Objectives EXPERIMENT 5 Bioelectric Measurements 1) Generate periodic signals with a Signal Generator and display on an Oscilloscope. 2) Investigate a Differential Amplifier to see small signals in a noisy environment. 3) Observe characteristics of the heart muscle potential (EKG) Introduction Many biological systems, ranging from the single cell to the human body, produce electrical signals that can be detected and recorded by sensitive electronic equipment. In recent years, the study of these signals has played an increasingly important role in the biological sciences, particularly in human medicine. Recently, there has been much interest in the electrical characteristics of plants. Even though research in this area is still in its infancy, there seems to be some evidence that plants change their electrical characteristics in response to changes in the environment. While a complete explanation of the origins of electric phenomena in biological systems is not possible here, we will introduce the very basic concept of electricity produced by ionic diffusion. Through this experiment you will gain the basic knowledge of bioelectric measurements. The differential amplifier used in this experiment employs precautions necessary for obtaining meaningful data safely from biological systems. When the heart is at rest, the inside of the heart muscle cells are negatively charged and the exterior of the cells are positively charged. The physics term used to describe this situation states that the cells are polarized. Depolarization and repolarization of the heart muscle cells causes the heart to contract and blood to be pumped throughout your system. Depolarization is accomplished when some of the positively charged ions move through the cell membrane, resulting in a lower potential difference between the exterior and interior of the heart muscle cells. Shortly after depolarization, positive ions move back to their original location and the heart cells are repolarized. The electrical potential displays the following characteristics in time: A small depolarization ("Pwave") of the atrium chambers of the heart is followed by a depolarization (in quick succession, "Q-, R-, and S-wave") of the two ventricle chambers of the heart. Shortly afterward, there is a repolarization ("T-wave") of the two ventricle chambers. The atria are repolarized at the same time as the ventricles are depolarizing and are therefore this electrical signal is obscured by the much larger ventricle depolarization. Heart beat monitors, e.g., in a heart patient hospital room, display these electrical signals (electrocardiogram, EKG) on a screen. Here we use an oscilloscope, a general instrument for displaying electrical signals. To detect the electrical signals, we use adhesive, disposable foam, single use EKG electrodes which contain a hydrogel to reduce the resistance of your skin. To eliminate the possibility of an electrical shock, an optical coupler in the amplifier converts the body potentials to an optical signal, which is then converted back to electrical signals. The optical coupler completely isolates the human from any form of high voltage, such as a problem with the 110V AC power in the oscilloscope. 1
Electrical Components used in this Laboratory Signal Generator (Frequency resets if this instrument is turn off) A signal generator can produce a periodic WAVEFORM. The buttons set the waveform to sinusoid, ramp, or square-wave, the AMPLITUDE is set by the MIN - MAX knob, and the FREQUENCY set by the ADJUST knob. In this lab we will use only the sinusoidal and squarewave shapes. We will "calibrate" our understanding of how the oscilloscope and amplifier operate with controlled signals from the signal generator and later from an RC circuit. Oscilloscope (Use Oscilloscope in AC mode, top row button, read in LC Display) The oscilloscope allows one to view an electrical signal that varies in time. A bright spot moves across the screen at a rate determined by the "sweep rate" or time-base (TB) control. With no input signal, the spot can be adjusted to put the sweeping spot halfway up the visible grid creating a bright line at the middle of the screen. During the sweep, any electrical voltage input to the oscilloscope (input connector labeled A or B) will deflect the spot vertically (up +, or down -) in proportion to the voltage, thus tracing out the shape of the voltage signal in time. For a sinusoidal signal varying between ±V 0, the oscilloscope time base can be adjusted to show any number of oscillations of this signal. The parameters of a sine wave signal are 2
