The document provides an overview of AD/DA conversion techniques. It discusses what AD and DA converters are, how they work, and some of their common applications. It then covers various conversion methods like successive approximation, flash, integrating, sigma-delta, as well as digital coding methods and sources of error in waveform digitization. Examples of AD/DA applications at CERN are also presented involving applications from low to high speed. Key circuits like sample and hold, R-2R ladders for DACs, and basic ADC designs are reviewed.

1. AD/DA Conversion Techniques
-
An Overview
J. G. Pett
 Introductory tutorial lecture for :-
‘Analogue and digital techniques in
closed-loop regulation applications’
17/09/2002
 for terminology see Analog Devices Inc.

2. AD/DA
 Introduction to the subject
 Understanding conversion methods
 Methods
 Parameters
 The past, the present and the future

3. Introduction
 What are AD/DA Converters
 What are they used for
 Why do you need to know how they work
 Digital coding methods
 Waveform digitising
 CERN examples

4. What are AD/DA
Converters (1)
 An Analog to Digital converter [AD or ADC]
is an electronic circuit which accepts an
analog input signal (usually a voltage) and
produces a corresponding digital number at
the output
 An Digital to Analog converter [DA or DAC]
is an electronic circuit which accepts a
digital number at its input and produces a
corresponding analog signal (usually a
voltage) at the output
 They exist as modules, ICs, or fully
integrated inside other parts, e.g. µCs

5. Photos

6. What are AD/DA Converters (2)
ADC 1 DAC 1
ADC 2
COMPUTER
or µP/µC
12
16
16
Digital
discrete time world
Analog
continuous time world
Analog
continuous time world
The
Real
World
The
Real
World
Typical AD & DA Application
+/-10v
+/-5v
+/-10v

7. What are they used for
 Any time a real world analog signal is
connected to a digital system
 CD players, GSMs, DVMs, Digital Camcorders
etc, etc
 CERN control systems & instruments
 HOWEVER, each application has particular
needs
 Resolution - number of bits
 Speed and Accuracy
 Level of input/output waveforms
 Cost etc

8. Why do you need to know
how they work
 Because the theoretical course you will
shortly undertake assumes perfect
converter products - BUT
 Practical converters have :
 Many conversion methods - why
 Trade-offs between resolution and speeds +
delays
 Different methods of “sampling” the
waveforms
 A large number of basic and method-dependent
error sources
 Manufacturers specifications which ‘differ’ -
AND
 Almost all converters need some analog ‘signal
conditioning’ which is application dependent

9. Digital coding methods (1)
 8,10,12,14,16,18, 20-24bits?
 Most/Least significant bit
MSB/LSB
 Uni-polar, bipolar, straight
binary, 2’s complement -
invert MSB
 Parallel I/O or serial [delay]
 Bytes or words
 Double buffering
 Digital ‘breakthrough’
 Digital correction methods
 Time skewing & jitter
0v
+10v
-10v
0000 FFFF
8000
AD/DA Transfer Characteristic
0000 7FFF
FFFF
8000

10. Digital coding methods (2)
 Resolution = 2n-1 [n = number of bits]
n 2n 1bit ppm [1x10-6]
 8bits 256 3906
 10bits 1024 976
 12bits 4096 244
 14bits 16384 61
 16bits 65536 15
 18bits 262144 3.8
 20bits 1,048576 0.95
 22bits 4,194304 0.24
 24bits 16,777216 0.06

11. Waveform digitising (1)
 A waveform is ‘digitised’ (sampled) at a constant
rate D t
 Each such sample represents the instantaneous
amplitude at the instant of sampling
 Between samples the value remains constant [zero
order hold]
 What errors can occur in this process ?
time
Digital
value

12. Waveform digitising (2)
 A & B show aliasing in the time domain
 C & D show a different case in the frequency
domain
- it is important to understand these effects
A
B
C
D

