PREPARED BY V.SANDHIYA LECT/ ECE UNIT- 3 APPLICATIONS OF OP-AMP 1

2 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

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

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

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

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

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

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

9 A/D Conversion Techniques n Counter or Tracking ADC n Successive Approximation ADC u Most Commonly Used n Dual Slop Integrating ADC n Voltage to Frequency ADC n Parallel or Flash ADC u Fast Conversion n Software Implementation n Shaft Encoder

10 Counter Type ADC n Block diagram n Waveform n Operation u Reset and Start Counter u DAC convert Digital output of Counter to Analog signal u Compare Analog input and Output of DAC F Vi < V DAC Continue counting F Vi = V DAC Stop counting u Digital Output = Output of Counter n Disadvantage u Conversion time is varied F 2 n Clock Period for Full Scale input

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

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

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

14 Dual Slope Integrating ADC n Operation u Integrate u Reset and integrate u Thus u  n Applications u DPM(Digital Panel Meter), DMM(Digital Multimeter), … n Excellent Noise Rejection u High frequency noise cancelled out by integration u Proper T 1 eliminates line noise u Easy to obtain good resolution n Low Speed u If T 1 = 60Hz, converter throughput rate < 30 samples/s

15 Voltage to Frequency ADC n VFC (Voltage to Frequency Converter) u Convert analog input voltage to train of pulses n Counter u Generates Digital output by counting pulses over a fixed interval of time n Low Speed n Good Noise Immunity n High resolution u For slow varying signal u With long conversion time n Applicable to remote data sensing in noisy environments u Digital transmission over a long distance

16 Parallel or Flash ADC n Very High speed conversion u Up to 100MHz for 8 bit resolution u Video, Radar, Digital Oscilloscope n Single Step Conversion u 2 n –1 comparator u Precision Resistive Network u Encoder n Resolution is limited u Large number of comparator in IC n Homework #5-1 u 어떻게 동시에 비교가 되는지를 설명하라.

17 Interface Software n Memory Mapped Transfers u ADC is assigned in Memory Space F MRD, MWR signal F MOV instruction u More complex decoding logic n I/O Mapped Transfers u ADC is in I/O Space F IOR, IOW signal F IN, OUT instruction u More Simple decoding logic n DMA (Direct Memory Access) u CPU release system bus by the request of DMA u DMA controller carried out data transfer by generating the required addresses and control signals u The system bus control reverts back to CPU when data transfer is finished n DMA is useful u High Speed u High volume data transfer F Disk Drive interface

18 DAS (Data Acquisition System) n DAS performs the complete function of converting the raw outputs from one or more sensors into equivalent digital signals usable for further processing, control, or displaying applications n Applications u Simple monitoring of a single analog variable u Control and Monitoring of hundreds of parameters in a nuclear plant

19 Single Channel System n Transducer u Generate signal of low amplitude, mixed with undesirable noise n Amplifier, Filters u Amplify u Remove noise u Linearize n S/H (Sample and Hold) u Reduce uncertainty error in the converted output when input changes are fast compared to the conversion time u In Multi-channel system F To hold a sample from one channel while multiplexer proceed to sample next one F Simultaneous sampling of two signal

20 Sample and Hold Circuits n Care in selecting hold capacitor Ch u Low Value F Reduces acquisition time F Increase Droop u High Value F Minimize Droop F Increase acquisition time u Choose capacitor to get a best acquisition time while keeping the droop per conversion below 1 LSB

21 Multi-channel System n Analog multiplexer and a ADC u Low cost n Local ADCs and digital multiplexer u Higher sampling rate

22 How to select and use an ADC n Range of commercially available ADCs n Guidelines for using ADCs u Use the full input range of the ADC u Use a good source of reference signal u Look out for fast input signal changes u Keep analog and digital grounds separate u Minimize interference and loading problem

23 Commercially available monolithic ADCs

24 Commercially available hybrid ADCs

25 Accuracy Calculation n Better than 1% accuracy is ensured n Actual accuracy with smooth input signal at room temperature will be better than 0.5%