Analog to Digital Conversion

1. Analog to Digital Conversion ADC Essentials A/D Conversion Techniques Interfacing the ADC to the IBM PC DAS (Data Acquisition Systems) How to select and use an ADC A low cost DAS for the IBM PC

2. Why ADC ? • Digital Signal Processing is more popular • Easy to implement, modify, … • Low cost • Data from real world are typically Analog • Needs conversion system • from raw measurements to digital data • Consists of • Amplifier, Filters • Sample and Hold Circuit, Multiplexer • ADC Chap 0

3. Basic I/O Relationship ADC is Rationing System x = Analog input / Reference Fraction: 0 ~ 1 n bits ADC Number of discrete output level : 2n Quantum LSB size Q = LSB = FS / 2n Quantization Error 1/2 LSB Reduced by increasing n ADC Essentials Chap 0

4. Offset Error Gain Error Can be eliminated by initial adjustments Integral Linearity Error Differential Linearity Error Nonlinear Error Hard to remove Converter Errors Chap 0

5. Converter Resolution The smallest change required in the analog input of an ADC to change its output code by one level Converter Accuracy The difference between the actual input voltage and the full-scale weighted equivalent of the binary output code Maximum sum of all converter errors including quantization error Conversion Time Required time (tc) before the converter can provide valid output data Converter Throughput Rate The number of times the input signal can be sampled maintaining full accuracy Inverse of the total time required for one successful conversion Inverse of Conversion time if No S/H(Sample and Hold) circuit is used Terminologies Chap 0

6. Input voltage change during the conversion process introduces an undesirable uncertainty Full conversion accuracy is realized only if this uncertainty is kept low below the converter’s resolution Rate of Change x tc  resolution Example 8-bit ADC Conversion Time: 100sec Sinusoidal input Rate of change Let FS = 2A Limited to Low frequency of 12.4 Hz Few Applications More on Conversion Time Chap 0

7. S/H (Sample and Hold) Analog circuits that quickly samples the input signal on command and then holds it relatively constant while the ADC performs conversion Aperture time (ta) Time delay occurs in S/H circuits between the time the hold command is received and the instant the actual transition to the hold mode takes place Typically, few nsec Example 20 nsec aperture time Reasonably good for 100sec converter S/H increase Performance Chap 0

8. Typically, Differential or Single-ended input signal of a single polarity Typical Input Range 0 ~ 10V and 0 ~ 5V If Actual input signal does not span Full Input range Some of the converter output code never used Waste of converter dynamic range Greater relative effects of the converter errors on output Matching input signal and input range Prescaling input signal using OP Amp In a final stage of preconditioning circuit By proportionally scaling down the reference signal If reference signal is adjustable Analog Input Signal Chap 0

9. Using unipolar converter when input signal is bipolar Scaling down the input Adding an offset Bipolar Converter If polarity information in output is desired Bipolar input range Typically, 0 ~ 5V Bipolar Output 2’s Complement Offset Binary Sign Magnitude … Input signal is scaled and an offset is added Converting bipolar to unipolar Add offset scaled Chap 0

10. I/O of typical ADC ADC output Number of bits 8 and 12 bits are typical 10, 14, 16 bits also available Typically natural binary BCD (3½ BCD) For digital panel meter, and digital multimeter Errors in reference signal From Initial Adjustment Drift with time and temperature Cause Gain error in Transfer characteristics To realize full accuracy of ADC Precise and stable reference is crucial Typically, precision IC voltage reference is used 5ppm/C ~ 100ppm/C Outputs and Analog Reference Signal Chap 0

11. Start From CPU Initiate the conversion process BUSY / EOC To CPU Conversion is in progress 0=Busy: In progress 1=EOC: End of Conversion HBE / LBE From CPU To read Output word after EOC HBE High Byte Enable LBE Low Byte Enable Control Signals Chap 0

12. 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 Chap 0

13. Block diagram Waveform 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 Disadvantage Conversion time is varied 2n Clock Period for Full Scale input Counter Type ADC Chap 0

14. Tracking or Servo Type Using Up/Down Counter to track input signal continuously For slow varying input Can be used as S/H circuit By stopping desired instant Digital Output Long Hold Time Disabling UP (Down) control, Converter generate Minimum (Maximum) value reached by input signal over a given period Tracking Type ADC Chap 0

15. 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 Block Diagram Successive Approximation ADC Chap 0

16. Circuit waveform Logic Flow Conversion Time n clock for n-bit ADC Fixed conversion time Serial Output is easily generated Bit decision are made in serial order Successive Approximation ADC Chap 0

17. Operation Integrate Reset and integrate Thus  Applications DPM(Digital Panel Meter), DMM(Digital Multimeter), … Excellent Noise Rejection High frequency noise cancelled out by integration Proper T1 eliminates line noise Easy to obtain good resolution Low Speed If T1 = 60Hz, converter throughput rate < 30 samples/s Dual Slope Integrating ADC Chap 0

18. 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 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 Voltage to Frequency ADC Chap 0

19. Very High speed conversion Up to 100MHz for 8 bit resolution Video, Radar, Digital Oscilloscope Single Step Conversion 2n –1 comparator Precision Resistive Network Encoder Resolution is limited Large number of comparator in IC . Parallel or Flash ADC Chap 0