Analog%20to%20Digital%20Converters

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Analog to Digital Converters

• Slow (Ramp)
• Medium (Successive Approx)
• Fast (Flash)
• Oversampling (S-D)

Key components Comparitors
Sample-and-Hold D/A converters
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Basic A/D Structure

Sample And Hold
Comparitors(s)
Digital Outputs
Analog Input
-
D/A(s)
Digital Control
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Comparitors

• quantizing unit of ADCs

Nonideal aspects Input offset voltage (static
characteristic) Propagation time delay -
Bandwidth (linear) - Slew rate (nonlinear)
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Slow-Speed A/D Converters
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Successive Approximation
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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.
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Successive Approximation
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5bit Successive Approximation ADC
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More Details on the DAC
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Pipeline Algorithmic ADC
Each stage x by 2, or by
Vref
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Self-Calibrating ADC
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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.
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Interpolating ADCs
Must get the gain within ½ LSB accurate.
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Folding ADCs
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Schematic of a 5bit Folding ADC
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Folding Circuits
Folding and interpolation ADCs offer the most
resolution at high speeds (8 bits at 200MHz)
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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.
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Two-Step Flash ADC
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Simple 4-bit Flash Converter
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Pipelined Flash ADC
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Part of a Two-Step ADC
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PIPELINE ADC WITH DIGITAL ERROR CORRECTION
The ADC of the first stage uses 16 equal
capacitors instead of 4 binary weighted for more
accuracy
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12-BIT PIPELINE ADC
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