specified on the left in the figure below. On the right is how the signal appears on the oscilloscope screen. If it is not centered vertically, the amplitude, V 0, can be found taking V pp /2. Consider the vertical scale (rocker labeled A and "V to mv", with LCD display) adjusted to show 1V per division and the Time Base (rocker labeled TB and "S to µs") adjusted to show 1ms per division. Before lab check that you can verify that a sinusoidal signal with Amplitude, V0 = 2.5V and a Period, τ = 6ms, is shown on an oscilloscope screen below on the right. If one were to change the sweep rate (TB) to 2ms/division, a little more than four periods of oscillation would show on the screen. Change the vertical scale (rocker A) to 2V/division, and the signal would reach only 1.25 divisions up and down. Change the vertical scale to 0.5V/division, and the extremes of the signal would be cut off at the top and bottom of the oscilloscope grid. Other helpful oscilloscope controls will be described in the Procedure section. Differential Amplifier An amplifier is an electrical device that typically increases the amplitude of an electrical signal. The amplifiers in your cell-phone amplify tiny digital signals picked up by an antenna. The heart generates somewhat larger analog signals to contract its muscles. However, there are many similar EM signals in our environment that can generate similar signals, such as the 60Hz AC current flowing in wires in the walls, that powers all of our electrical devices or battery chargers. A differential amplifier is designed to amplify only signals that have opposite polarities but with a similar amplitude at two locations, such as generated by the heart. In addition it removes most signals with the same polarity and amplitude at both locations. The amplification factor is known as the differential GAIN and the ability to remove background signals is known as the common-mode rejection. The amplifier used here has a differential gain close to 100. This means any signal that has opposite polarities at the two locations will be amplified by a factor of 100! This amplifier also removes similar (common mode) signals at the two locations and passes only a few % of its amplitude. Procedure (use the attached worksheet to record your measurements) In the experiment, a signal of order 0.5V will be observed and then attenuated by about a factor of 100, down to 5mV, invisible on the oscilloscope without a scale change. This level of signal is comparable to that generated by the heart. Putting this signal into the differential amplifier can return it to close to its original amplitude. The differential amplifier will do this for the small heart muscle signals, as well as remove much of the observed and measured background electrical noise. 3
1. Set Signal Generator output voltage via an Oscilloscope Set the signal generator at 60Hz and have it produce a sine wave. Place the red and black plugs into corresponding jacks on the signal generator, and attach the BNC cable end on the input BNC connector for channel-a on the oscilloscope. Using the oscilloscope to view the signal, adjust the signal generator amplitude to 0.5V and leave it at this value throughout the experiment. A sketch and measurement of the period and amplitude of the signal, and the two oscilloscope scales used, are recorded in Question 1a. To get a reasonable view of the signal, set the oscilloscope to these values, V-scale: 0.2V/div, TB: 10ms/div. Adjustment is by the V-scale (rocker labeled A V -- mv ), and Timebase (rocker labeled TB S -- µs ) switches, however, a press of the auto set button may be sufficient. Please do not adjust any other oscilloscope settings without first asking your TA. 2. View an attenuated signal The circuit picture for Steps 2 & 3 is shown on the right. Do not connect the red and black wires to the amplifier just yet. Using two T-splitters (sliver colored connectors), insert the attenuator (blue cylinder) between the signal generator and the cable to the oscilloscope's channel-a BNC connector, as shown in the picture on the right. The attenuator reduces the signal amplitude to a value similar to the heartbeat signal. Try to observe this attenuated signal on the oscilloscope without adjusting any scales. You should find that the signal is too small to see without changing the V-scale (rocker switch labeled V -- mv ). Change the voltage sensitivity to ~10mV/division and observe the signal. Measure and record (Question 1b) the period and amplitude of the attenuated signal by using the cursors on the scope. Check that the attenuated sine wave has a <10mV amplitude. You may want to press the lock button on the scope to freeze the display (don t forget to push the lock button again to unlock the display afterwards). To control the CURSORS use the 4 buttons located and the bottom of the scope screen. Press cursors on, then V ctrl or T ctrl to control the voltage or time base cursors respectively. 3. Observe the Amplifier's Voltage Gain Use only an amplifier that has been recently charged at the front of the room. On the back of the amplifier the switches should be set for low gain and AC coupling. Leaving one end of the BNC cable attached to the oscilloscope, remove the BNC cable end from the attenuator & T, and reconnect it to the BNC output on the front of the amplifier. Using BNC-to-jacks (Red & Black) on the T-splitter, connect the amplifier using the red and black wires as shown in the picture above and on the right. In this configuration, the waveform generator signal is sent through the attenuator, into the amplifier and out to the oscilloscope. Try to place the amplifier away from all power cords, wall outlets, oscilloscope, and signal generator to reduce noise pickup. Adjust the oscilloscope settings and obtain a reasonable displayed signal (see step #2). Measure and record (Question 1c) the amplitude of the signal and the oscilloscope 4
