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What this board does

This is a software-defined mixed-signal instrument. Rather than buying a separate function generator and oscilloscope, it implements both in one compact PCB using the STM32's on-chip ADC and DAC.

The BNC connectors are the standard lab-instrument interface — they let the board connect directly to other instruments (scopes, generators, DUTs) over 50 Ω coax, so it drops into a professional bench setup.

Schematic architecture — four sub-sheets

The schematic is split into four hierarchical sub-sheets in KiCad, each owning one functional block. It keeps every sheet focused and lets the top level show the system architecture without drowning in component detail.

⚡ Power — USB regulation

The board is powered entirely from the USB Micro B connector (5 V at up to 500 mA on VBUS), stepped down to 3.3 V for the MCU and signal chain.

📊 ADC — signal input & conditioning

Raw signals at a BNC input can't go straight into the MCU's ADC — they need conditioning first:

🎵 DAC — signal generation & output

The STM32's internal DAC has a multi-kΩ output impedance. Drive a 50 Ω cable/load directly and the signal nearly vanishes in the divider, so a buffer amplifier sits between the DAC and the BNC output.

The DAC output is also a stepped (zero-order-hold) waveform, so a reconstruction low-pass filter at or below f_s/2 smooths the steps into a clean analog signal instead of a high-harmonic staircase.

🖥️ MCU — STM32 core and USB

The STM32, its support components (crystal, decoupling, reset), and the USB interface. The MCU's DMA controller is key here: it lets the ADC fill a buffer and the DAC drain a buffer without CPU involvement, so the CPU can run FFTs, USB and a display while DMA streams the real-time ADC/DAC at a precise, timer-clocked sample rate.

Mixed-signal design principles applied

A mixed-signal PCB carries both noisy digital circuits (MCU, USB, oscillators) and sensitive analog ones (ADC/DAC paths, op-amp buffers, BNC connectors). The whole challenge is keeping the digital noise out of the analog path.

🌐 Ground-plane strategy — the most critical decision

I used a single solid ground plane with careful placement, rather than split analog/digital planes joined at a star point:

• Split planes (star ground) stop digital return currents crossing the analog ground, but the seam creates impedance discontinuities and the star point is easy to get wrong.
• One solid plane with analog components physically separated from digital ones lets return currents — which at high frequency flow directly under their trace — stay in their own regions naturally. This is the Henry-Ott / modern-SI recommendation, and what I used.

📡 BNC connector — the 50 Ω standard

Key learnings

• Mixed-signal partitioning — where digital noise enters the analog path and how ground strategy controls it.
• Anti-aliasing filter design from the sample rate, and the Nyquist/aliasing intuition behind it.
• DAC output buffering — why output impedance matters and how an op-amp follower fixes loading.
• BNC selection and the meaning of 50 Ω impedance matching in bench instrumentation.
• Hierarchical schematic design in KiCad — splitting a complex design into clean functional sub-sheets.
• USB CDC on STM32 and DMA-driven ADC/DAC for precise, CPU-independent sample timing.