Last time we covered turning digital numbers into analog voltages with DACs. Now let's go the other direction: converting real-world analog signals into digital values your microcontroller can process. This is the job of the Analog-to-Digital Converter (ADC).

An ADC does two things: it samples the analog signal at discrete moments in time, and it quantizes each sample into a digital code. Both steps introduce limitations you need to understand.

Resolution and LSB size. An N-bit ADC divides its reference voltage into 2^N steps. For a 10-bit ADC with a 3.3V reference, each step (one LSB) is:

LSB = 3.3V / 1024 ≈ 3.22 mV

This means you can't distinguish voltage differences smaller than about 3.2 mV. If you're reading a thermistor circuit that produces 10 mV/°C, your temperature resolution is roughly 0.3°C — which may or may not be good enough. Upgrading to a 12-bit ADC (4096 steps) cuts the LSB to 0.81 mV and gets you to ~0.08°C resolution. This is exactly the kind of back-of-envelope math you should do before selecting an ADC.

Sampling rate and Nyquist. The Nyquist theorem says you must sample at at least twice the highest frequency present in your signal. In practice, sample at 5–10× the signal bandwidth to get clean reconstructions. If you're digitizing audio up to 20 kHz, a 44.1 kHz sample rate is the theoretical minimum — which is exactly why CD audio uses that rate. For a slowly varying temperature sensor, even 10 samples per second is overkill.

Common ADC architectures:

• Successive Approximation (SAR) — the workhorse. Medium speed (up to a few MSPS), good resolution (12–18 bits), low power. This is what's inside most microcontrollers (STM32, ATmega, ESP32).
• Delta-Sigma (ΔΣ) — trades speed for resolution. Excellent for 16–24 bit precision at low sample rates. Used in digital multimeters, load cells, and precision instrumentation. The ADS1115 is a popular 16-bit ΔΣ converter for hobbyist projects.
• Flash — extremely fast (GSPS range) but expensive and power-hungry. Used in oscilloscopes and RF systems. You won't encounter these in typical projects.

Practical tips for clean ADC readings:

• Decouple the ADC's reference voltage pin with a 100 nF ceramic capacitor right at the pin. Noisy reference = noisy readings.
• Keep analog signal traces short and away from digital switching signals (SPI clocks, PWM lines).
• Use a simple RC anti-aliasing filter on the input. Even a 100Ω resistor and 100 nF cap (cutoff ~16 kHz) prevents high-frequency noise from folding into your measurement band.
• Average multiple samples in software. Averaging 16 readings and shifting right by 4 effectively gives you a cheap noise reduction of ~4× (two extra effective bits).

Real-world example: You want to measure a 0–5V battery voltage with a 3.3V microcontroller's built-in 12-bit SAR ADC. Use a resistor divider (e.g., 10kΩ and 6.8kΩ) to scale 5V down to ~2.02V, well within the 0–3.3V input range. Add a 100 nF cap at the ADC pin to stabilize the reading. Your LSB is 0.81 mV at the ADC, which maps to about 2 mV at the battery — plenty of resolution for battery monitoring.