• BME 210
  • Lab 9: EMG Project

    1. Background

    You have just landed a job at a startup company that aims to make next-generation prosthetics that can be controlled with electromyograms (EMGs). Having recently completed BME 210 at NC State University, you now possess all the background necessary to complete this project (whether you know it or not). Your group’s task will be to build the EMG pre-amplifier and capture an EMG signal on a computer, which fits into the overall scheme of things as shown below in Figure 1.

    Figure 1. The EMG will be amplified and filtered before being sampled by an A/D converter. A computer will analyze and interpret the EMG and send the signal through a D/A converter to actuate a servo motor.

    You will spend the next two lab sections building, characterizing, and demonstrating a Bio-instrumentation amplifier. The bio-instrumentation amplifier is the foundation for all devices that acquire signals from the body. The amplifier is shown in Figure 2 and consists of three stages. Stage one is a series of buffers that prevent loading of the amplifier. Stage two is a differential amplifier that amplifies the difference between the two inputs. Stage three is a band pass filter that also amplifies the signal. The high-pass (low corner) is determined by the 1 \(\mu\)F capacitor and 3.3 M\(\Omega\) resistor. The low-pass (high corner) frequency is determined by the 0.01 \(\mu\)F capacitor and 150 k\(\Omega\) resistor.

    2. Procedure

    2.1. Week 1

    Figure 2. Basic instrumentation amplifier.
    1. Construct the circuit shown in Figure 2, omitting switch S1. Measure and record all component values as you build the circuit. If needed, consult the op-amp pin diagram. See Figure 3 for information on connecting the potentiometer.

      Figure 3. Attach the potentiometer to the circuit by connecting one of the end leads to the (+) op-amp input and the other two leads to ground.
    2. Determine amplifier common mode gain \(G_\mathrm{c}\) at 60 Hz by connecting the two inputs \(V_\mathrm{a}\) and \(V_\mathrm{b}\) together and driving them with a common mode voltage \(V_\mathrm{c}\) from the signal generator. Connect the signal generator ground to the amplifier ground. Use \(V_\mathrm{c}\) = 0.2 \(\mathrm{V_{pp}}\) initially. You may need to increase the amplitude if no amplifier output signal is visible on the oscilloscope.

    3. Attach one channel of the oscilloscope to the signal generator output and the other channel to the amplifier output.

    4. By adjusting the CMRR pot (potentiometer, or adjustable resistor), determine the maximal and minimal common mode gain \(G_\mathrm{c}\). Record the maximal and minimal gain. Set the pot for minimal common mode gain and leave it at this setting for the remainder of the project. Minimizing common mode gain will minimize the amount of 60 Hz interference from power lines, lights, and other electrical equipment.

    5. If you get finished early, proceed to Week 2 tasks.

    2.2 Week 2

    1. Find the bio-instrumentation amplifier differential gain \(G_\mathrm{d}\) at 60 Hz by connecting one of the two amplifier inputs to ground. It is difficult to obtain a 1 mV signal from a typical signal generator so we use a 100:1 attenuator. Build a 100:1 voltage divider and hook it to your amplifier as shown in Figure 4. Observe the amplifier output on the oscilloscope; make sure coupling is set to AC. Adjust the signal generator amplitude to yield a visible sine wave at the output (i.e., on the oscilloscope). Determine the differential gain of the amplifier at 60 Hz.

      Figure 4. The gain is very high for tha instrumentation amplifier, so a voltage divider is needed to attenuate the input signal or else it will be saturated at the output.
    2. Determine the bio-instrumentation amplifier frequency response by varying the signal generator frequency from a midband of 60 Hz, to high and low frequencies. When the output observed on the oscilloscope is reduced to 0.707 of its value at 60 Hz, write down these high and low corner (cut-off or break) frequencies.

      Note! Before proceeding, disconnect the DC power supply and attach 9V batteries to power your circuit.

    3. Attach the Ag/AgCl electrodes to your upper arm, centered over your bicep. Space the electrodes about one inch apart. Connect a third (ground) lead to your right ankle. Disconnect the signal generator and connect the two wrist electrodes to each of the instrumentation amplifier inputs. Connect your ankle electrode to ground. Show the TA your connection.

    4. Connect the output of the amplifier to analog input 0 (AI0+) of the A/D converter. Save the Labview file BME_210_Final_Project.VI to your desktop. Double-click to load LabView and set the sample period to 1 sec and the sample rate to twice your amplifier high corner frequency.

    5. Capture the amplifier output while your right arm is relaxed. Sketch the frequency response. Show the TA.

    6. Clench your right fist and capture the amplifier output again. Monitor your signal on the screen and try to record a satisfactory EMG signal. Sketch the frequency response. Show your TA once you have an acceptable signal.

    3. Results

    1. Model the bioinstrumentation amplifier in MultiSim using a frequency sweep. Use the component values that you measured in lab when you built your circuit.
    2. Provide a screenshot of your MultiSim Circuit, and graphs of your simulation outputs.
    3. Record the maximal and minimal \(G_\mathrm{c}\) for the circuit you built in Section 2.1.
    4. Record \(G_\mathrm{d}\) for the circuit you built in section Section 2.2. Compare to simulated results.
    5. Record the calculated and measured high and low corner frequencies of the bioinstrumentation amplifier.
    6. Qualitatively describe the difference in the frequency response before and after clenching your fist.

    Last updated:
    January 6, 2018