ADC & DAC — turning signals into numbers

In one line

An analog-to-digital converter (ADC) measures a continuous voltage many times a second and turns each measurement into a number. A DAC does the reverse. The ADC is the doorway every biosignal walks through to become data.

The two knobs that decide quality

• Sampling rate (samples per second, Hz) — how often you measure. The Nyquist rule: to capture a signal you must sample at more than twice its highest frequency. Sample too slowly and fast features fold into false low-frequency junk (aliasing) — which on an ECG can invent or hide things. An ECG is sampled at ~250–1000 Hz; audio (a digital stethoscope) at ~8–44 kHz.
• Resolution (bits) — how finely each sample is graded. An 8-bit ADC has 256 steps; a 12-bit one has 4096; a 24-bit one (used in ECG/EEG) resolves microvolts. More bits = finer detail, but only as good as the noise floor allows.

Where it shows up in digital health

• Every measured vital — the sensor → amplifier → ADC chain inside an ECG, pulse oximeter, BP cuff or CGM. The numbers it produces are what eventually become FHIR Observations.
• DACs drive outputs: the tone of an alarm, the waveform a stimulator delivers, the drive signal to an actuator.

Watch for

An anti-aliasing filter (a low-pass filter before the ADC) is mandatory, not optional — it removes frequencies above half the sampling rate so they can't alias. Skipping it is a classic, hard-to-debug source of corrupted readings.

Key takeaways

• An ADC turns a continuous voltage into numbers (sampling); a DAC does the reverse.
• Two quality knobs: sampling rate (Nyquist: > 2× the highest frequency) and resolution (bits).
• Under-sampling causes aliasing — false features that can invent or hide things on an ECG.
• An anti-aliasing filter before the ADC is mandatory; the ADC is the doorway every biosignal becomes data through.