sar adc design
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
| Component | Function | Critical Parameter |
|---|---|---|
| Capacitor DAC | Generates comparison voltages | Matching (< 0.1% for 10-bit) |
| Comparator | Compares Vin vs DAC output | Offset, noise, speed |
| SAR Logic | Binary search controller | Switching sequence |
| Sample/Hold | Captures input voltage | Bandwidth, settling |
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
| Architecture | Speed | Resolution | Power | Area |
|---|---|---|---|---|
| SAR | 1-100 MSPS | 8-18 bit | Very Low | Small |
| Pipeline | 100 MSPS-1 GSPS | 8-14 bit | Medium | Large |
| Flash | 1-10 GSPS | 4-8 bit | High | Very Large |
| Sigma-Delta | < 10 MSPS | 16-24 bit | Low | Medium |
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.