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A Level     Signal     >ADC Flash<     ADC Ramp     DAC Summing     Distortion     Problems     Regenerator     Sample and Hold     Sampling     Serial and Parallel     signal to noise ratio    

Signal ADC Flash


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Flash ADC

High speed conversion of an analogue signal into a digital signal.

Advantages

Disadvantages

  • Fast because the speed is limited only by the comparators and exclusive or gates.
  • Suitable for digitising video because of its speed
  • Fairly easy to understand because it only requires comparators, exclusive OR gates and diodes (used as OR gates).
  • Expensive
  • High component count (not too much of a problem with custom designed ICs).
  • Very hard to build from discrete components because of the large number of components needed.

To calculate the resolution of the ADC ...

To calculate the number of comparators (or exclusive OR gates) needed ...

Uses XOR and Diode OR Gates

Flash Analogue to Digital Converter

XOR and Diode OR gates replaced with a priority encoder

Flash ADC with Priority Encoder

This is a two bit flash ADC (not very useful!) 8 or 16 bits would be much more use but also very much more complex.

Look here for a very similar three bit comparator that has about double the component count. Each extra bit added doubles the component count again. An 8 bit converter would need 255 comparators and exclusive or gates.

Half Flash ADC

An 8 bit converter would need 28 - 1 comparators = 255. This complexity makes the chip expensive and more difficult to make.

With some loss of speed, it's possible to use two 4 bit flash converters with some additional circuitry. This is the circuit diagram.

Half Flash Analogue to Digital Converter

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