Analog-to-Digital Conversion
An ADC samples an analog signal and encodes it as a number; its architecture sets the trade-off between speed, resolution, and latency.
The Measurement Core
An analog-to-digital converter (ADC) is the device that turns a voltage into a digital code. It combines two operations: sampling in time, usually via a sample-and-hold that freezes the input, and quantization in amplitude, mapping the held voltage to one of a finite set of codes. Every digital measurement of a physical quantity passes through this step.
Major Architectures
- Successive approximation (SAR): moderate speed, good resolution, low latency, one sample at a time
- Delta-sigma: very high resolution for slow signals, using oversampling and noise shaping, higher latency
- Pipelined: high sample rates at moderate resolution, with fixed pipeline latency
- Flash: very fast, lower resolution, used where speed dominates
Speed, Resolution, Latency
These architectures exist because no single design maximizes everything. Higher resolution generally costs speed or latency; higher speed costs resolution or power. A delta-sigma converter can reach many bits but only for signals far below its clock rate and with conversion latency from its filtering. A SAR converter gives a result quickly with low latency, which suits per-sample control loops. Matching the architecture to the loop's rate and precision needs is the core selection decision.
Latency Matters for Control
In a feedback loop, conversion latency is part of the sensor-to-actuator delay. A converter that averages heavily for resolution may add delay that erodes phase margin. This is why control front ends often favor low-latency SAR converters over higher-resolution delta-sigma parts, accepting fewer bits in exchange for prompt data.
Front-End Requirements
An ADC only performs to specification with proper support: a stable, low-noise voltage reference sets absolute accuracy; an anti-alias filter must precede it; and clean power and grounding preserve its effective resolution. The converter is one link; its accuracy is bounded by the weakest element of the surrounding analog design.