Analog to Digital Converters

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# Analog to Digital Converters - PowerPoint PPT Presentation

Analog to Digital Converters. Slow (Ramp) Medium (Successive Approx) Fast (Flash) Oversampling ( S-D ). Key components: Comparitors Sample-and-Hold D/A converters. Basic A/D Structure. +. Sample And Hold. Comparitors(s). Digital Outputs. Analog Input. -. D/A(s).

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## Analog to Digital Converters

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Presentation Transcript
Analog to Digital Converters
• Slow (Ramp)
• Medium (Successive Approx)
• Fast (Flash)
• Oversampling (S-D)

Key components:

Comparitors

Sample-and-Hold

D/A converters

Basic A/D Structure

+

Sample

And Hold

Comparitors(s)

Digital Outputs

-

D/A(s)

Digital Control

Comparitors

Nonideal aspects:

• Input offset voltage (static characteristic)

• Propagation time delay

- Bandwidth (linear)

- Slew rate (nonlinear)

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.

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.

Each stage: x by 2,

+ or – by Vref

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.

Must get the gain within ½ LSB accurate.

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.

PIPELINE ADC WITH DIGITAL ERROR CORRECTION

The ADC of the first stage uses 16 equal capacitors instead of 4 binary weighted for more accuracy