Analog to digital conversion


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Analog to digital conversion

  1. 1. Presented By: Roll No:
  2. 2. Presentation Topic: Analog to Digital Conversion (ADC)
  3. 3. Analog-to-Digital Conversion Terminology analog: continuously valued signal, such as temperature or speed, with infinite possible values in between digital: discretely valued signal, such as integers, encoded in binary analog-to-digital converter: ADC, A/D, A2D; converts an analog signal to a digital signal
  4. 4. Analog Signals Analog signals – directly measurable quantities in terms of some other quantity Examples: • Thermometer – mercury height rises as temperature rises • Car Speedometer – Needle moves farther right as you accelerate
  5. 5. Digital Signals Digital Signals – have only two states. For digital computers, we refer to binary states, 0 and 1. “1” can be on, “0” can be off. Examples: • Light switch can be either on or off • Door to a room is either open or closed
  6. 6. ADC Basic Principle: • The basic principle of operation is to use the comparator principle to determine whether or not to turn on a particular bit of the binary number output. • It is typical for an ADC to use a digital-toanalog converter (DAC) to determine one of the inputs to the comparator.
  7. 7. Quantization • Quantization is the process of converting the sampled continuousValued signals into discrete-valued data ©Alex Doboli 2006
  8. 8. Quantizing The number of possible states that the converter can output is: N=2n where n is the number of bits in the AD converter Example: For a 3 bit A/D converter, N=23=8. Analog quantization size: Q=(V max -V min)/N = (10V – 0V)/8 = 1.25V
  9. 9. Analog  Digital Conversion 2-Step Process: • Quantizing - breaking down analog value is a set of finite states • Encoding - assigning a digital word or number to each state and matching it to the input signal
  10. 10. Step 1: Quantizing Example: You have 0-10V signals. Separate them into a set of discrete states with 1.25V increments. (How did we get 1.25V? (Discussed in previous slide) Output States Discrete Voltage Ranges (V) 0 0.00-1.25 1 1.25-2.50 2 2.50-3.75 3 3.75-5.00 4 5.00-6.25 5 6.25-7.50 6 7.50-8.75 7 8.75-10.0
  11. 11. Step 2. Encoding • Here we assign the digital value (binary number) to each state for the computer to read. Output States Output Binary Equivalent 0 000 1 001 2 010 3 011 4 100 5 101 6 110 7 111
  12. 12. Sampling • It is a process of taking a sufficient number of discrete values at point on a waveform that will define the shape of waveform. • The more samples you take, the more accurately you will define the waveform. • It converts analog signal into series of impulses, each representing amplitude of the signal at given point…….
  13. 13. Sampling Collect sufficient data for correctly representing a continuous-time signal ©Alex Doboli 2006
  14. 14. 3 Basic Types • Flash ADC • Digital-Ramp/Dual slope/Counter slope ADC • Successive Approximation ADC
  15. 15. 1-> Flash ADC • Consists of a series of comparators, each one comparing the input signal to a unique reference voltage. • The comparator outputs connect to the inputs of a priority encoder circuit, which produces a binary output
  16. 16. 3 bit Flash ADC Circuit
  17. 17. How Flash Works • As the analog input voltage exceeds the reference voltage at each comparator, the comparator outputs will sequentially saturate to a high state. • The priority encoder generates a binary number based on the highest-order active input, ignoring all other active inputs.
  18. 18. ADC Output
  19. 19. Flash Advantages • Simplest in terms of operational theory • Most efficient in terms of speed, very fast limited only in terms of comparator and gate propagation delays Disadvantages • Lower resolution • Expensive • For each additional output bit, the number of comparators is doubled i.e. for 8 bits, 256 comparators needed
  20. 20. 2-> Dual Slope ADC • Also known as Counter-Ramp or Digital Ramp ADC • A dual slope ADC is commonly used in measurement instruments (such as DVM’s). ADC 1.21
  21. 21. Dual Slope ADC circuit Input Oscillator Switch Control Logic Counter VReference Registers Digital Output ADC 1.22
  22. 22. Dual Slope Function • The Dual Slope ADC functions in this manner: – When an analog value is applied the capacitor begins to charge in a linear manner and the oscillator passes to the counter. – The counter continues to count until it reaches a predetermined value. Once this value is reached the count stops and the counter is reset. The control logic switches the input to the first comparator to a reference voltage, providing a discharge path for the capacitor. – As the capacitor discharges the counter counts. – When the capacitor voltage reaches the reference voltage the count stops and the value is stored in the register. ADC 1.23
  23. 23. Successive approximation ADC • Much faster than the digital ramp ADC because it uses digital logic to converge on the value closest to the input voltage. • A comparator and a DAC are used in the process.
  24. 24. Successive Approximation ADC • A Successive Approximation Register (SAR) is added to the circuit • Instead of counting up in binary sequence, this register counts by trying all values of bits starting with the MSB and finishing at the LSB. • The register monitors the comparators output to see if the binary count is greater or less than the analog signal input and adjusts the bits accordingly
  25. 25. Successive Approximation ADC Circuit
  26. 26. Output
  27. 27. ADC Types Comparison ADC Resolution Comparison Dual Slope Flash Successive Approx 0 5 10 15 Resolution (Bits) 20 Type Speed (relative) Cost (relative) Dual Slope Slow Med Flash Very Fast High Successive Approx Medium – Fast Low 25
  28. 28. Examples of A/D Applications • Microphones - take your voice varying pressure waves in the air and convert them into varying electrical signals • Strain Gages - determines the amount of strain (change in dimensions) when a stress is applied • Thermocouple – temperature measuring device converts thermal energy to electric energy • Voltmeters • Digital Multimeters