settings. What is your estimate for the Gain (Question 1d) G = Output-V 0 / Input-V 0 (Input-V 0 from step #2) of the amplifier? If you can't get a reasonable sine wave signal, ask your TA for help. 4. View electrical noise on your body. The people in the room are exposed to large AC noise from the power wiring in the walls. This noise would overwhelm the heart signals without the differential amplifier. Using the amplifier without the differential feature we can view this noise. To observe the amplified noise signal on your body, remove the red plug from the amplifier (the one from the red side of attenuator jack) and insert a new red plug wire into the amplifier jack and hold the conductor of the plug at the other end. You may want to freeze the trace on the oscilloscope by pressing the lock button. Draw this signal in the box for Question 2, or take a cell phone picture of the noise signal on the oscilloscope display and paste it in the box. What is the amplitude and period of this noise signal (Question 2)? When done remove this extra red wire. 5. View a 1Hz signal powering an RC circuit Oscilloscope Signal Generator Circuit Board Red A TB Red 1.0mF A Black 50Ω Black BNC R/B split BNC to A a) Set the Signal Generator to produce a SQUARE WAVE with a frequency of ~1Hz, but do not change the Amplitude (0.5V) of the signal from steps 1-4. b) Following the circuit schematic above, take one red and one black wire and connect the outputs of the Signal Generator to the circuit board (RED to capacitor, and BLACK to the resistor) to form a circuit. c) Use the connector that splits the coax cable into a ground (Black) and a signal (Red) wire to attach the Black wire to the ground side of the circuit and the Red wire between the resistor and capacitor. Adjust the oscilloscope Time Base (TB ~ 0.2s) to view the 1Hz signal. Measure and record the period and amplitude of this signal (Question 3). After finishing this measurement remove the connecting wires from the RC circuit and TURN OFF the signal generator. 6. View your heartbeat signal If the amplifier has been on for more than 30 minutes, recharge it at the front of the room. Note, the amplifier is powered by a charged capacitor and there is an optical coupling between the inputs and the output thus isolating the oscilloscope power from the "patient". Attach the 3 electrodes (single use sticky contacts) to one lab partner sitting on a stool that is not Coax Cable 5
close to the oscilloscope as shown in the figure above. Make sure they are stuck down well, or replace them if loose. The red electrodes should be placed high on the arm (above the elbow) or at the ends of the shoulders (under clothing is OK) and the black electrode to your ankle. Before attaching the wires with red plugs to the amplifier, twist the wires together so that as they leave the amplifier (no more than 5 twists) and stay together until they have to split to attach to the person. The "patient" must remain very still with arms loose at the sides and feet off the floor. From step #5, the oscilloscope should have a time base (TB) appropriate for ~1Hz signals and the amplifier should be producing a visible heartbeat signal. If not, try adjusting the V-scale to view the signal. If you do not have a heartbeat signal, ask your TA for help. Draw the heartbeat signal in the box for Question 4, or take a cell phone picture of the signal on the oscilloscope display and paste it in the box. Record the period and frequency of the first lab partner's heartbeat signal (Question 4). Attach new sticky contacts to the second lab partner (arms loose and feet off the floor) and repeat the display of the heartbeat signal. Draw the heartbeat signal in the box for Question 5, or take a cell phone picture of the signal on the oscilloscope display and paste it in the box. Record the amplitude and frequency of the second lab partner's heartbeat signal (Question 4), or a third lab partner if there is one. 6
Name: Worksheet (each student) Lab Partner: BioElectric Measurements Section: Experiment 5 (Spring 2017) Spaces below: Oscilloscope scales or Measurement Value ± Uncertainty & Units 1a. Initial signal Period: Initial signal Amplitude V 0 : Sketch signal (carefully) _------------------------------------------------------------------------------------------------------------------- 1b) Attenuated signal Period: Attenuated signal Amplitude V 0 : Sketch signal (carefully) _-------------------------------------------------------- ----------------------------------------------------- ------ 1c) Amplified signal Amplitude V 0 : Output V0 (Question 1c) 1d) Amplifier GAIN = = ± Input V0 (Question 1b) _------------------------------------------------------------------------------------------------------------------- 7
2) Period of the Noise signal: Compare this period to the period of the signal with frequency = 60 Hz in Question 1. Sketch signal (carefully) -------------------------------------------------------------------------------------------------------------------- 3) RC-circuit Resistor signal Period RC-circuit Resistor signal Amplitude V 0 Oscilloscope scales used to make these measurements: -------------------------------------------------------------------------------------------------------------------- 4. Heartbeat Measurements (Use the same Time Base scale as in Question 3) Heartbeat Sketch Lab Partner A Heartbeat Sketch Lab Partner B/C A. Heartbeat signal Period Heartbeat Frequency Heartbeat Frequency (new units) ± ± Hz ± beats/min. B/C. Heartbeat signal Period Heartbeat Frequency Heartbeat Frequency (new units) ± ± Hz ± beats/min. 8