- 2069 Asar · 7 marks
Compare successive approximation and flash ADCs
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.
Compare performance of successive approximation and flash type A/D converters in terms of resolution, conversion time and cost.
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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