AD/DA Conversion Techniques: An Overview

1. AD/DA Conversion Techniques-An OverviewJ. G. Pett • Introductory tutorial lecture for :-‘Analogue and digital techniques inclosed-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. 16 16 12 What are AD/DA Converters (2) Analog continuous time world Digital discrete time world Analog continuous time world +/-10v +/-10v ADC 1 DAC 1 COMPUTER or µP/µC The Real World The Real World +/-5v ADC 2 Typical AD & DA Application

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) AD/DA Transfer Characteristic • 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 +10v 0v -10v FFFF 8000 0000 7FFF 0000 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 ? Digital value time

12. Waveform digitising (2) C • 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 D B

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 binaryAll 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,

20. Some very simple ideas ‘Digitally set’ potentiometer Comparator • 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] DAC ADC Vref Unknown voltage dial = Vdac equal

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

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) Simple ramp and comparator ADC • 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 Unknown analog input start Binary output

24. Basic ADC circuits (2) • This ADC circuit is limited and rarely usedWHY - • 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 Vref Flash Vref Half-Flash • The fastest process <50nsecs • Limited resolution typically 8 - 10bits • Half-flash technique is cheaper analog input analog input

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