GT ECE 6414 - Analog to Digital Converters (25 pages)

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Analog to Digital Converters



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Analog to Digital Converters

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Pages:
25
School:
Georgia Institute of Technology
Course:
Ece 6414 - Analog Integrate Sys Dgn

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Analog to Digital Converters Slow Ramp Medium Successive Approx Fast Flash Oversampling Key components Comparitors Sample and Hold D A converters Sample And Hold Comparitors s D A s Digital Outputs Analog Input Basic A D Structure Digital Control Comparitors quantizing unit of ADCs Nonideal aspects Input offset voltage static characteristic Propagation time delay Bandwidth linear Slew rate nonlinear Slow Speed A D Converters Successive Approximation Successive Approximation Successive Approximation Algorithm 1 Start with the MSB bit and work toward the LSB bit 2 Guess the MSB bit as 1 3 Apply the digital word 10000 to a DAC 4 Compare the DAC output with the sampled analog input voltage 5 If the DAC output is greater keep the guess of 1 If the DAC output is less change the guess to 0 6 Repeat for the next MSB If the number of bits is N the time for conversion will be NT where T is the clock period Successive Approximation 5bit Successive Approximation ADC More Details on the DAC Pipeline Algorithmic ADC Each stage x by 2 or by Vref Self Calibrating ADC Parallel Flash A D Converter Number of comparator required is 2N 1 Typical sampling frequencies can be as high as 400MHz for 6 bits in sub micron CMOS technology Interpolating ADCs Must get the gain within LSB accurate Folding ADCs Schematic of a 5bit Folding ADC Folding Circuits Folding and interpolation ADCs offer the most resolution at high speeds 8 bits at 200MHz Need discussion for floating gate Flash ADCs If no offset at all then sizing of devices can be optimized for speed Therefore small input transistors highest speed and lowest input capacitance if smallest cap in a typical 0 35um process gate input capacitance is approximately 1fF therefore a 6bit would have an input capacitance of 100fF with parasitics accounted for which is small enough would need 1kOhm output resistance for a S H to settle in 1ns Do we need S H block here Need pictures here Two Step Flash ADC Simple 4 bit Flash Converter Pipelined Flash



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