Analog to Digital conversion

1. Analog to Digital conversion

2. Introduction • The process of converting an analog signal into an equivalent digital signal is known as Analog to Digital (AD) conversion. • The conversion time depends upon the frequency of input clock signal.

3. Analog to digital conversion 03 07 10 14 09 02 00 04

4. Conversion Methods • Ladder Comparison • Successive Approximation • Slope Integration • Flash Comparison

5. Ladder Comparison

6. Successive Approximation

7. Slope integration • Charge a capacitor at constant current • Count clock ticks • Stop when the capacitor voltage matches the input • Cannot achieve high resolution • Capacitor and/or comparator Vin Counting time StartConversion StartConversion Enable S Q R N-bit Output C Counter IN Clk Oscillator

8. Flash comparison • If N is the number of bits in the output word…. • Then 2N comparators will be required. • With modern microelectronics this is quite possible, but will be expensive.

9. ADC Essentials Basic I/O Relationship • ADC is Rationing System • x = Analog input / Reference • Fraction: 0 ~ 1

10. ADC Essentials Quantization Error • 1/2 LSB • Reduced by increasing n n bits ADC • Number of discrete output level : 2n • Quantum • LSB size • Q = LSB = FS / 2n

11. Converter Errors Offset Error Gain Error

12. Converter Errors Integral Linearity Error Differential Linearity Error

13. A/D conversion Techniques • Counter or Tracking ADC • Successive Approximation ADC • Most Commonly Used • Dual Slop Integrating ADC • Voltage to Frequency ADC • Parallel or Flash ADC • Fast Conversion • Software Implementation • Shaft Encoder

14. Counter type ADC Block Diagram Wave form

15. Counter type ADC Operation • Reset and Start Counter • DAC convert Digital output of Counter to Analog signal • Compare Analog input and Output of DAC • Vi < VDAC • Continue counting • Vi = VDAC • Stop counting • Digital Output = Output of Counter

16. Tracking type ADC Tracking or servo type • Using Up/Down Counter to track input signal continuously • For slow varying input

17. Successive Approximation ADC Block Diagram • Most Commonly used in medium to high speed Converters • Based on approximating the input signal with binary code and then successively revising this approximation until best approximation is achieved • SAR(Successive Approximation Register) holds the current binary value

18. Successive Approximation ADC Circuit wave form Logic Flow

19. Dual slope integrating ADC Operation • Integrate • Reset and integrate • Thus • 

20. Dual slope integrating ADC Low speed • If T1 = 60Hz, converter throughput rate < 30 samples/s Excellent Noise Rejection • High frequency noise cancelled out by integration • Proper T1 eliminates line noise • Easy to obtain good resolution

21. Voltage to Frequency ADC VFC (Voltage to Frequency Converter) • Convert analog input voltage to train of pulses Counter • Generates Digital output by counting pulses over a fixed interval of time

22. Voltage to Frequency ADC • Low speed • Good noise immunity • High resolution • For slow varying signal • With long conversion time • Applicable to remote data sensing in noisy environments • Digital transmission over a long distance

23. Parallel or Flash ADC Single step Conversion • 2n –1 comparator • Precision Resistive Network • Encoder Very high speed conversion • Up to 100MHz for 8 bit resolution • Video, Radar, Digital Oscilloscope Resolution is limited • Large number of comparator in IC

24. Software Implementation Implementing software using Microprocessor • Counting • Shifting • Inverting • Code conversion • ….. Limited practical use • Availability if Good performance with very responsible cost

25. Shaft Encoder Electromechanical ADC • Convert shaft angle to digital output Encoding • Optical or magnetic sensor Applications • Machine tools, industrial robotics, Numerical control

26. Shaft Encoder Binary Encoder • Misalignment of mechanism causes large error. Ex: 011 -> 111 (180 deg) Gray Encoder • Misalignment causes of 1 LSB error.

27. The End ……. Thank You ……