SAR ADC (Successive Approximation Register Analog-to-Digital Converter) is the most widely used ADC architecture that converts analog voltages to digital codes through a binary search algorithm — offering the best combination of moderate speed (1-100 MSPS), medium-to-high resolution (8-18 bits), low power consumption, and compact area that makes it the default choice for SoC-embedded data conversion.

How SAR ADC Works

1. Sample: Track-and-hold circuit captures the input voltage (Vin). 2. Compare MSB: Internal DAC set to Vref/2. Comparator checks: Is Vin > Vref/2?

• Yes → MSB = 1, keep Vref/2. No → MSB = 0, remove Vref/2.

3. Compare MSB-1: DAC adds/subtracts Vref/4. Compare again. 4. Repeat: N comparisons for N-bit resolution. 5. Output: N-bit digital code after N clock cycles.

Key Components

Capacitive DAC (CDAC)

• Binary-weighted capacitor array: C, C/2, C/4, ... C/2^N.
• Charge redistribution: Switch capacitor plates between Vin, Vref, and GND.
• Advantage: Capacitors in CMOS are more linear and match better than resistors.
• Bottom-plate sampling: Reduces charge injection error.

SAR ADC Advantages

• Low Power: Only 1 comparator decision per bit per sample → minimal switching.
• Power scales with: $P \propto C_{total} \times V_{ref}^2 \times f_s$.
• State-of-art: < 10 fJ/conversion-step (Walden FOM).
• Compact Area: No op-amps needed (unlike pipeline ADC).
• Scalable with CMOS: Better performance at smaller nodes (smaller caps = less power).

SAR ADC vs. Other Architectures

Advanced SAR Techniques

• Time-Interleaved SAR: Multiple SAR channels sampling at offset times → aggregate bandwidth multiplied.
• Noise-Shaping SAR: Embed sigma-delta noise shaping in SAR loop → higher ENOB without oversampling penalty.
• Redundant Bit SAR: Extra comparison bits relax comparator speed requirements.

SAR ADC is the workhorse data converter of the semiconductor industry — its elegant binary search algorithm delivers the optimal power-resolution-speed tradeoff that has made it the most prevalent ADC architecture in modern SoCs, from IoT sensors to 5G transceivers.