Today I decided to solder the EKG design from Instrumentation amp from op amps fine for EKG onto one of my instrumentation amp protoboards, leaving out the instrumentation amp chip—I wanted to see how much trouble it would be. As it turned out, the build was fairly straightforward, but a little tedious. There are only dedicated spaces for 8 resistors on the board, but there are 9 resistors in the design I used, so one had to go elsewhere on the board. I deliberately left out the low-pass filter on this implementation (eliminating one capacitor), which did not make a huge difference—I ended up with about 58µV peak-to-peak of 60Hz noise on my input signal (compared to about 40µV in the previous design with a capacitor for low-pass filtering), which is fairly small compared to the 870µV R spike or the 220µV T wave. The 60Hz interference was large enough to interfere with the P wave and make it difficult to see whether or not there was a U wave. Of course, these measurements were made in my bedroom/lab, which has a lot less 60Hz interference than the lab the students work in. I’ll have to take the board into work and see how bad the interference is in that space.
Using a digital filter to remove the 60Hz noise reduced the 60Hz interference to under 100nV peak-to-peak (way lower than other noise components), producing very nice waveforms, even when sampling at 360 Hz. I’ll probably want to include a digital filter Python script in the book so that people can see the cleaned up signals, even if there isn’t room in the course to design digital filters.
I still have to decide whether to have students do the EKG amplifier without the INA126P chip, using only op amps. Wiring up the bigger circuit takes time, and I’m not sure that 6 hours of lab will be enough time for students to debug their design and get it soldered—it took them long enough to solder the EKG with the INA126P chip, which has fewer components and fewer wires to route. It took me quite a while to solder up the board, so it would probably take the students far too long. Is the pedagogic value of designing and building a 2-op-amp instrumentation amp worth the time? I do want the students to end up with an EKG to take home, as it is a tangible artifact that can demo the function of. I’m thinking that I could even drop the soldering of the pressure-sensor amp (since they don’t take home pressure sensors), and add soldering of the microphone pre-amp. If I do that, I’ll probably want to redesign the protoboard again, making it an op-amp protoboard with no instrumentation amp slot, but with more resistor spaces.
Cutting one part that costs about $2.70 and the $1.90 thermometer might justify my switching back to the resistor assortment I used in Winter 2013: 1120 piece resistor assortment for $17.39 instead of 1280 piece resistor assortment (currently $10.65) without raising the lab fee. Why would I want fewer resistors at a higher price? The 1120-piece assortment is 10 each of 112 values, while the 1280-piece assortment is 20 each of 64 values. Also the 64 values don’t seem to be very repeatable from set to set, and some sets has duplicates (so only 62 or 63 different values). The 112-value sets seem more reliably useful. A hobbyist might be better off going one step further to the 3700-piece resistor assortment (25 each of 148 values), but I can’t justify the $31.48 price for my class. (The extra $14 would probably raise the lab fee.)