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Transcript
Analog-Digital Converters In presenting Order: Josh Navikonis Moiz H Mike Hochman Brian Post ME 6405 9/29/2009 Agenda Introduction to ADC Types of ADC Characteristics of ADC in MC9S12C Application and Selection of ADC Introduction of ADC What is ADC? Why is ADC important? How does it work? What is ADC? ADC (Analog to Digital Converter) is an electronic device that converts a continuous analog input signal to discrete digital numbers (binary) Analog Real world signals that contain noise Continuous in time Digital Discrete in time and value Binary digits that contain values 0 or 1 Why is ADC Important? All microcontrollers store information using digital logic Compress information to digital form for efficient storage Medium for storing digital data is more robust Digital data transfer is more efficient Digital data is easily reproducible Provides a link between real-world signals and data storage How ADC Works 2 Stages: Sampling Sample-Hold Circuit Aliasing Quantizing and Encoding Resolution Binary output Sampling Reduction of a continuous signal to a discrete signal Achieved through sampling and holding circuit Switch ON – sampling of signal (time to charge capacitor w/ Vin) Switch OFF - voltage stored in capacitor (hold operation) Must hold sampled value constant for digital conversion Simple Sample and Hold Circuit Response of Sample and Hold Circuit Sampling Sampling rate depends on clock frequency Use Nyquist Criterion Increasing sampling rate increases accuracy of conversion Possibility of aliasing Sampling Signal: Tw 1 Sampling Period: Ts f s Nyquist Criterion: fs 2 f max Aliasing High and low frequency samples are indistinguishable Results in improper conversion of the input signal Usually exists when Nyquist Criterion is violated Can exist even when: Prevented through the use of Low-Pass (Anti-aliasing) Filters fs 2 f max Quantizing and Encoding Approximates a continuous range of values and replaces it with a binary number Error is introduced between input voltage and output binary representation Error depends on the resolution of the ADC Resolution Maximum value of quantization error Error is reduced with more available memory Vrange=Input Voltage Range n= # bits of ADC resolution Vrange /( 2n 1) Example: Vrange 7.0V n3 1V 7V /( 23 1) Resolution Qerror resolution / 2 .5V Resolution Increase in resolution improves the accuracy of the conversion Minimum voltage step recognized by ADC Analog Signal Digitized Signal- High Resolution Digitized Signal- Low Resolution Types of A/D Converters Presenter : Moiz H Flash A/D Converter Successive Approximation A/D Converter Example of Successive Approximation Dual Slope A/D Converter Delta – Sigma A/D Converter Elements of a Flash A/D Converter Encoder Comparator FLASH A/D CONVERTER Resolution 23-1 = 7 Comparators 3 Bit Digital Output Flash A/D Converter Contd. Pros • Fastest (in the order of nano seconds) • Simple operational theory • Speed is limited only by gate and comparator propagation delay Cons • Each additional bit of resolution requires twice the number of comparators •Expensive • Prone to produce glitches in the output Elements of Dual-Slope ADC Integrator Dual-Slope ADC * Elements of the Successive Approximation ADC Successive Approximation Register Digital to Analog Converter Takes in a Combination of Bits SUCESSIVE APPROXIMATION A/D CONVERTER Example Show the timing waveforms that would occur in SAR ADC when converting an analog voltage of 6.84V to 8-bit binary, assume that the full scale input voltage of the DAC is 10V. Vin = 6.84 V Vref = 10 V 5 7.5 7.5 6.25 6.25 6.875 6.5625 6.5625 6.71875 6.796875 6.796875 6.8359375 6.8359375 6.84 V DAC Input DAC Vout Cumulative Voltage D7 5.0000 5.0000 D6 2.5000 7.5000 D5 1.2500 8.7500 D4 0.6250 9.3750 D3 0.3125 9.6875 D2 0.15625 9.84375 D1 0.078125 9.921875 D0 0.0390625 9.9609375 Dual Slope A/D Converter Contd. Pros • High accuracy • Fewer adverse affects from noise Cons • Slow • Accuracy is dependent on the use of precision external components Delta-Sigma ADC Delta-Sigma ADC contd. #1 Delta-Sigma Modulator Delta-Sigma ADC contd. #2 Digital Filter Decimator Sigma-Delta A/D Converter Contd. Pros •High Resolution •No need of precision components Cons • Slow due to over sampling • Good for low bandwidth ADC Comparison Type Speed(relative) Cost(Relative) Dual Slope Slow Med Flash Very fast High Successive approx Medium fast Low Sigma-Delta Slow Low ATD10B8C on MC9S12C32 Presented by: Michael Hochman MC9S12C32 Block Diagram ATD10B8C Block Diagram ATD10B8C Key Features Resolution Conversion Time 8/10 bit (manually chosen) 7 usec, 10 bit Successive Approximation ADC architecture 8-channel multiplexed inputs External trigger control Conversion modes Single or continuous sampling Single or multiple channels ATD10B8C External Pins 12 external pins AN7 / ETRIG / PAD7 AN6/PAD6 – AN0/PAD0 Analog input General purpose digital I/O VRH, VRL Analog input channel 7 External trigger for ADC General purpose digital I/O High and low reference voltages for ADC VDDA, VSSA Power supplies for analog circuitry ATD10B8C Registers 6 Control Registers ($0080 - $0085) 2 Status Registers ($0086, $008B) Formatted results (2 bytes) 1 Digital