Analog to Digital Converters (ADC) Lecture 7 Rferences U. Tietze, Ch.Schenk, Electronics Circuits Handbook for Design and Applications, Springer,2010 Advertisement materials and Application notes of: Linear Technology, Analog Device, Texas Instruments, National Semiconductors NEV Corporation Types of AD converters Direct (voltage comparison) parallel flash converter, serial or pipeline (2-step, 3-step,.), successive approximation. Indirect (voltage time - counter) tracking converter, compensation converters(singe-,dual-, four-slop converter), oversampling converter (Δ-Σ converters), U/F converters. 1
AC direct, parallel, flash Features: sampling frequency (>100MHz to 10GHz), low resolution (4,6,8,10 bits), high number of comparators 2 n -1 high power dissipation, technology: bipolar or ECL, Comparator and storage flip-flop - a simplified example comparator latch flip-flop Example of a flash converter AD9054A 2
Example of a flash converter AD9054A cont. Features (compared to flash ): lower number comparators higher conversion time more popular (cheaper) higher resolution 2-step converter (direct, serial, pipeline) 3-step converter (direct, serial, pipeline) 3
Example of a pipeline converter ADS5221 12-Bit, 65MSPS Sampling, +3.3V FEATURES HIGH SNR: 70dB HIGH SFDR: 88dBFS LOW POWER: 285mW INTERNAL/EXTERNAL REFERENCE OPTION SINGLE-ENDED OR FULLY DIFFERENTIAL ANALOG INPUT FLEXIBLE DUTY CYCLE ADJUST CIRCUITRY LOW DNL: 0.5LSB SINGLE +3.3V SUPPLY OPERATION TQFP-48 APPLICATIONS WIRELESS LOCAL LOOP COMMUNICATIONS MEDICAL IMAGING PORTABLE INSTRUMENTATION n-step converter (direct, serial, pipeline) Features: Number of comparators = number of bits higher (compared to flash ) conversion time less popular Successive approximation AD converters (direct) 4
Successive approximation AD converters - principle of operation Example of successive approximation converter -AD7854 Compare with flash Example of successive approximation AD converter - AD7854 (cont.) 5
Features: one comparator high conversion time less popular Tracking converter (counting method conversion; delta modulator ) Compensation converter-single slop converter (indirect, counting method) τ T n n N = 2 = 2 u U in REF y V i τ T Features: result depend on: comparator offset linearity of saw-tooth gen. clock (crystal) Compensation converter (indirect) - dual slop converter 6
Compensation converter (indirect) - dual slop converter (cont.) Advantages: result does not depend on: linearity of the integrator, time constant, clock frequency, comparator offset, some noise is suppressed. U t = 2 in U REF t1 Auto-zero conversion (auto zero amplifier) Integrator Amplifier Comparator U in To control circuit 1. For K4 and K3 on, the capacitor C is charged to a voltage U k equal to the integrator offset. 2. When K4 and K3 off the voltage U k cancels the integrator offset. Dual slope with auto-zero example ICL7106/7107 7
Dual slope with auto-zero example ICL7106/7107(cont.) Dual slope with auto-zero example ICL7106/7107(cont.) BCD output cod Dual slope with auto-zero example ICL7106/7107(cont.) AZ cap Auto pol. 8
Indirect converter---four - slope Phase 0 1 2 3 4 U R GND U R U i U R Four slope converter- principle of correction numerical result result (U REF) U REF out = result offset result U REF offset 123 = ( ) (1) 1442 443 (2) 144444424444444 3 (3) (2) (1) (3) result offset = U result( U ) offset 4443 REF 14442 0...1 REF measured voltage offset (U REF) Indirect, integrating converters -dual and four slope Result is independent on: time constant of the integrator integrator nonlinearity, comparator offset (integrate and de-intergrate phase) clock frequency (result = ratio of pulse number) high frequency noise (averaging effect) gain error temperature drifts four slope dual slope 9
Types of AD converters Direct (voltage comparison) parallel flash converter, serial or pipeline (2-step, 3-step,.), successive approximation. Indirect (voltage time - counter) tracking converter, compensation converters(singe-,dual-, four-slop converter), oversampling converter (Δ-Σ converters), U/F converters. Δ-Σ converter oversampling converter V AD for example 44,29 MHz for example 44,1 khz 16 bit V DA!!!!!!!! Sampling frequency f OSR is much greater than reading frequency f S Δ-Σ converter - principle of operation f OSR V i V AD V DA 10
Oversampling Δ-Σ converter -general principle Samplig frequency f OSR f S <<f OSR Resolution N N+x Noise shaping in Δ-Σ converter Δ-Σ converter linear model 11
Common converter N U bits Nout 1 fos f f 20log OS f = 6dB / bit S S f OS /f S bit 2 0.5 4 1 256 4 First order Δ-Σ converter N bits U Nout 1 fos f S f 20log OS f S = 6dB / bit 3 3 f OS /f S bit 2 1.5 4 3 256 12 3dr order Δ-Σ converter U Nout 5 1 fos f S N bits f 20log OS f S = 6dB / bit f OS /f S bit 2 2.5 4 5 256 20 5 12
number od bits (resolution) Oversampling 2nd order -Σ converter 1st order -Σ converter traditional converter oversampling ratio Noise shaping in Δ-Σ converter Example of Δ-Σ converter ADS1110 13
Lab setup of pressure sensor Example of Δ-Σ converter AD7705/7706 Example of Δ-Σ converter AD7705/7706 (cont.) 14
resolution (bits) General comparison of AD converters 24 20 16 12 8 4 dual slop -Σ counting successive approximation pipeline flash 0 1 1k 1M 1G sampling frequency V/f Voltage-frequency transducers -features Long distance transmission Possible supply by means of transmission cable Accuracy comparable to 14 bits AD converter Easy galvanic isolation Low sampling frequency!!!!!!!! isolator f or T measurement Voltage-frequency transducers AD654 15
Voltage-frequency transducers AD654 (cont.) Voltage-frequency transducers AD537 Przetworniki U/f AD537 16
Summary of AD converters Direct (voltage comparison) parallel flash converter, serial or pipeline (2-step,...), successive approximation. Indirect (voltage time - counter) tracking converter, compensation converters (singe-,dual-, four-slop converter), oversampling converter (Δ-Σ converters), U/F converters. AD converters comparison Summary of AD converters(cont). Parameters of ADC: resolution, offset, scale error, integrated nonlinearity, differential nonlinearity, sampling rate, From previous lecture: DA converters Flow rate sampling frequency vs. resolution f SNR[ db] B[ Hz] 6n FR[ bits / s] 1442 3443 3 flow rateof a channel S 2 = n f S high resolution low sampling rate high sampling rate low resolution!!!!!!!!!!!!!!!!!! 17
resolution (bits) General comparison of AD converters 24 20 16 12 8 4 counting dual slop -Σ successive approximation pipeline flash 0 1 1k 1M 1G sampling rate References 1. W. Tietze, Ch. Schenk, Electronic circuits. Handbook for Design and Applications, Springer 2002. 2. P. Horowitz, H. Hill, The Art of Electronics, Cambridge Univ. Press 1989. 3. Application notes: Analog Device Inc., Texas Instrument Inc. Test questions: Name types of AD converters, describe the typical speed and resolution. What is the principle of oversampling? 18