PCB Design · Analog Electronics · Schematic Capture

4-Bit DAC on a One-Inch PCB

A binary-weighted resistor digital-to-analog converter taken from LTspice simulation through schematic capture and a two-layer surface-mount layout in Fusion 360 — then built, and used to play digital audio from a laptop through a speaker.

Role Circuit design, simulation, schematic capture, and PCB layout

Timeline Fall 2023

Team Two-person project

PCB Layout Fusion 360 LTspice Surface Mount Op-Amp Design LM1458 Two-Layer Audio

The board

Small, dense, and all surface-mount.

1″ × 1″ Board outline — the full circuit on one square inch

2 Copper layers, routed across nine vias

100% Surface-mount — 1206 resistors and an SOIC dual op amp

~40 mV Output step per binary count in simulation

PCB design

Schematic to routed board.

The schematic was captured in Fusion 360 and laid out on a one-inch, two-layer board. Every component is surface-mount.

Fusion 360 schematic of the 4-bit DAC: a five-pin input header feeding four weighted resistors into the first LM1458 op amp as an inverting summer, a second inverting stage with 10k and 1k resistors, a three-pin power header, a four-pin output header, and a right-angle audio jack on the output. — Schematic capture. A five-pin header brings in the four bits, four weighted resistors sum them at the first op amp, a second stage re-inverts and scales the result, and the output lands on an audio jack.

Two-layer PCB layout of the DAC on a one-inch board: red top-layer traces and blue bottom-layer traces linked by vias, an LM1458 SOIC package, seven 1206 resistors, three pin headers, a right-angle audio jack footprint, and four corner mounting holes. — The routed board — top copper in red, bottom in blue, nine vias between them. Four corner holes mount the board to a chassis alongside the Arduino that drives it; two smaller holes secure the audio jack.

Layout decisions

The size constraint drove most of the choices. Fitting an SOIC dual op amp, seven resistors, three headers, and a right-angle audio jack into one square inch meant going entirely surface-mount, with 1206 resistors as the smallest package that could still be hand-soldered reliably. The traces occupy both copper layers and cross through nine vias to reach every component without a dead end.

A ground pour was deliberately left off. For a four-bit converter at audio frequencies there is no return-path or thermal case for one, and omitting it kept the two-layer routing simpler to verify by eye.

Three headers separate the board's roles. A three-pin header supplies ground and the ±12 V rails to the op amps. A five-pin header takes the four input bits plus ground. A four-pin header exposes the output of each op amp stage for probing — so the summing stage and the scaling stage can be checked independently on a bench.

Circuit design

Weighting four bits with resistors.

A binary-weighted DAC treats each bit as its own input and gives it a resistor sized so that bit 3 carries twice the weight of bit 2, which carries twice the weight of bit 1, and so on. An inverting summing amplifier adds them.

With a 2.55 kΩ feedback resistor setting the top-bit gain at one, the remaining inputs needed 5.1 kΩ, 10 kΩ, and 20 kΩ to weight bits 2, 1, and 0 at one half, one quarter, and one eighth. Those were chosen from a stock list of available values rather than the exact ideal ratios — close enough that the step size stays uniform across the range. A second inverting stage with a 10 kΩ and 1 kΩ pair flips the summed output back to positive and scales it by one tenth to a level an audio input can take. Both op amps run on ±12 V rails.

LTspice schematic of the DAC: four pulse voltage sources driving 20k, 10k, 5.1k and 2.55k resistors into an inverting summing amplifier with a 2.55k feedback resistor, followed by an inverting amplifier with 10k input and 1k feedback, powered from plus and minus 12 volts. — LTspice model. The four sources are pulse functions timed so the binary input counts up by one each second.

LTspice simulation output: a staircase waveform over sixteen seconds, stepping up by roughly 40 millivolts each second as the binary input increments from 0 to 15. — Simulated output over sixteen seconds — a clean staircase rising about 40 mV per count, confirming the resistor weighting before any hardware was ordered.

Result

Digital audio in, sound out.

The board was assembled, tested stage by stage from the probe header, and then put to use: digital audio from a laptop was fed through the DAC and played out through a speaker via the on-board audio jack.

Simulating first paid off. The LTspice model predicted the step size and the output polarity before a single part was bought, so bring-up was a matter of confirming the board matched the model rather than debugging an unknown.

Documentation

Design report.

Read the design report (PDF)

Covers the circuit design and LTspice simulation, the Fusion 360 schematic and layout, and the component selection behind the one-inch board.