: An inductor is a electronic device that stores energy in the
form of a magnetic field. An inductor consists of a wire loop or coil.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram

LED: A light-emitting diode (LED) is a diode that emits lights when
electrically biased in the forward direction of the p-n junction.

Transistor: A transistor is a device used to amplify or switch signals. It
allows a small current or voltage to control the flow of a much larger
current.

Op-Amp: It is a dc high gain electronic voltage amplifier with
differential inputs and single output.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram
3. Laws

Ohm’s Law: The direct current flowing in a conductor is directly
proportional to the potential difference between its ends. It is usually
formulated as V = IR, where V is the potential difference, I is the current
and R is the resistance of the conductor.

Kirchhoff’s Current Law (KCL): The sum of currents flowing towards
a point is equal to the sum of currents flowing away from that point.
So, from the above diagram we can see that
IA = IB + I C + ID
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram

Kirchhoff’s Voltage Law (KVL): It states that total voltage around a
closed loop must be “zero”.
4. Design
Your Project is to design an amplifier circuit for measuring an electromyogram
(EMG). This circuit will consist of resistors, operational amplifiers (or op-amps),
power supply, wire clips, a protoboard, and electrodes.
The circuit for measuring EMG’s is shown in Fig. 2. The electrodes are attached
to the biceps on the upper arm where they pick up tiny voltages generated by
nerves and muscle. Because the power in these signals is minute, attaching the
electrodes directly to a circuit that draws current will make the voltage go to zero.
(see Ohm’s Law). The solution to this problem is to use pre-amps that draw
approximately zero current from the electrodes while outputting the same tiny
voltages and multiplies that by a much higher current level. The pre-amp is
followed by a differential amplifier that measures the difference between the
electrode voltages and multiplies that difference by five hundred. We use a
differential amplifier because it eliminates any noise voltages from external
sources that are common to both electrodes.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram
Fig. 2. Schematic Diagram of the electromyogram circuit
5. Analyze OP-AMP circuits
In this section we will learn about the Op-amp.
Figure of an Op-Amp
The op-amp is an amplifying device that has specific characteristics and can be
used in many applications. Let’s start with an ideal amplifier, this means that
every amplifier is designed to try to match these specifications as much as
possible. First characteristic of an op-amp is that it’s input resistance is infinite,
that means that current is zero at its input. Furthermore, the second characteristic
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram
is that the Output terminal should have zero resistance that means that it should
have maximum current.
Since we learned the requirements for an op-amp we can introduce feedback.
Feedback is necessary for amplifying a signal. It determines by how much a
signal should be increased to match our requirements. This feedback loop is
necessary and will determine the gain of our amplification. In our case we will
increase current in the pre-amp.
If we need only an increase in current we don’t necessarily need to change the
amplitude of the voltage. We will then have an amplifier of gain (Av) of 1. This is
why we call the current amplification stage, pre-amps. We are preparing the
signal so that our differential amplifier can amplify it. Here is a picture of the preamp.
Fig 3: Picture of the pre-amp
Differential amplifiers have a dual role in this system. First they will amplify the
amplitude of the tiny voltages generated from muscles by a gain factor of 500.
Also, the differential amplifier will filter any noise (signal disturbances) generated
by our circuit. The equation that satisfies these characteristics is expressed in Eq.
1.
R
R 
V3   4 V2  2 V1 
R1 
 R3
Eq. 1: Differential Amplifier Equation
6. Build Electromyogram Circuit
You will find a pre-built circuit in your station. This circuit will contain the pre-amp
and differential amplifier section. You will need to find values for the differential
amplifier resistors that will result in a gain of 500.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram
Use Eq. 1 to find resistor values to obtain the gain of 500 for the differential
amplifier circuit. After finding the resistor values place the resistors in the
protoboard as shown in Fig 4.
Fig 4: Picture of the built circuit
These instructions have been written for use with the following items:
HP 54645D Oscilloscope (Digital Lab)
HP E3631A Power Supply (Digital Lab)
EMG Circuit from ECE1270 Lab
(3) Kendall ECG Conductive Skin Electrode (733)
(3) Electrode Lead Wires (ours have been modified for use with solder-less breadboards)
Connecting and Configuring the Power Supply
with the EMG Circuit:
Connect three wires as shown in Figure (5) to the power supply and then to the EMG
Circuit. Connect a BNC/Alligator wire to ch1 of the oscilloscope and then to the EMG
circuit as shown in Figure (6).
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram
Figure 5
Figure 6
Turn on the Power Supply and press the +25v button (yellow outline), use the rotary dial
(white outline) to select 9 volts, press the -25v button (blue outline) and do the same. If
possible limit the current on the power supply to 100mA.
Figure 7
Figure 8
Figure 9
Figure 10
Next, place the electrode and probes to the arm as shown above in Figures (8)-(10).
Connect the other ends of the probes to the circuit as shown below in Figure’s (11)-(13).
Ignore “Checkpoint 1” in Figure (13) for this lab.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram
Figure 11
Figure 12
Figure 13 Picture of the built EMG circuit
When everything is connected, press the Output on/off button outlined in black as shown
in Figure (5). The oscilloscope should now show a signal.
To set up the scope press the main button (red outline) Figure (7), press the button
corresponding to Roll on the screen (blue outline) Figure (7).
Now test the circuit. You should be able to see a waveform similar to Figure (14).
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
Design of Electromyogram
Figure 14. Screenshot of EMG waveform output
Fun Fact: A special case of the EMG is the ECG (electrocardiogram), often referred to
as the EKG (elektrokardiogram) due to its German origins, which is an EMG
measurement of the heart. ECG signals are quite strong – often 3-4 mV in amplitude.
This twice the voltage of a typical EMG signal for large muscles, such as the bicep.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu