4-Bit Digital to Analog Converter

Project Overview

I built a 4-bit digital-to-analog converter (DAC) that takes four digital inputs (from an Arduino) and converts them into an analog output suitable for basic audio playback. The design uses a weighted-resistor ladder feeding an op-amp stage to create a stepped analog voltage (16 levels), followed by a second op-amp stage to set polarity and amplitude. The work covered LTspice simulation, EAGLE PCB design, PCB assembly, and bench testing with function generator/audio input, logic analyzer, and oscilloscope.

System Specifications

Resolution: 4 bits (16 quantization levels).
Digital input interface: 4 GPIO lines from Arduino, LSB→MSB weighted via resistor values.
Core topology: Weighted-resistor ladder → inverting op-amp → second inverting op-amp (restores upward staircase and sets drive level).
Output interface: Audio jack (post-amp); intended to drive basic audio demonstrations (not a power amp).
Active components: LM1458D dual op-amp.
Power: Headered rails for op-amp ± supply (plus ground) and separate headers for bit inputs and board power.
PCB: 1″×1″, double-sided; 1206 SMD passives; minimal vias; autorouted after DRC tuning.
Bill of materials / cost: ≈$20 total (OSH Park board ~$5; expedited shipping & misc. ~$15).
Intended signal chain (audio demo): Audio source → level shifter (DC offset) → Arduino (bitstream) → 4-bit DAC → speaker (distorted demo audio).

Design & Architecture

The DAC is built with a weighted-resistor ladder that converts 4 digital inputs (from Arduino GPIOs) into proportional voltage levels.
An inverting op-amp stage sums the resistor currents, producing a stepped analog output (16 discrete levels).
A second inverting op-amp stage restores the staircase polarity and sets the final output amplitude for audio demo use.
The design was simulated in LTspice to validate waveforms before hardware.
The circuit was then laid out in EAGLE PCB, using a compact 1″×1″ two-layer board with SMD resistors/capacitors and a dual op-amp (LM1458D).
The board includes headers for Arduino bit inputs, ± supply rails, and the final analog output jack.
The signal chain is: Arduino bitstream → resistor ladder → op-amp 1 (summing) → op-amp 2 (gain/polarity) → audio jack.

LTSpice Circuit Design

PCB Layout

Top View of PCB

Schematic Diagram

3D CAD Model of PCB Chassis

Build Gallery

Project Demo

Results & Lessons Learned

Results

Simulation:
- After first op-amp: stepped downward staircase.
- After second op-amp: stepped upward staircase with expected 16 levels.
Assembly & bench test: Reflowed SMD parts; through-hole parts hand-soldered. On the scope, the assembled DAC’s gain did not match simulation due to an incorrect resistor value, leading to wrong amplification.
Logic analyzer (Arduino): Confirmed four weighted square-wave bitlines, indicating correct digital sequencing for LSB→MSB.
Audio demo: With level shifting → Arduino → DAC → speaker, playback produced a clearly distorted/quantized sound consistent with 4-bit reconstruction.

Lessons Learned

Component accuracy matters: A single wrong resistor value altered the overall gain—double-check BOM, labeling, and placements before reflow.
Sim vs. reality: Simulations predicted ideal staircases; bench behavior exposed real-world sensitivities (tolerances, op-amp limits, gain-setting errors).
Signal conditioning is essential: AC-coupled sources require proper level shifting prior to digital quantization to avoid clipping/logic errors.
Design for testability: Provide easy probe points for each stage (ladder node, op-amp outputs) to isolate issues quickly. (Implied by your multi-stage tests.)
Time management & iteration: More schedule slack would have helped absorb debugging and validation cycles.

Documentation

Copy of Final Report EE113 (1) (4) (1).pdf

Full 4-Bit Digital to Analog Converter Report

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