Interview Assignment

The hardest part of this project was making design decisions. If the goal were simply to solve the scenario described in the assignment introduction, a basic multimeter would have sufficed. So with little to no design, monetary, or other constraints, I decided to base my choices on how interesting I found the solutions, of course only when there was no clear direction provided in the task.

Task 1

The 4–20 mA interface was not specified, so I assumed a sensor with an integrated transmitter and chose a 100 Ω shunt resistor, expecting a 0.4 V to 2 V voltage output (if the transmitter in the sensor could not provide a 2V voltage for example the shunt resistor would have to be changed). If the sensor were just a passive component, a transmitter circuit would be required.

I attempted to build the circuit using components I had available to make the process more interesting. The result is this schematic: CurrentSensorUSB_prototype.pdf. I didn’t have any components to achieve galvanic isolation between the MCU and the current measurement, but I was able to use the circuit to test the Python data acquisition software. I wrote the firmware in Rust because I had never tried embedded Rust before README.md.

The prototype circuit didn’t fully meet the requirements outlined in the task, so I created another schematic: CurrentSensorUSB.pdf, along with a BOM: CurrentSensorUSB.csv. A more detailed explanation of the component choices can be found here: README.md. I couldn’t write the firmware for this circuit because I didn’t have the hardware or toolchain to test it.

Task 2

The software folder contains a Python project with a CLI tool that can acquire a data stream over serial and generate a report based on the acquired data. An example report generated using the prototype board can be found here: report.pdf. More information on this task can be found in the README in the software directory: README.md.

Task 3

1. Procurement and Documentation

• Bill of Materials (BOM):

  • List each component’s reference designator, manufacturer part number, quantity, package, and tolerance. (BOM)

• Supplier Selection:

  • Order from authorized distributors (e.g., Digi‑Key, Mouser).
  • Verify lead times (aim for < 8 weeks) and minimum reel‑quantities match production volume.

• Files for Suppliers:

  • PCB Fabricator: Gerber RS‑274X package (copper, mask, silkscreen, drill), board outline (DXF/PDF), impedance spec (90 Ω differential for USB lanes).
  • Assembly House: BOM, pick‑and‑place (centroid) file, paste‑mask Gerbers, assembly drawing PDFs.

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2. Manufacturing Steps

1. PCB Fabrication (DFM Review)

   • Ensure trace/space, via sizes, and 2.5 mm isolation clearance meet standards.
   • Four‑layer stack‑up (2 signal, 2 power/ground) for stable analog and controlled‑impedance USB. (Optional: separate isolation layers, though I didn’t have time to finish that layout, and it wasn’t required in the task description.)

2. Assembly (PCBA)

   • Provide a laser‑cut stencil for solder paste.
   • Match the reflow profile to the most temperature‑sensitive IC.
   • Perform post‑reflow AOI to catch solder defects. Since no QFN parts are used, X‑ray inspection is not necessary.

3. Firmware Provisioning

   • Use a simple SWD/JTAG fixture and ST‑Link (or equivalent) to program each MCU with the final Rust firmware binary.
   • It may be advisable to build a small dev board with a USB bootloader and debugger for a better firmware development experience (the current design has only SWD headers).

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3. Testing and Calibration

• Incoming Inspection (IQC):

  • Sample critical parts (shunt resistor, op‑amp, isolator) to verify values and functionality.

• In‑Process Checks:

  • Bare‑Board Test: Use flying‑probe or ICT to catch opens/shorts.
  • Power‑Up Smoke Test: Limit current to ~100 mA, verify no excessive draw or heating.

• End‑of‑Line Functional Test (FCT):

  1. USB Enumeration: Ensure the board appears as a CDC/Serial device; verify VID/PID and basic loopback.

  2. Current‑Loop Accuracy:

     • Use a 4–20 mA precision source (≤ 0.01 % accuracy) to apply five setpoints (4, 8, 12, 16, 20 mA).
     • Read back ADC counts via USB; compute offset and gain error.
     • Adapt the Python data acquisition tool to create automatic reports (see report_20250518_223138.pdf).
     • The MCU should communicate the firmware version and serial number on startup to build that identifier into the report.

  3. Calibration:

     • Adjust offset (at 4 mA) and gain (at 20 mA) by programming compensation coefficients into flash. (Trim resistors could also be added to the board for calibration.)
     • Verify mid‑point (e.g., 12 mA) to confirm linearity within tolerance.

  4. Isolation Test:

     • Perform a hipot test between the USB side and the sensor side to confirm isolation and leakage requirements.

• Traceability:

  • Assign each board a serial number; record calibration data (offset/gain) and test results in a simple CSV or database, with links to the generated reports.

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4. Required Equipment & Files Summary

Category	Files/Supplies	Equipment
PCB Fab	Gerbers, drill file, board outline (DXF/PDF), impedance spec	—
PCB Assembly	BOM (incl. alternates), pick‑and‑place, paste‑mask Gerbers	Pick‑and‑place machine, reflow oven, AOI
Firmware/Programming	Final Rust .bin/.hex, programming script, README	SWD/JTAG programmer (ST‑Link), PC
Functional Testing	Test‑fixture drawing, FCT script (Python), calibration doc	4–20 mA precision source (e.g., Keithley), 6½‑digit DMM, oscilloscope, USB protocol analyzer, hipot tester