7/23 Data Acquisition and Analog-digital Conversion Computer Science & Engineering Department Arizona State University Tempe, AZ Dr. Yann-Hang Lee

set Data Acquisition  Data acquisition (DAQ) refers to the process of capturing data from a real-world physical system (Eg. Transducers) and presenting the data in a form that is readily available to a digital processing system.  Acronyms: ADC  Analog to Digital Converter DAC  Digital to Analog Converter  Topics Covered: -  Architecture of a typical DAQ System.  Range, Resolution, Conversion Rate, Data Transfer, Differential Non-Linearity etc.  Circuits: ADC (Dual-Slope Integrating, Successive Approximation, Flash (Parallel)) and DAC (R-2R Ladder).

set Architecture of a typical DAQ System  Transducer: Converts non-electric input signals into proportionate analog electrical signals.  Actuator: Converts electrical signals to proportionate physical quantities. ADC DAC Real World. Eg., Force, Velocity, Light etc. Analog Output Analog Input Analog Voltage/Current Digital Voltage/ Current Transducer Actuator DAQ Hardware

set Architecture of a typical DAQ System  ADC is used to convert analog electrical signals to proportional digital values. This digital data can then be processed, recorded and visualized in a digital computer.  DAC is used to convert a value represented in digital code by the computer, to a proportionate analog voltage/current signal. This signal can then be used to control a real-world device through an actuator.  Example: Cruise control of an automobile. Speedometer interfaced to a Transducer  DAQ  Microcontroller.

set Key Terminologies: ADC  Full-Scale Voltage Range: The difference between the maximum voltage and minimum voltage that the ADC is converting.  Resolution: The number of bits the ADC uses to represent the digital data determines the resolution.  Example: A 3-bit ADC. For a full-scale voltage of 10 volts, the resolution will be 10/(2 3 )=1.25volts.  Differential sensing:  In the regular single sensing mode, the ADC samples the difference between the applied signal and ground pin.  In differential mode, the input signal is applied between a signal line and its own special reference, the signal return path is through this reference signal and not the regular ground pin.  Differential mode is used when noise on input line is high, or the voltages are extremely low (< 1 volt).

set Key Terminologies: ADC (Resolution)  However, if we increase the number of bits to 12, then resolution becomes 10/(2 12 )=2.44 mV. Volts Sampling Time Digital Output 4/8 8/8FS LSB

set Key Terminologies: ADC (Sample Rate)  Sampling Rate or Conversion Rate: Data is acquired by the ADC by using a process called sampling.  Instantaneous values from the analog signal are sampled at discrete time intervals. The rate at which signals are sampled is called the sampling frequency.  Higher the rate of sampling, better is the quality of conversion. The minimum sampling frequency must be at least twice the maximum frequency of the input analog signal (Nyquist Rate). Sampling at a lower rate will lead to aliasing.

set Key Terminologies: ADC (Sample Rate) Time Voltage  Red wave: Original Signal (34.1 KHz)  Black wave: Undersampled waveform (Fs=10KHz)

set How fast should ADC be? Examples Applications No. of Conversions per second (Approx.) Monitor and Control Telephone Voice8000 CD-Quality Audio85, 42.5, 21.3 Kilo Video2*10^6 Radar100…1000*(10^6)  Large rate of collecting data by the DAQ can result in a transfer bottleneck because of the limited memory bandwidth and the lack of processor availability for such data transfers  DMA bus master transfers data directly to RAM without any need for processor intervention.

set ADC Hardware  ADC architectures can be divided into three categories as shown below: -  Integrating, Successive Approximation and Flash architectures are the most popular. Low Speed, High Accuracy Medium Speed, Medium Accuracy High speed, Low-to- medium Accuracy Integrating Oversampling Successive Approximation, Algorithmic Flash, Two-step, Interpolating, Folding, Pipelined, Time- interleaved

set Dual-Slope Integrating ADC- Operation  Phase I : C1 is charged with S1 connected to –V in with charging duration equal to T1=(2 N )/f clk.  At the end of Phase I,

set Dual-Slope Integrating ADC - Operation  Phase II : At the start, the counter is reset and S1 is connected to V ref. C1 discharges in time T2, proportional to the value it was charged in Phase I. Counter counts until V x is less than zero. This count value is the digitized value of V in.  The counter output is proportional to V in, all other quantities in the above equation are constant.  Merits and Demerits: -  100 microseconds (slow), Very Accurate, Low Cost, Long life.

