The Flash ADC, also called parallel ADC, is very easy to understand. It works by comparing the input voltage – i.e. the analog signal – to a reference voltage, which would be the maximum value achieved by the analog signal. For example, if the reference voltage is of 5 volts, this means that the peak of the analog signal would be 5 volts. On an 8-bit ADC when the input signal reached 5 volts we would find a 255 (11111111) value on the ADC output, i.e. the maximum value possible.
Then the voltage reference is lowered thru a resistor network and other comparators added, so the input voltage (analog signal) can be compared to other values.
On Figure 6 you can see a 3-bit Flash ADC. The comparison is done thru an op amp. All resistors have the same value.
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Figure 6: Flash ADC.
The priority encoder can be done using XOR gates and a series of diodes and resistors, like shown on Figure 7, or a single chip like 74148 (3-line to 8-line priority encoder).
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Figure 7: Flash ADC.
Even though Flash ADC uses a very simple design, it requires a lot of components. The number of required comparers is 2^n-1, where n is the number of output bits. Thus for an eight-bit Flash ADC 255 comparers would be necessary, and for a 16-bit Flash ADC, 65,535!
On the other hand, Flash ADC is the fastest ADC type available. The digital equivalent of the analog signal will be available right away at it output (it will only have the propagation delay inserted by the logic gates) – hence the name “flash”.
Another advantage of Flash ADC is that you can create an ADC with non-linear output. Usually ADCs have a linear output, i.e. each digital number corresponds to a fixed voltage increase on the analog input. For example, on the 3-bit ADC shown above with a Vref of 5 V, each digital number would represent 625 mV (5 V / 2^3). So 0 V = 000, 0.625 V = 001, 1.250 V = 010 and so on up to 5 V = 111.
Since Flash ADC comparisons are set by a set of resistors, one could set different values for the resistors in order to obtain a non-linear output, i.e. one value would represent a different voltage step from the other values.
