Lab11_OpAmpFilterBuild2311c

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Apr 3, 2024

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EECE 2510 – Circuits and Signals: Biomedical Applications Lab 11 Op-Amp Active Filters, Design, Measurement, and Limitations Introduction: As discussed in class, Op Amps are useful building blocks in many sensing and measurement applications. To measure the ECG signal in the coming weeks, we will be using them to amplify small signals, to reject common-mode signals (which means that, if we measure signal waveforms as a voltage on two electrodes, we will want to attenuate components that are common to both electrodes and amplify the components that are different between the two electrodes), and to filter out unwanted high and low-frequency noise and interference. Today, we will explore the high pass filter that we discussed in class. Building and Testing Active Filters. 1. Use the 1490 Operational Amplifier chip in an active high pass filter configuration. (see Figure 1). Color code your wiring as described in the last op-amp lab. Q1: After the circuit is working, take a photo for your lab report. 2. Figure 1: First order active high pass filter +5 V -5 V

࠵?࠵?࠵? ࠵? ! = 20 ࠵?࠵?, ࠵? " = 0.1 μ ࠵?, ࠵? " = 10 ࠵?࠵?, ࠵? # = 10࠵?Ω . Note that the capacitor should be coded 104 (in very fine print!). This code means 10x10 4 pF or 0.1 μ ࠵? . So, for example 473 would be 47x10 3 pF or 0.047 μ ࠵? . (You could also use 2 0.047 μ ࠵? capacitors in parallel if you don’t have the 0.1 μ ࠵? capacitor.) Note that the tolerance of these capacitors is usually either ± 10 or ± 20%, so your measured cut off frequency might be a bit different than your design frequency. Q2: What are the theoretical in-band gain , the cutoff angular frequency ω c (rad/sec), and the corresponding cutoff frequency f c (Hz) of the circuit? What is the magnitude of the transfer function of this circuit at the cutoff frequency ? 3. Set the function generator to produce a 1 V p-p amplitude sine wave with a 5 kHz frequency. Connect the input (signal generator) to CH 1 on the oscilloscope and the output signal to CH 2. Remember that the negative terminals of analog input 1 and analog input 2 go to ground for normal oscilloscope measurements. Measure the output of your filter at 5 kHz, 500 Hz, 50 Hz, and 5 Hz, changing the time and voltage scales as necessary to see the result. Q3: What is the measured magnitude of the transfer function, |H(ω)|, of your circuit at each frequency ? Is the 5kHz result what you expect to observe well above the cut off frequency? Qualitatively, do you see the filter reducing the amplitude of the signal at lower frequencies? Hint: You can use “measure” on the oscilloscope to quickly record the p-p values for the waveforms on CH 1 and CH2, but the noise that the measurement adds will give an overestimate of the size of the wave, especially as the signal gets smaller. The cursors give a more accurate measurement when the cursor is placed in the middle of the blurry oscilloscope trace. 4. Now measure the magnitude of the transfer function (also called the frequency response) of your circuit |H(ω)|. Measure from 5 Hz to 500 kHz, manually changing the frequency of the waveform and then measuring the results, including enough points to enable you to construct a reasonable plot of |H(ω)|. Using the 1- 2-10 steps of the signal generator for 3 points/decade is reasonable. Be sure to take some extra measurements around the cutoff frequency, f c, to capture f c accurately. So, in the end, you will have 3 points/decade for 5 decades + a few extra points near the cut off frequency, or about 20 points total to plot. In this process, you will experimentally determine the cutoff frequency (which, just for your reference, is also known as the 3dB frequency or the corner frequency) of your circuit. Q4: Use your measurements to manually plot the Bode magnitude plot. Remember that this is a plot of the magnitude of the transfer function, using a log scale on the horizontal (frequency) axis and dB (also a log scale) on the vertical (voltage) axis. Does the cutoff frequency agree with the theoretical value? You should take enough high frequency measurements to observe that the gain gets smaller again at the highest frequencies. Discuss the reasons for this with the instructor or a TA and record

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