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).
• Input offset voltage (static characteristic)
• Propagation time delay
- Bandwidth (linear)
- Slew rate (nonlinear)
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.
Each stage: x by 2,
+ or – by Vref
Number of comparator required is 2N-1
Typical sampling frequencies
can be as high as 400MHz
for 6-bits in sub-micron
Must get the gain within ½ LSB accurate.
Folding and interpolation ADCs offer the most resolution at high speeds (≈8 bits at 200MHz)
If no offset at all, then sizing of devices can be optimized for
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.
The ADC of the first stage uses 16 equal capacitors instead of 4 binary weighted for more accuracy