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DAC Architectures

DAC architecture determines output speed, monotonicity, glitch behavior, resolution, output type, power, and cost. Many embedded systems only need a simple voltage-output DAC, but audio, instrumentation, calibration, and high-speed waveform generation require more careful architecture selection.

Learning Objectives

By the end of this lesson, you should be able to:

  • describe common DAC architectures;
  • compare resistor-string, R-2R, current-steering, sigma-delta, and PWM-based DACs;
  • understand monotonicity, glitches, and settling tradeoffs;
  • choose a DAC type for typical embedded applications;
  • identify architecture-specific layout and firmware concerns.

Resistor-String DAC

A resistor-string DAC divides the reference into many equal steps and selects one tap with switches. It is naturally monotonic when the string is well matched.

Strengths:

  • good monotonic behavior;
  • simple voltage output;
  • low glitch compared with some architectures.

Limitations:

  • large resistor count for high resolution;
  • switch resistance and output buffering matter;
  • slower for high-speed applications.

R-2R Ladder DAC

An R-2R ladder uses only two resistor values, R and 2R, switched according to the digital code. It is compact and common in integrated DACs.

Strengths:

  • efficient resistor count;
  • good for moderate speed and resolution;
  • can support voltage or current outputs.

Limitations:

  • resistor matching controls linearity;
  • major-code transitions can glitch;
  • output impedance and buffering must be checked.

Current-Steering DAC

Current-steering DACs switch matched current sources into an output node. They are common in high-speed waveform generation and communications.

Strengths:

  • high update rate;
  • good for RF, video, and arbitrary waveform outputs;
  • differential outputs are common.

Limitations:

  • layout and clock quality are critical;
  • output often needs a transformer, resistor load, or amplifier;
  • glitches and spurs affect spectral purity.

Sigma-Delta DAC

A sigma-delta DAC uses high-rate one-bit or multi-bit modulation followed by filtering. Audio DACs often use this principle.

Strengths:

  • high resolution in the audio or low-frequency band;
  • relaxed analog matching compared with some precision ladders;
  • digital filtering can shape noise out of band.

Limitations:

  • requires reconstruction filtering;
  • out-of-band noise must be managed;
  • latency may matter in closed-loop systems.

PWM as a DAC

PWM is not a true DAC by itself, but a timer output plus low-pass filter can create an average voltage:

$$
V_\text{AVG}=D V_\text{HIGH}
$$

where D is duty cycle from 0 to 1. PWM is cheap and useful for slow control voltages, LED dimming, and simple setpoints.

Architecture Comparison

Architecture Best fit Watch out
resistor string monotonic setpoints area at high resolution
R-2R ladder general voltage/current DAC matching and glitches
current steering high-speed waveforms layout, spurs, output load
sigma-delta audio, high resolution filtering and latency
PWM plus filter low-cost slow outputs ripple and response time
multiplying DAC programmable gain/reference reference bandwidth

Monotonicity and Linearity

A DAC is monotonic if increasing the digital code never decreases the analog output. Monotonicity is essential in control loops because a non-monotonic step can drive the loop in the wrong direction.

Linearity errors include INL and DNL, similar to ADCs. Precision systems also care about offset, gain, drift, noise, and reference quality.

Common Mistakes

  • Using PWM for a low-noise precision reference without checking ripple.
  • Selecting a high-speed current-output DAC without a proper output network.
  • Ignoring major-carry glitches in waveform or control applications.
  • Assuming all DACs are monotonic to their full resolution.
  • Forgetting that reference noise appears at the output.

Summary

DAC architectures trade resolution, speed, monotonicity, glitch energy, output type, and cost. Resistor-string DACs are simple and monotonic, R-2R ladders are compact, current-steering DACs are fast, sigma-delta DACs are strong for audio and high resolution, and PWM plus filtering can serve slow low-cost outputs.

Further Reading

Mind Map

mindmap root((DAC Architectures)) Resistor string Tap selector Monotonic Simple voltage output Large at high bits R2R ladder R and 2R network Compact Matching critical Major glitches Current steering Fast updates Differential outputs RF waveforms Layout sensitive Sigma delta Oversampled output Noise shaping Needs filter Audio common PWM Vavg equals duty times Vhigh Cheap timer output Ripple tradeoff Slow setpoints Common mistakes Ignoring monotonicity No ripple check Weak reference Wrong output load