I learned the value of 0 Ω resistors while on co-op during the summer and fall of 2008. My official title was NPI (New Product Introduction) Electrical Intern and I worked in the department in charge of transitioning new products from the R&D stage to production. My responsibilities included building and documenting the electrical test fixtures that were used out on the production floor to test products at various stages of completion. Being at the end of my second of five years in school, I knew very little about anything at the time, and this being my first real world engineering experience, I was not the one who designed these fixtures.
However, being able to work with the man who did design the circuitry for these fixtures was, I can honestly say, awesome. Over the six months I worked with him I came to think of his as the Yoda of circuit design. He was an old-time analog guru and I have no idea how long he’s been designing circuits but it seemed like he knew everything. Whenever there was a complex board to test or debug Yoda and I would sit together in his cube,measuring various voltages and what not and he would always patiently explain to me how something worked or why a certain portion of the layout had to be a certain way. I learned a lot from Yoda about design and troubleshooting, but my favorite trick he used was to stick 0 Ω resistors and unpopulated passive component footprints in key areas all over the board.
Figure 1 above shows an example circuit containing the techniques I learned from Yoda. Capacitor C1 and Resistor R3 would be marked NP, for Non-Populated, on the schematic by Yoda indicating to myself that the components were not to be soldered down when building the board. If, when checking output of the op-amp stage, the output looked a little noisy, placing a capacitor in the feedback path to filter out noise above a certain frequency wouldn’t require a complex rework. The component was simply soldered down on the empty pads and testing could continue.
As testing would go on for the op-amp stage, the empty pads where R3 should be were convenient places to place a multi-meter or oscilloscope probe. Only after the op-amp was working correctly would I be told to solder a 0 Ω resistor in place for R3 and testing would then continue on down the signal chain.
When it came time to finalize the schematic, if C1 was needed, its value would be inserted into the schematic and the NP marker removed, otherwise it would be deleted. R3 was typically left in place because if a problem arose with U1 in the future the resistor could simply be tombstoned, thus isolating the op-amp from the rest of the circuit for testing. This in and of itself was a useful debugging technique for if the problem disappeared after removing R3 then we knew something down the line was affecting the loading on U1 and the op-amp was probably fine.
Another thing I admired Yoda for was that he always came up with multiple circuit blocks to implement a desired function. The two or three most feasible of these blocks would be placed on the test fixture PCB in parallel with each other and 0 Ω resistors were put in series with their inputs and outputs as shown below in Figure 2. His most likely choice for that given feature was soldered down first and tested. Should that block not perform to spec, time was saved because the PCB did not need to be re-spun, just move the 0 Ω’s to an auxiliary block and keep going.
Yoda got away with this because most of the time there wasn’t a tight area requirement for the test fixtures. Placing an extra op-amp or two on the board could be done with only a minimal cost increase. Typically, electrical components went into a standard sized project box that was machined with the proper holes for connectors and any other mechanical parts and that was about it. So long as the PCB fit in the box all was good. This freedom to experiment with new and/or different options for various circuits not only made testing and debugging easier, it also made Yoda a better designer because when it came time to design a new product and its required test fixtures, he already had a sense on what type of circuit might work for a given application.
While these techniques may seem like common sense, I for one am glad I got to learn about them while out in the field. The use of 0 Ω resistors and empty pads in key places are not the type of topics that get covered in core EE courses and I probably would not have learned them otherwise while in school. Currently, I am making great use of these techniques while designing and testing the circuit for my thesis. Having these tricks up my sleeve has been quite handy during the whole process. Thanks Yoda.