---
title: "Flash type ADC and its demerits"
subject: "Advanced Electronics"
chapter: 3
key: "semester_4/advanced_electronics:flash-adc-demerits"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/flash-adc-demerits
site: BCT Engineering
---

# Flash type ADC and its demerits

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

- Asked in: 2071 Shrawan, 3+3+1 marks
- Video: [Flash ADC (Parallel ADC) and Half-Flash ADC Explained](https://www.youtube.com/watch?v=NASkjo7s8f4) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/flash-adc-demerits
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-3

## Question

Draw and explain the circuit diagram of flash type ADC. What are the demerits of flash type ADC over other ADCs?

## Short answer

Flash ADC requires 2^n-1 comparators, leading to very large hardware, high cost, power, and chip area that grow exponentially with bits. It has limited practical resolution (usually ≤8 bits) due to offset and mismatch, high input capacitance from driving all comparators, and can produce sparkle codes at high speed.

## Answer

A flash (parallel or simultaneous) ADC is the fastest type of ADC. It compares the input with all quantization levels at the same time, using 2ⁿ − 1 comparators, and a priority encoder gives the binary output in one step.

#### Circuit (2-bit / 3-level example)
```text
 +Vref
   |
   R  -------(-)C3
   |  3Vref/4 (+)<-Vin -->+
   R  -------(-)C2        |
   |  2Vref/4 (+)<-Vin -->+ PRIORITY
   R  -------(-)C1        | ENCODER -> B1 B0
   |  1Vref/4 (+)<-Vin -->+
   R
   |
  GND
```

A 3-bit flash ADC uses 8 equal resistors and 7 comparators in the same way.

#### Working
1. The resistor divider makes the reference levels Vref/4, 2Vref/4 and 3Vref/4 (for n bits, steps of Vref/2ⁿ).
2. Vin goes to the + input of every comparator at the same time.
3. Each comparator whose reference is below Vin gives 1; the others give 0. The comparator outputs form a thermometer code.
4. The priority encoder converts the thermometer code into binary.

| Vin range | C3 C2 C1 | B1 B0 |
|---|---|---|
| < Vref/4 | 0 0 0 | 00 |
| Vref/4 – Vref/2 | 0 0 1 | 01 |
| Vref/2 – 3Vref/4 | 0 1 1 | 10 |
| > 3Vref/4 | 1 1 1 | 11 |

The conversion time is only one comparator delay plus the encoder delay (nanoseconds). No clock-by-clock search is needed, which is why it is called "flash".

#### Demerits compared with other ADCs
- **Very large hardware:** it needs 2ⁿ − 1 comparators and 2ⁿ resistors. An 8-bit flash needs 255 comparators, while SAR or dual slope ADCs need only one.
- **High cost, power and chip area**, which grow exponentially with the number of bits.
- **Limited resolution** in practice (usually 8 bits or less). Comparator offsets and resistor mismatch limit accuracy.
- **High input capacitance**, because Vin drives all comparators. This loads the source.
- It can produce sparkle codes and bubbles in the thermometer code at high speed.

## Check yourself

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

1. An n-bit flash ADC needs how many comparators?
   A. 2^n - 1
   B. n
   C. 2^n
   D. n^2
   Answer: A. An 8-bit flash needs 255 comparators. (Key point: Comparators)

2. Why is flash called the fastest ADC?
   A. It counts clock pulses
   B. Vin is compared with all levels at once, so conversion is one comparator plus encoder delay
   C. It uses successive approximation
   D. It integrates the input
   Answer: B. No clock-by-clock search is needed. (Key point: Speed)

3. What does the priority encoder do?
   A. Generates reference voltages
   B. Integrates Vin
   C. Converts the thermometer code into binary
   D. Counts clock pulses
   Answer: C. Comparators below Vin output 1; the encoder gives the binary result. (Key point: Encoder)

4. Which is a demerit of flash ADC?
   A. Slow conversion
   B. Uses only one comparator
   C. Unlimited resolution
   D. Hardware, cost and power grow exponentially with bits
   Answer: D. This limits practical resolution to about 8 bits, with high input capacitance too. (Key point: Demerit)

## Related in Advanced Electronics

- [Compare successive approximation and flash ADCs](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Acompare-sar-flash-adc) — Advanced Electronics, chapter 3
- [Dual slope ADC block diagram; integral linearity](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Adual-slope-integral-linearity) — Advanced Electronics, chapter 3
- [Dual slope ADC working; why accurate](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Adual-slope-adc-accurate) — Advanced Electronics, chapter 3, asked 3×
- [Count up/down and tracking type ADC](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Acount-up-down-tracking-adc) — Advanced Electronics, chapter 3

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