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Flash Analog to Digital Conversion

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As the number of comparators increases, one rapidly reaches a point of diminishing returns. Every time one wishes to add one more bit to the ADC output, one doubles the number of comparators. Since each comparator has some input capacitance (and the wiring feeding the comparators has capacitance as well!), the maximum speed drops because RC increases. This collides with the greater settling time needed to improve the number of bits of accuracy. Thus, beyond 8 bits, flash converters face severe limitations. Nevertheless, in the fastest digital instruments, in high speed cameras, and in communications systems, digitization rates of over 1 GHz have been realized, while several hundred MHz digitization is routine. Using the arguments we have already made, assuming a source resistance of 50Ω (quite common), input capacitance for each comparator of 1 pF, and ignoring speed limitations in the comparator (dream on!), the fastest a flash converter can digitize is:

Number of bits Maximum Digitization Rate (Hz)
29 GHz
1.8 GHz
450 MHz
200 MHz
113 MHz

Let's critically consider this table. Silicon transistors can not switch at 29 GHz; the fastest switches can run at less than 10 GHz. GaAs can reach 30 GHz, but is not commonly used in consumer electronics. For the highest numbers of bits (>8), there is an additional error source that we haven't yet considered. A flash converter is no more accurate than the resistors in the divider chain. To be useful, a 16 bit converter needs the divider chain to be accurate to 1 part in 65536. The best resistors one can purchase are ~ 0.01% accurate at a single temperature (and change resistance by 5 parts per million per degree Celsius). 0.01% is about 12 bit resolution. This hints at a more general problem: high resolution simply can't count on precision of resistors to set digitization precision. We ill find that some of the other ADC designs depend on component precision, others not.



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