1. Simple ADC structures

2. Some ADC generates internal Vref, some require externally provided Vref
Sampling clock is provided externally, but need to generate the various phases of clocks internally
EOC (end of conversion) indicates that output data is ready
Digital output data can go to a data bus, or to a buffer or memory for block transfer

3. The Comparator: A 1-Bit ADC
Widely used as a building block in many higher resolution ADCs
Comparator’s resolution needs to be able to support overall high resolution ADC
Without hysteresis, a 1-bit ADC is inherently linear, but with hysteresis, it is not
Hysteresis size should be less than required resolution
Constant offset does not harm linearity by itself, but may cause internal signals to go out of range, thus causing errors

4. High Speed ADC Architectures
Flash ADC
N bit string DAC
2N – 1 comparators
Thermometer to binary encoder
Logic latch
Clock signal
Vref to the DAC

5. Chapter 17 Figure 24

6. Chapter 17 Figure 25

7. Chapter 17 Figure 26

8. Chapter 17 Figure 31

9. Chapter 17 Figure 32

10. Chapter 17 Figure 33

11. Successive Approximation ADCs
For an N-bit R samples per sec SAR ADC
SHA should be N-bit accurate, at R SPS
DAC must be N-bit accurate, at > N*R SPS
Comparator must have N-bit resolution at > N*R rate
Logic must work at > N*R rate
Used in high resolution medium speed applications

12. Chapter 17 Figure 05

13. Example timing diagram
At “convert start”, SHA grabs a sample and hold its value
Set DAC MSB to 1 as test bit, rest bits set to 0,
DAC output compared to sample held
If comparator output = 1, keep test bit as 1, else set test bit = 0
If test bit is LSB, reset “busy” and signal end of conversion
Else, move test bit to next lower bit, and set it to 1, generate DAC output
At end of conversion, DAC input code sent out as ADC output code

14. Chapter 17 Figure 04

15. Charge redistribution implementation
In the track mode, AIN is constantly charging and discharging all the capacitors, Q = C_tot * Vin
When SIN and Sc open, Q = C_tot * Vin
If S1, S2, S3, and S4 all connected to ground, Q = C_tot * Vin,  V_A = - Vin
If S1 is switch to Vref, Q = C_tot * Vin = C*(Vref – V_A) – (C/2 + C/4 +…)*V_A  V_A = – Vin + Vref/2
If comparator output is 1, V_A < 0, leave S1 at Vref; else, switch S1 to gnd;
Next test S2, S3, … in similar way