set Successive Approximation-Operation  Follows the concept of Binary Search Algorithm.  Begins by comparing analog input with midpoint voltage, i.e., SAR sets MSB to 1. If analog input is greater, MSB remains unchanged and in the next cycle, the SAR sets the next bit in line and the cycle repeats until all the bits are covered.

set Successive Approximation - Example  A simple example:  Step 1>  Step 2>  Step 3>  Final Value:  Merits and Demerits: -  Good combination of simple circuit and reasonable conversion time.  1-10 microseconds. 000 D/A ValueAnalog Value (4*0.625v)=2.5 V1.7 V (2*0.625v)=1.25 V1.7 V (3*0.625v)=1.875 V1.7 V

set Flash (Parallel) ADC - Circuit

set Flash (Parallel) ADC - Operation  Input signal is fed to all the comparators. The other input is connected to different nodes of a resistor string.  When V ri is larger than V in, a “one” is output.  The only NAND gate that will have low output is the one having one at both inputs. This happens when there is a transition from one to zero at the comparator outputs. All other NAND-gate outputs will be one. This makes it easy for the encoder.  Merits and Demerits: -  Extremely fast, 4-50 ns.  Occupies large chip area, exponential order of complexity.

set DAC Circuit - R-2R Ladder  When the above ladder circuit is analyzed, we can see that the resistance “as seen” at the arrows shown, is always 2R irrespective of the number of stages.  This gives the current relationships shown.  Thus, the ladder circuit can be used to obtain binary weighted currents. RRR2R V ref 2R V ref /2R V ref /16RV ref /8RV ref /4R

set DAC: R-2R Ladder Circuit  The ladder circuit connected to an Op-amp to perform D/A conversion.  The switches are controlled by the digital value. The switch positions are shown when bit equals one.

set R-2R Ladder Circuit - Operation  When the bit is high, (MSB on extreme left), the current through the branch will pass through to resistor Rf, if bit is low, the current flows to ground.  Therefore, only when the bit is high a binary weighted current will flow through Rf. The output voltage is equal to the drop across Rf. Hence, a voltage proportional to the digital value will appear at the output.  Merits: -  Only two resistor values required.  Fully modular, stages can be added or deleted easily.

set ADC in MCF5211  Two separate and complete ADCs of 12-bit resolution  Internal multiplex to select two of 8 inputs  Single conversion time of 8.5 ADC clock cycles, a dditional conversion time of 6 ADC clock cycles  Maximum ADC clock frequency of 5.0 MHz, 200ns period

set Single Ended or Differential  Single ended  upper switch is closed and bottom switch select V ref  Differential  upper switch is open and bottom switch selects V-  differential pairs: AN0-1, AN2-3, AN4-5, and AN6-7

set Operation Mode of ADC  Conversion process is either initiated by a sync signal from one of two input pins (SYNCx) or by writing 1 to a STARTn bit.  Operation modes:  a single scan and halt  a scan whenever triggered  scan sequence repeatedly until manually stopped.  an ordered list of the analog input channels defined in ADLST1/2  sequential (8 channels) or parallel (AN0-AN3 and AN4-AN7)  12-bit samples + offset  result registers (ADRSLT0-7)  Conversion clock for sampling  the IPBus clock (system clock) and the clock divisor bits within the CTRL2  should be between 100 kHz and 5.0 MHz.  Reference voltages V REFH and V REFL : all analog inputs are measured against.

set ADC and Resistive Screen Touch Panel  Resistive touch screens embed a thin glass substrate between two layers of a plastic overlay  The surface of the glass substrate has a tin oxide coating onto which a slight electrical current is constantly applied  When pressure is applied to the outside of the overlay, the two surfaces touch and a ground occurs  The x- and y-coordinates of the ground (touched area) are detected and registered with the control device  Cheap and popular  Scratches and long time wear & tear, makes it inaccurate

set Touch Panel Controller: TI ADS7843  SPI interface  receive command and respond samples  Pen-down interrupt and 2 addition analog inputs

set Touch Panel Controller: TI ADS7843  Successive Approximation Register (SAR) ADC  Analog inputs to the converter are provided via a four-channel multiplexer

set Touch Panel Controller: TI ADS7843  Configuration of switches to measure Y position  X+  +IN, Y+  +Vcc, Y-  -IN and GND