Lab modules

AXL-003 — Fast waveform generator PMOD module (8-bit R-2R DAC)

Catalog #3 in the AruviX lab line, level L4 (instrument). Pairs with AXL-002 as its own test source; DDS at MHz rates where the MCP4725 lab (axo-010) tops out at audio.

Circuit review & bench-test guide →

Components#

Every placed component, with the reason it is on the board. Extracted directly from the schematic, so this cannot drift from the design.

Signals#

Net labels used in the schematic, which are also the names the HDL top level should use.

3V3 D0 D1 D2 D3 D4 D5 D6 D7 GND LAD0 LAD1 LAD2 LAD3 LAD4 LAD5 LAD6 LAD7 WAVE_BUF WAVE_OUT

Production status#

Source: boards/lab-modules/axl-003-wavegen

Catalog #3 in the AruviX lab line (../README.md), level L4 (instrument). Pairs with AXL-002 as its own test source; DDS at MHz rates where the MCP4725 lab (axo-010) tops out at audio.

Status: schematic generated, ERC clean (0/0), netlist reviewed. PCB layout not started. Not released.

What it teaches#

The dds_fast core: a phase accumulator + waveform table driving a parallel DAC every fabric clock. Update-rate math: at a 25 MHz sample clock, Nyquist allows <12.5 MHz, but with only ~2.5 samples per cycle and ladder/buffer settling, ~10 MHz is the usable ceiling — measure the roll-off and the images on AXL-002. Second lesson: R-2R accuracy vs resistor tolerance — with 1% parts the MSB leg error alone can reach ~2 LSB at 8 bits; measure INL/DNL and see why real DACs use trimmed networks.

Design#

• R-2R ladder, R = 1 kΩ / 2R = 2 kΩ (both E24, 1%, 0603, Yageo RC0603FR-071KL / RC0603FR-072KL): 8× 2k bit legs, 7× 1k chain, 1× 2k LSB terminator = 16 resistors. Full scale 0–3.3 V.
• D0–D7 are host-driven, so no series resistors on the data lines (line rule applies to module-driven lines only).
• OPA356AIDR unity buffer (SOIC-8 D variant; +in from ladder, −in tied to out) — the review-recommended R-2R buffer (200 MHz CMOS RRIO, stock symbol + footprint). 100 nF + 10 µF decoupling.
• Output through 100 Ω (back-termination / short protection) to a 1×02 header and a BNC (BNC_Amphenol_B6252HB-NPP3G-50_Horizontal, stock footprint verified — same check as AXL-002, so the BNC is fitted).

Regenerate#

python3 generate_design.py

Remaining for release#

• [ ] PCB layout (keep ladder legs matched-length), DRC, fab outputs
• [ ] hdl/: dds_fast core, golden-model testbench, ECP5 demo
• [ ] BOM export and availability check, design-review checklist