Digital-to-Analog Conversion
A DAC turns a numeric command into a continuous voltage or current, forming the output stage that lets a digital controller act on the physical world.
From Number to Signal
A digital-to-analog converter (DAC) is the reverse of an ADC: it maps a digital code to a proportional analog voltage or current. In a control system it is often the last electronic step before the power stage that drives an actuator. The DAC's resolution, speed, and glitch behavior determine how faithfully the controller's intent becomes a physical drive level.
The Staircase Output and Reconstruction
A DAC updated at the sample rate produces a staircase: it holds each value until the next update. This zero-order hold introduces a small delay of half a sample period on average and adds high-frequency images of the signal. A reconstruction filter, an analog low-pass after the DAC, smooths the staircase and removes the images, recovering a clean continuous signal for the actuator.
Key Characteristics
- Resolution in bits, setting the smallest output step
- Settling time, how fast the output reaches a new value within tolerance
- Glitch energy, transient spikes when codes change, worst at major transitions
- Monotonicity, guaranteeing output never decreases as the code increases
Why Update Timing Matters
As with the input side, the instant the DAC updates should be precise. Timing the output from a hardware event, rather than whenever software writes the register, keeps output jitter low so the actuator sees a clean command. Monotonicity matters in feedback: a non-monotonic DAC can make a small commanded increase produce a decrease, which can destabilize a loop.
Alternatives
Not every actuator needs a linear DAC. Pulse-width modulation drives many loads efficiently by switching full-on and full-off and varying the duty cycle, letting the load's own inertia or an inductor average the result. This is common for motors, heaters, and switching supplies, where it avoids dissipating power in a linear stage. Whether a true DAC or PWM is used, the actuator interface must convert the digital command into physical action with predictable timing and defined safe behavior on fault.