Gas station without pumps

2013 March 11

Twenty-sixth day of circuits class

Filed under: Circuits course — gasstationwithoutpumps @ 20:53
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I started today with feedback on the writing.  The students have been getting much better about the content and organization of their reports, and the writing is pretty good from at least half the class, but everyone is having trouble with not checking the details, particularly in the schematics.  I’d really like to give some A’s this quarter, but I can’t give an A to a lab report that has a power-ground short in a schematic or a missing connection.

After the writing feedback, I talked a bit about safety in biomedical equipment (battery operation, opto-isolators, isolation transformers, wireless, …).  I also talked about how skin was an excellent insulator at low frequencies and low voltages, but how at high frequencies the skin capacitance is no barrier, and that at high voltages, like with Automatic Electric Defibrillators (AEDs) the skin dielectric is broken down.  Even with EKGs, running at low voltages, they need to add series current-limiting resistors to all their electrode leads, to ensure that currents remain below 50µA, even if the leads are accidentally shorted to the highest voltage in the system.

After the safety discussion, I gave them some information about the AC signal they were trying to amplify (±0.5–1mV, with a DC offset of ±300mV, and a frequency range of 0.1Hz to 50Hz). Rather than covering the block diagram as a whole class, I had them split into 3 groups of 4 and try to design the block diagram for the EKG.  I circulated around the room and answered questions. I did tell the class that every design I’ve seen for an EKG puts the high-pass filter after the first-stage instrumentation amp.  I’m not sure why that is standard, but it certainly seems to be.

The main point (which some groups came to quickly and others slowly), is that the ±300mV offset constrains how much gain you can ask the first stage for, so that a second stage is essential.  One group asked if they could do a particular gain in one op amp, so I got a chance to remind them of gain-bandwidth product (the frequencies here are so low that the gain was easily feasible).

Over the weekend the whole class had gathered to work together (mainly on rewriting lab reports—I offered the class in the syllabus that they could rewrite any lab report whose grade they were not happy with).  They sent me a photo of the class working together—I think this is the first time I’ve seen a spontaneously formed study group consisting of an entire class.  Today I got 15 redone lab reports (about 2 weeks worth of grading, with more expected on Wednesday, not to mention the class-D amplifier reports that are due on Wednesday.

Since we are not having a final exam for the course (the quizzes and the 50 pages of design reports from the students is enough to evaluate them on), the students asked if we could meet during the exam period for beer (with the constraint that it must be a place that serves non-alcoholic beverages to those under 21, since not all the students are drinking age).  I agreed that we could, but I’ve not thought of an ideal location.  Perhaps Cafe Pergolesi would do—they serve beer and wine, but are primarily a café serving coffee, tea, and hot chocolate.  I don’t know how much of a hipster hangout it is on Tuesday afternoons, but I suspect that a dozen of us could occupy one of the “geek” rooms.  I had planned to make a detailed survey for the students to fill out about things that were good and bad about the course—they’ve been pretty good about giving me feedback as we go along, but I’m planning on redesigning the course somewhat and revising the course approval forms, so I’d like to know which aspects of the course worked best (and worst) from their perspective at the end of the course.  I’ll also be asking them to send me PDFs of all their lab reports, so that I can show them to the EE faculty to try to convince the faculty that this applied circuits course is acceptable preparation for the signals and systems course and the bioelectronics course.

I think I’ll be spending tomorrow on 3 things:

  • trying out a different EKG circuit that uses active feedback of the common-mode signal.
  • grading redone lab reports—if I can get half the stack done tomorrow I’ll probably be able to handle the load for the rest of the week.
  • trying to catch up in physics with my son—it kind of slid for the past 2 weeks as both he and I were crazy-busy.

Wednesday I have a guest lecturer coming in to talk about action potentials and excitable cells, which should be good preparation for understanding the EKGs they will be making on Thursday—I’ve not really talked about where the voltages they’ll be measuring come from.

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