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

3
ADC Essentials

• 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

• Basic I/O Relationship
• ADC is Rationing System
• x Analog input /
• Reference
• Fraction 0 1

4
Converter Errors

• Integral Linearity Error
• Differential Linearity Error
• Nonlinear Error
• Hard to remove

• Offset Error
• Gain Error
• Can be eliminated by initial adjustments

5
Terminologies

• 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

6
More on Conversion Time

• 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

• 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 converters
  resolution
• Rate of Change x tc ? resolution

7
S/H increase Performance

• 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

8
Analog Input Signal

• 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

9
Converting bipolar to unipolar

• Input signal is scaled and an offset is added

• 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
• 2s Complement
• Offset Binary
• Sign Magnitude

Add offset
scaled
10
Outputs and Analog Reference Signal

• 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

11
Control Signals

• HBE / LBE
• From CPU
• To read Output word after EOC
• HBE
• High Byte Enable
• LBE
• Low Byte Enable

• Start
• From CPU
• Initiate the conversion process
• BUSY / EOC
• To CPU
• Conversion is in progress
• 0Busy In progress
• 1EOC End of Conversion

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

13
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 lt VDAC
• Continue counting
• Vi VDAC
• Stop counting
• Digital Output Output of Counter
• Disadvantage
• Conversion time is varied
• 2n Clock Period for Full Scale input

• Block diagram
• Waveform

14
Tracking Type ADC

• 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

15
Successive Approximation ADC

• 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

16
Successive Approximation ADC

• 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

17
Dual Slope Integrating ADC

• 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 lt 30
  samples/s

• Operation
• Integrate
• Reset and integrate
• Thus
• ?
• Applications
• DPM(Digital Panel Meter), DMM(Digital
  Multimeter),

18
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

• 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

19
Parallel or Flash ADC

• 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

• Homework 5-1
• ??? ??? ??? ???? ????.

20
Software Implementation

• Implementation with software using microprocessor
• Counting
• Shifting
• Inverting
• Code Conversion

• Limited Practical Use
• Availability of Good performance with very
  reasonable Cost

21
Shaft Encoder

• Binary Encoder
• Misalignment of mechanism causes large error
• Ex 011 ? 111 (180deg)
• Gray Encoder
• Misalignment causes 1 LSB error

• Elctromechanical ADC
• Convert shaft angle to digital output
• Encoding
• Optical or Magnetic Sensor
• Applications
• Machine tools, Industrial robotics, Numerical
  control

22
Interfacing the ADC to the IBM PC

• Interface Operations
• Most-recent-data Scheme
• At end of conversion it updates an output FIFO
• Automatically start new conversion
• CPU read FIFO to acquire most recent data
• Start-and-wait Scheme
• CPU initiate conversion every time it needs new
  data
• CPU check EOC until conversion is finished

• Using CPU Interrupt
• CPU initiate conversion every time it needs new
  data
• CPU can proceed to do other thing
• ADC interrupt CPU when conversion is complete
• CPU goes to ISR
• See Chapter 3, For more information about 8259A

23
Interface Software

• DMA (Direct Memory Access)
• CPU release system bus by the request of DMA
• DMA controller carried out data transfer by
  generating the required addresses and control
  signals
• The system bus control reverts back to CPU when
  data transfer is finished
• DMA is useful
• High Speed
• High volume data transfer
• Disk Drive interface

• Memory Mapped Transfers
• ADC is assigned in Memory Space
• MRD, MWR signal
• MOV instruction
• More complex decoding logic
• I/O Mapped Transfers
• ADC is in I/O Space
• IOR, IOW signal
• IN, OUT instruction
• More Simple decoding logic

24
Interface Hardware

• Parallel Data Format
• Three state output buffer in ADC
• To Interface ADC
• CPU Decoding logic
• To generate Chip Select signal
• To generate Start Signal
• To Check EOC signal

• Serial Data Format
• Asynchronous Serial transmission to send data
  over long distance to a monitoring station
• UART is commonly used
• Interfacing 10 or 12 bit ADC
• Transfer data in chunks of 8 bits one after
  another

25
DAS (Data Acquisition System)

• 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

• Applications
• Simple monitoring of a single analog variable
• Control and Monitoring of hundreds of parameters
  in a nuclear plant

26
Single Channel System

• S/H (Sample and Hold)
• Reduce uncertainty error in the converted output
  when input changes are fast compared to the
  conversion time
• In Multi-channel system
• To hold a sample from one channel while
  multiplexer proceed to sample next one
• Simultaneous sampling of two signal

• Transducer
• Generate signal of low amplitude, mixed with
  undesirable noise
• Amplifier, Filters
• Amplify
• Remove noise
• Linearize

27
Sample and Hold Circuits

• Care in selecting hold capacitor Ch
• Low Value
• Reduces acquisition time
• Increase Droop
• High Value
• Minimize Droop
• Increase acquisition time
• Choose capacitor to get a best acquisition time
  while keeping the droop per conversion below 1 LSB

28
Commercially Available S/H
29
Multi-channel System

• Analog multiplexer and a ADC
• Low cost

• Local ADCs and digital multiplexer
• Higher sampling rate

30
How to select and use an ADC

• Range of commercially available ADCs

• Guidelines for using ADCs
• Use the full input range of the ADC
• Use a good source of reference signal
• Look out for fast input signal changes
• Keep analog and digital grounds separate
• Minimize interference and loading problem

31
Commercially available monolithic ADCs
32
Commercially available hybrid ADCs
33
A low cost DAS for the IBM PC

• Generating clock
• For starting ADC conversion
• For causing interrupt
• Make a pulse stream from TCLK with short pulses
  of duration ½ x BCLK/4
• TCLK from 8253 Timer/Counter
• Wide pulse

• Multi-channel system
• Less than 100
• ADC0816 from National Semiconductor
• Constant, repetitive rate
• 1000 samples/s

34
ADC circuit for PC prototype board
SCSLCT (Start Conversion SeLeCT) Latched trough
port 30CH SCSLCT H Selection of 30AH
(/E10) start conversion SCSLCT L
TCLK start conversion
INTSLCT (INTerrupt SeLeCT) Latched trough port
30CH INTSLCT H EOC cause IRQ2 INTSLCT L
No Interrupt CPU read Status register
(Port 309H) to check EOC
35
Status Register

• For polling TCLK and EOC signal
• Port 309H (/E9)
• Polling of EOC results in a low level after the
  data from ADC have been read

36
Throughput rate calculation
4.77MHz / 8 596KHz
37
Accuracy Calculation

• Better than 1 accuracy is ensured
• Actual accuracy with smooth input signal at room
  temperature will be better than 0.5

38
Basic Program for Controlling ADC
Sampling rate lt 200 samples/s Because OUT and IN
instruction in Basic takes 5ms
39
C Programming for Controlling ADC

• Sampling from ADC channel 1 at 5ms interval and
  sending each sampled data point to the DAC

40
Homework 5-2

• Prototype board? ?? ?? ???? ?? C program? ????
  ??? ????
• ?? ?? Outp(CNTRL,5)? ???? ??? ?? ?? ??? ?????
  ?.