Flash type ADC and its demerits

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)

 +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.

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.