13. Waveform digitising errors
 For a DAC
 output waveform is a ‘distorted’ version of original
 higher frequencies not reproduced - aliasing ?
 ‘average shape’ displaced in time
 ‘sharp’ edges need filtering
 For an ADC
 converter sampling errors
 with a ‘sample & hold’ circuit ahead of the converter?
 integrating action during part, or all of the sample-time
?
 conversion time
 data ‘available’ delay
 aliasing - [ is multiplication of input spectrum and
fs]
…[must ‘remove’ all spectrum > fs/2 before
sampling]

14. Sampling rate
 Nyquist rate = 2x highest frequency of
interest
 Practically, - always sample at least 5x, or
higher
 Ensure ADCs have input filtering [anti-alias]
where necessary [large hf signals]
 Filter DAC outputs to remove higher
frequencies and switching ‘glitches’
 ‘Over-sampling’ converters sample x4 to
x500 - this may reduce above problems
and/or extend resolution

15. CERN examples
 Many PLCs with analog values, such as
temperature, to measure : 10 - 12bit <10kHz
 PS, SPS, LHC control instrumentation, such
as power converter control, regulation and
monitoring : 16 - 22bit <1kHz
 Beam instrumentation, experiments : high
speed: 10 - 12bit 25ns
 ETC ETC

16. Photos
1969
ISR Beam-Transfer DAC
[5 decimal decades]
Relay switching
Kelvin-Varley divider
1973
ISR Main Bends DAC
[16bit binary
All electronic switching

17. Photos
ADC Sigma-Delta 1998
1989
LEP 16bit Hybrid DAC

18. Understanding Conversion
Methods

19. AD/DA Methods
 Some very simple ideas
 DAC circuits
 Basic ADC circuits
 Successive approximation, flash - S&H
 Integrating - single/dual/multi slope
 Charge balance, D

20. Some very simple ideas
 ADC =
 precise reference voltage
 comparison of divider value with unknown [analog input]
 “digitally adjustable” divider or potentiometer [output
value]
 DAC =
 precise reference voltage ……. {multiplying dac}
 “digitally adjustable” divider or potentiometer [input
value]
 optional output amplifier of pot. value [analog output]
=
‘Digitally set’
potentiometer
dial
Comparator
equal
Vref
Unknown
voltage
DAC ADC
Vdac

21. DAC circuits (1)
 Summation of binary weighted currents
 Modern DACs use the ‘R-2R ladder’
Simplified binary weighted resistor DAC
8.75V
9.375
max.
R - 2R ladder DAC

22. DAC circuits (2)
 Important circuit concepts
 Resistor tracking - temp. & time > ratios
 Switch is part of R [on & off resistance]
 Limits for tracking and adjustment
 Switch transition times - glitches
 Switched current sources are faster
 Other DAC methods
 DC performance not needed for all uses
 Different ladders, Caps. as well as Resistors
 PWM, F>V
 Sigma-Delta
 Performance cannot be better than the Reference
- {multiplying DAC concept}

23. Basic ADC circuits (1)
 Digitising begins with a ‘start’ pulse
 DAC is ramped up from zero
 counter stopped by comparator when Vin = DAC out
 ADC output is counter value
 Tracking ADC
Simple ramp and comparator ADC
start Binary output
Unknown
analog
input

24. Basic ADC circuits (2)
 This ADC circuit is limited and rarely used
WHY -
 slow
 variable time to give result
 input signal can vary during digitising
 Successive Approximation ADC solves these
problems - using
 complex logic to test and retain each DAC bit
 a sample and hold circuit ahead of the
comparator

25. Successive Approximation
ADC
 Fast process - 1 -
100µsecs
 Result always n clocks
after start
 Used extensively for
12-16bit DAQ systems

26. Flash ADC
 The fastest process <50nsecs
 Limited resolution typically 8 -
10bits
 Half-flash technique is cheaper
Flash
Half-Flash
analog
input
analog
input
Vref
Vref

27. Sample & Hold Circuit (1)
 Essential for defining the ‘exact’ moment of
sampling
 Circuit introduces other error sources [ see (2) ]
LF398

28. Sample & Hold Circuit (2)
Storage Capacitor Waveform