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Flash ADC

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

• Used with Video signals.
• Not needed with slower signals like Audio and human interface devices.

To calculate the resolution of the ADC ...

• The ADC below detects four different input levels
• The resolution is Vref / 4
• An 8 bit ADC can resolve 2⁸ levels = 256 levels so the resolution is Vref / 256

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

• 2^N - 1 comparators are needed where N is the number of bits in the output.
• In the example below, there is a two bit output so 2² - 1 = 3 comparators needed.
• For a four bit converter, 2⁴ - 1 = 15 comparators needed.
• For an 8 bit converter, 2⁸ - 1 = 255 comparators needed. (hard to implement)
• For a 16 bit converter, 2¹⁶ - 1 = 65535 comparators needed. (Some CPU processors have millions of gates so this is entirely possible.)

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.

• The circuit above has three comparators.
• All the resistors are equal. 1 to 10k would be OK.
• Each comparator is fed a proportion of the reference voltage from Vref.
• If the input voltage (Vin) is too low, all the comparators will be turned off.
• If Vin is a little higher, only the bottom comparator will turn on.
• If Vin is a little high still, the bottom two comparators will turn on.
• If Vin is high enough, all the comparators will turn on.
• If all the comparators are off, the output will be 0 0. This is zero in binary.
• If the bottom comparator is on, the output will be 0 1. This is one in binary.
• If the bottom two comparators are on, the output will be 1 0. This is two in binary.
• If all the comparators are on, the output will be 1 1. This is three in binary.
• Adding extra bits is simple. More comparators are needed and the output logic gets more complex too.

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.

• The analogue input is converted using the first 4 bit Flash ADC.
• This gives an approximate binary value ( bits 4, 5, 6 and 7 ) which is converted back into an analogue voltage by the DAC.
• This approximate value is subtracted from the original signal and what's left is converted to digital by the second Flash ADC ( bits 0, 1, 2 and 3 ).
• This 8 bit value is stored in the latch when the clock line goes high.
• Due to the time delay introduced by the extra circuitry, a sample and hold circuit on the input would be essential.