---
title: "Compare successive approximation and flash ADCs"
subject: "Advanced Electronics"
chapter: 3
key: "semester_4/advanced_electronics:compare-sar-flash-adc"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/compare-sar-flash-adc
site: BCT Engineering
---

# Compare successive approximation and flash ADCs

Advanced Electronics · Chapter 3: Digital-to-Analog and Analog-to-Digital Conversion

- Asked in: 2069 Asar, 7 marks
- Video: [Analog-to-Digital Converters (ADC) - Flash ADC and Successive Approximation Register ADC](https://www.youtube.com/watch?v=75GcoQ9_LFI) — Institute of Electronics - TU Graz
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/compare-sar-flash-adc
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-3

## Question

Compare performance of successive approximation and flash type A/D converters in terms of resolution, conversion time and cost.

## Short answer

SAR ADC uses one comparator and binary search; resolution up to 18 bits, conversion time is n clock cycles, low cost. Flash ADC uses 2ⁿ−1 comparators, converts in one step (fastest), resolution typically 4–8 bits, cost and power are high.

## Answer

The successive approximation (SAR) ADC finds the digital code bit by bit using a binary search, with one comparator and a DAC. The flash (parallel) ADC compares the input with all levels at once, using 2ⁿ − 1 comparators.

#### Comparison
| Parameter | Successive approximation ADC | Flash ADC |
|---|---|---|
| Principle | Binary search, one bit per clock | All levels compared at once |
| Comparators | 1 | 2ⁿ − 1 (255 for 8 bits) |
| Other parts | n-bit DAC, SAR register, control logic | 2ⁿ resistor divider, priority encoder |
| Conversion time | n clock periods (fixed), e.g. 8 bits at 1 MHz = 8 µs | One comparator + encoder delay (ns), fastest type |
| Speed dependence | Grows linearly with n | Independent of n |
| Practical resolution | High: 8 to 18 bits | Low: usually 4 to 8 bits |
| Resolution limit | DAC accuracy and comparator | Comparator count, offsets and resistor matching |
| Hardware growth | Linear with n | Exponential with n |
| Cost | Low to moderate | High (large chip area, many comparators) |
| Power | Low | High |
| Input capacitance | Low (one comparator) | High (all comparators in parallel) |
| Sample-and-hold | Needed (input must stay constant for n clocks) | Usually not needed |
| Typical use | Data acquisition, microcontroller ADCs, instruments | Video digitizing, radar, oscilloscopes, high-speed communication |

#### Summary
- **Resolution:** SAR is better. It easily gives 10 to 16 bits. Each extra bit in a flash ADC doubles the number of comparators, so flash ADCs stay at about 8 bits or less.
- **Conversion time:** flash is better. It converts in one step (tens of ns). SAR needs n clock cycles, which is still much faster than counter or dual slope types.
- **Cost:** SAR is much cheaper. Flash cost, power and area grow as 2ⁿ.

For example, an 8-bit SAR uses 1 comparator and needs 8 clocks. An 8-bit flash uses 255 comparators and converts in a single step.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How many comparators does an n-bit flash ADC need?
   A. n
   B. 2^n - 1
   C. 1
   D. 2n
   Answer: B. A flash ADC compares the input with every level at once, so it needs 2^n - 1 comparators (255 for 8 bits). (Key point: Flash comparator count)

2. An 8-bit SAR ADC uses a 1 MHz clock. What is its conversion time?
   A. 1 us
   B. 256 us
   C. 8 us
   D. 255 us
   Answer: C. A SAR ADC finds one bit per clock, so n clocks are needed: 8 x 1 us = 8 us. (Key point: SAR conversion time)

3. Why do flash ADCs stay at about 8 bits or less while SAR ADCs reach 10 to 16 bits?
   A. Flash ADCs are slower
   B. Each extra flash bit doubles the number of comparators, while SAR hardware grows only linearly
   C. SAR ADCs use more comparators
   D. Flash ADCs cannot use a DAC
   Answer: B. Flash hardware grows as 2^n, so cost and area explode with resolution; SAR grows linearly with n. (Key point: Resolution comparison)

4. Which statement correctly compares the two converters?
   A. Flash is faster but costs more; SAR is cheaper but slower
   B. SAR is faster and costs more
   C. Both have the same conversion time
   D. Flash is cheaper and has higher resolution
   Answer: A. Flash converts in one step (ns) but needs many comparators; SAR is cheap and needs n clocks. (Key point: Cost and speed trade-off)

## Related in Advanced Electronics

- [Flash type ADC and its demerits](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aflash-adc-demerits) — Advanced Electronics, chapter 3

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