Input Enable Register ($008D) Allows for analog conversion of internal states 16 Conversion Result Registers ($0090 - $009F) General status information regarding ADC 2 Test Registers ($0088 - $0089) Configure general ADC operation Convert channels to digital inputs 1 Digital Port Data Register ($008F) Contains logic levels of digital input pins Control Register 2 Control Register 3 Control Register 4 Control Register 5 Single Channel Conversions Multi-channel Conversions Status Register 0 Status Register 1 Results Registers ATD Input Enable Register Port Data Register Setting up the ADC Applications For ADC What are some applications for Analog to Digital Converters? Measurements / Data Acquisition Control Systems PLCs (Programmable Logic Controllers) Sensor integration (Robotics) Cell Phones Video Devices Audio Devices Measurements / Data Acquisition What is Data Acquisition The sampling of the real world to generate data that can be manipulated by a computer (DSP) Digital Signal Processing first requires a digital signal Eg. Analysis of data from weather balloons by the National Weather Service NI X-Series Data Acquisition Card Control Systems e*(∆t) t R + - e S/H & ADC t ∆t e*(∆t) u*(∆t) Digital CPU Controller Clock Digital Control System Transducer 1001 0010 1010 0101 e* 0010 0101 0011 1011 e u*(∆t) Controller ∆t D/A & Hold Y u Plant The Old Way…. Analog Computers Comdyna GP6 The New Way e*(∆t) t t Controller 1001 0010 1010 0101 e* 0010 0101 0011 1011 e u*(∆t) ∆t ∆t Analog Output Analog Input ADC D/A Programmable Logic Controllers ADC in PLCs PLCs are the industry standard for automation tasks including: designed for: Motion Control Safety Systems multiple inputs and output arrangements extended temperature ranges immunity to electrical noise resistance to vibration and impact Most I/O are Boolean, however most PLC systems have an analog I/O module Rockwell PLC Analog I/O Module Sensor Integration (Robotics) Many robots use microprocessors ADC allows robots to interpret environmental cues and compensate If the algorithm needs to be changed it’s a simple matter of modifying the code Analog control systems require a complete circuit redesign Cell Phones Why Digital? Digital signals can be easily manipulated Digital phones convert your voice into binary information and then compress it This compression allows between three and 10 digital calls to occupy the space of a single analog call. The analog-to-digital and digital-toanalog conversion chips translate the outgoing audio signal from analog to digital and the incoming signal from digital back to analog Audio Devices Examples ADCs are integral to current music reproduction technology They sample audio streams and store the digital data on media like compact disks The current crop of AD converters utilized in music can sample at rates up to 192 kilohertz Sound Cards ADC From Sound Card Video Devices TV Tuners Analog video and audio signals are converted to digital signals for display to user Slingbox converts analog input stream and rebroadcasts it across the internet in digital form CCDs use ADCs to process image data Selection of an ADC Important Considerations: Input Type – Differential or Single Ended Resolution - Most Important Scaling - allows the user to divide or multiply the input voltage to more closely match the full scale range of the ADC Sample Rate - The sample rate must be at least twice the frequency the you are measuring, but 5 times is much better Channel Scan Rate - The channel scan rate is the maximum rate that the ADC can select a new channel and make a measurement. many ADCs have a relatively slow scan rate (when compared to the sample rate.) Eg. To achieve a sample rate of 600Hz on three channels, you will need a channel scan rate of at least 1.8kHz Example: Selecting an ADC We want to digitize a vibration signal measured by an accelerometer with the following characteristics (PCB 301A10): Sensitivity: (±2.0%) 100 mV/g Measurement Range: ±50 g pk Frequency Range: (±5%) 0.5 to 10000 Hz Select a satisfactory Analog to Digital Converter…. Example Continued Desired Signal: Sensitivity: (±2.0%) 100 mV/g Measurement Range: ±50 g pk Frequency Range: (±5%) 0.5 to 10000 Hz Solution Vrange resolution n 2 1 Freq: f s min 2 * f max Resolution: Minimum Sampling Ideal Sampling Freq: f s min 5 * f max n ln( 10 1) 0.1 6.66 bit 8 bit ln( 2) f s min 5 *10000Hz 50000Hz Choosing AD7892 From Analog Devices: The AD7892 is a high speed, low power, 12-bit A/D converter that operates from a single +5 V supply. The part contains a 1.47 µs successive approximation ADC, an on-chip track/hold amplifier, an internal +2.5 V reference and onchip versatile interface structures that allow both serial and parallel connection to a microprocessor. The part accepts an analog input range of ±10 V or ±5 V. Overvoltage protection on the analog inputs for the AD7892-1 and AD7892-3 allows the input voltage to go to ±17 V or ±7 V respectively without damaging the ports. References Cetinkunt, Sabri. Mechatronics 2007 www.me.gatech.edu/mechatronics_course en.wikipedia.org/ www.engineer.tamuk.edu/ www.scm.tees.ac.uk Bishop, Ron. Basic Microprocessors and the 6800 MC912SC Family Data Sheet MC912SC Reference Manual MC912SC Programming Reference Guide