ADC – Simultaneous Conversion

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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.