Question 8.2.1: In an N bit flash ADC, the analog voltage is fed simultaneou...

In an N bit flash ADC, the analog voltage is fed simultaneously to 2^{N}-1 comparators. The output of the comparators is then encoded to a binary format using digital circuits. Assume that the analog voltage source V_{\text {in }} (whose output is being converted to digital format) has a source resistance of 75 Ω as shown in the circuit diagram below and the input capacitance of each comparator is 8 pF. The input must settle to an accuracy of \frac{1}{2} LSB even for a full scale input change for proper conversion. Assume that the time taken by the thermometer to binary encoder is negligible. If the flash ADC has 8 bit resolution, which one of the following alternatives is closest to the maximum sampling rate?

(A) 1 megasamples per second

(B) 6 megasamples per second

(C) 64 megasamples per second

(D) 256 megasamples per second

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Given that accuracy= 0.5LSB for N bit flash counter even for Full scale voltage, then using Nyquist theorem where Fs is greater and equal to 2 Fm. So for
V_{\text {in }}=2^{N}-1,
Hence, the correct option is (A).

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