.SYSC 3203: Final Exam December 18, 2014 Carleton University, Systems and Computer Engineering

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1 .SYS 3203: Final Exam December 18, 2014 arleton University, Systems and omputer Engineering Instructions: - This exam has 5 pages and 9 questions (worth 100 points). Answer all questions. Marks are indicated. - You have 150 minutes to complete this exam. Write your answers in this booklet. - This is a closed book exam; however, you are permitted to bring one sheet of notes. - You are permitted to use a non network-connected calculator. - All components may be assumed ideal, unless stated otherwise. - You may need the following table of filter properties. N F s (40dB) F s (0dB) F s (80dB) f n G f n G f n G f n G FILTE = hebychev 0.5dB FILTE = hebychev 2.0dB Background: Speed skating uses special ice skates which contact the ice in a long flat line. The longer the blade, the faster the skate, up to a point where length would become an obstacle. Training for speed skating involves careful timing and balance of movements. You have been asked to help design equipment to improve training of speed skaters. The idea is a portable system which measures the timing of movements, weight placement onto the skates, and activity in the leg muscles. As shown in the figure, two sensors are used: 1) electrodes, placed on the calf muscle to measure the EMG, and 2) strain gauges, placed onto the posts of the skate to measure the distribution of weight onto the ice. Since all weight is transferred via the two posts, it is possible to measure the extent to which balance is forward or backward by the distribution of weight between the posts. A schematic diagram of the system is shown below Electrodes Amplifier Filter S/H Quantizer Encoder Strain Gauges Skate Amplifier lock

2 SYS 3203: Final Exam 2/5 1. (10 points) In order to measure the EMG signal, Ag/Agl electrodes are placed onto the skin of the calf muscle of the athlete. (a) What type of electrode is the Ag/Agl electrode? (b) Explain how the Ag/Agl electrode allows charge carried by ions in the body to be transferred into charge carried by electrons in a wire. What chemical reactions happen in or near the electrode? (c) What is a movement artefact, and how does it originate at an electrode? 2. (20 points) The first stage of measurement uses an instrumentation amplifier, for which the internal schematic is shown in the figure below. The values of the resistors are 1 = 2 = 3 = 4 = 10 kω, and A = B = 100 kω. V A A 2 1 G B 4 3 V B Looking at its specification sheet, you read that it has the following properties: Parameter M I OS I B V OS f T (G BW) S Value Units db na na µv khz V/µs At a particular instant in time, the difference voltage (from the EMG), V d = V A V B, is 5 mv peakto-peak. The common-mode voltage (mostly from the power-line interference), V cm = 1 2 (V A +V B ), is 0.5 V peak-to-peak. The output of the instrumentation amplifier is. (a) alculate the value of G such that the differential gain, G d = /V d, is 100. (b) What is the contribution to from the difference voltage at the input? (c) What is the contribution to from the common-mode voltage at the input? (d) Discuss whether the M of this amplifier is adequate. Would you recommend buying a more expensive amplifier, (with higher M), or a cheaper one (with lower M)? (e) If V A = V B = 1V, what current is flowing in 3? (f) For small inputs, with a maximum frequency content of 5 khz, would this instrumentation amplifier s properties limit its response? Why or why not?

3 SYS 3203: Final Exam 3/5 3. (10 points) It was determined that the common-mode interference is primarily at 0 Hz. In order to remove this effect, a high-pass filter is required with a cut-off frequency, f c, at 200 Hz which achieves 40 db of attenuation at 0 Hz. The acceptable passband deviation is 1.0 db. The figure below shows a Salen-Key high pass filter (assume the op-amp is ideal) V i (a) Sketch the filter requirements. (b) Using a hebychev filter approach, how many filter stages are required? alculate the gain and f c required at each filter stage, using a Salen-Key architecture. (c) Using a Salen-Key filter approach, show the design of the first stage of the filter. Use = 100 nf and 2 = 10 kω. 4. (10 points) In order to record the measured data, an analog-to-digital (AD) converter is used. As shown in the block diagram, it is consists of a S/H (sample and hold), a quantizer and an encoder. (a) What is the function of a S/H circuit? (b) What is the function of the clock? Why is it required to feed into both the AD and S/H circuit? (c) The AD range is from 2 V to +2 V, while the range of the amplified and filtered EMG signal is +50 mv to 50 mv. In order so ensure that the maximum quanitization error is 100 µv, what is the minimum number of AD converter bits, B? (d) The engineer who is writing the software to analyse the signals is concerned to detect whether the AD clips the input signals. Describe a strategy to detect clipping in the input signals.

4 SYS 3203: Final Exam 4/5 5. (10 points) In order to provide the clock for the AD circuit, we use a 555 timer as shown in the figure below. omponent values of 1 = 2 = 10 kω and = 1 nf are selected. V = 9 V. Vcc 1 2Vcc/3 Q Vcc/3 S Q- 7 (a) Sketch the waveform at pin and pin 3 for two periods of oscillation. (b) Label any times and voltage levels at which transitions occur. (c) Using the 555 output as the clock signal for the AD, what is the sampling frequency, f s?. (10 points) A strain gauge is used to measure the deformation of the skate post. (a) Sketch the layout of the conductive trace on the strain gauge such that it is sensitive to compression (and expansion) of the post, but not other deformations. (b) In order to calibrate the strain gauge, you place several masses, and measure the post length and resistance. What is the gauge factor (GF)? Test mass (kg) Post length (mm) esistance (Ω) (c) What does the term piezoresistive contribution to the gauge factor mean? What kind of strain gauge materials are piezoresistive? 7. (10 points) For the strain gauge sensor in the previous question, define the following instrument characteristics, and briefly describe a test which can be used to measure their value. (a) Accuracy. (b) epeatability. (c) Linearity.

5 SYS 3203: Final Exam 5/5 8. (10 points) Electrical safety is obviously an important factor in biomedical electronics. (a) Define the term let-go current. (b) Define the term threshold of perception. (c) How do the let-go current and threshold of perception change with the frequency of the applied electrical current? (d) Why is the let-go current higher (on average) for a subject with higher mass? 9. (10 points) You design the circuit given in the figure below. It receives an input signal, V i (t), as shown. V i (t) is minimum ( 2 V) at 1, 2 and 3 ms, and is maximum (+2 V) at 1.5 and 2.5 ms. 3V V i 7 V a OP97 4-3V 1 2 2V V i -2V t 1ms 2ms 3ms In this circuit, 1 = 20 kω, 2 = 10 kω, = 10 kω, and = 10 nf. Assume the op-amp is ideal, with positive and negative output saturation at V = 3 V and V EE = 3 V. (a) Sketch the output of the op-amp, V a (t), from 1 ms to 3 ms. On your axis, indicate times and voltage levels of any transitions. Indicate any time constants. (b) Sketch the output of - filter, (t), from 1 ms to 3 ms. On your axis, indicate times and voltage levels of any transitions. Indicate any time constants.

UNIVERSITY OF CALIFORNIA College of Engineering Department of Electrical Engineering and Computer Sciences

UNIVERSITY OF CALIFORNIA College of Engineering Department of Electrical Engineering and Computer Sciences EECS 145L: Electronic Transducer Laboratory FINAL EXAMINATION Fall 2013 You have three hours to