The maximum clock speed of a tracking ADC depends on the propagation delay of the DAC and the comparator. After every clock, the counter output has to propagate through the DAC and appear at the output. The compara- tor then takes some amount of time to respond to the change in DAC voltage, producing a new up/down control input to the counter. Tracking ADCs are not commonly available; in looking at the parts avail- able from Analog Devices, Maxim, and Burr-Brown (all three are manufac- turers of ADC components), not one tracking ADC is shown. This only makes sense: a successive approximation ADC with the same number of bits is faster. However, there is one case where a tracking ADC can be useful. If the input signal changes slowly with respect to the sampling clock, a tracking ADC may produce an output in fewer clocks than a successive approximation ADC. I saw a design once that implemented a tracking ADC in discrete hardware in exactly this situation. Flash ADC
bolic logic. As the meeting progressed and the lecturers explained the theory be- hind TM, I noticed my logician friend becoming increasingly restless. Looking more and more pained and shifting about constantly in his seat, he was finally un- able to resist. When the leaders called for questions at the end of the lecture, he raised his hand and gently but surely demolished the presentation we had just heard. In less than two minutes, he pointed out precisely where and why the lec- turers' complex argument was contradictory, illogical, and unsupportable. The ef- fect on the discussion leaders was devastating. After a confused silence, each attempted a weak reply only to halt midway to confer with his partner and finally to admit that my colleague's points were good ones "requiring further study." More interesting to me, though, was the effect upon the rest of the audience. At the end of the question period, the two recruiters were faced with a crowd of au-
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