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Simultaneous/Flash ADC is based on using a number of
comparators to simultaneously compare the analog voltage to be converted with a
set of predefined reference voltage levels and the process of such comparison
happens simultaneously. The number of comparators required for making an n-bit simultaneous
converter is given by 2n .
As an example consider a 2-bit simultaneous ADC. A schematic diagram
of such a system is shown below.
The analog signal to be digitized serves as one of the inputs
to each of the comparators. The second input for each of the comparators is a
reference input, different for each comparator. The reference voltages to be
used for comparators are in general V/2n, 2V/2n, 3V/2n,
4V/2n and so on. Here, V is the maximum amplitude of the
analogue signal that the A/D converter can digitize, and n is the number of
bits in the digitized output.
In the present case of a two-bit A/D converter, the reference
voltages for the three comparators will be V/4, V/2 and 3V/4. If we wanted a
three-bit output, the reference voltages would have been V/8, V/4, 3V/8, V/2,
5V/8, 3V/4 and 7V/8. As we see from above diagram that the output status of
various comparators depends upon the input analogue signal VA. For
instance, when the input VA lies between V/4 and V/2, the C1
output is HIGH whereas the C2 and C3 outputs are both
LOW. The possible results are shown in the table below.
The outputs of the comparators can then be fed to a coding
network to provide two bits that are digital equivalent of the input analog
voltage. The bits at the output of the coding network can then be entered into
a flip-flop register for storage.
One of the biggest disadvantages of a simultaneous ADC is
that, as the number of bits in the desired digital signal increases, the number
of comparators required for performing A/D conversion increases very rapidly,
and it may not be feasible to use this approach once the number of bits exceeds
six or so. The greatest advantage of this technique lies in its capability to
execute extremely fast analogue-to-digital conversion.