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Embedded Computer Systems Elec 296 - MyWeb at WIT

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					Elec 471 Embedded Computer Systems
  Chapter 7, Data Acquisition Systems


                By Prof. Tim Johnson, PE
           Wentworth Institute of Technology
                      Boston, MA
     Theory and Design for Mechanical Measurement
                   by Richard Figliola
                              Content
• Sampling concepts
    –   Sample Rate
    –   Alias Frequencies
    –   Amplitude Ambiguity
    –   Selecting sample rate and data number
•   Analogue Converters: ADC and DAC
•   Data Acquisition System Overview
•   Data Acquisition Component parts
•   Data Communications
    –   Single vs. Double Ended
    –   Serial: RS232, SPI, I2C
    –   Parallel: GPIB,
    –   Wireless
          Chapter introduction
• Concept: analogue signals are continuous in
  amplitude and time while digital signal are
  discrete (meaning countable).
• Digital signal are non-continuous in amplitude
  and time however individual data signals are valid
  representations for amplitude and time.
• Sampling is the process of making a continuous
  signal discrete.
• The problems and solutions for digitization are
  presented.
    Introduction to sampling




• The question that confronts the digital engineer is how
  well does the sampling represent the analogue signal?
• The answer depends upon :
    – the frequency content of the signal
    – The size of the period between sample (ADC frequency)
    – The total sample period of the measurement
           Sampling Concepts
• A Discrete Fourier Transform (DFT) conveys all
  the information needed to reconstruct a
  Fourier series of a continuous signal from a
  digital signal.
• Fourier analysis will provide some guidelines
  for sampling continuous data.
• Let δt be the interval between samples so the
  sample rate (sample frequency, fs) is
                      fs = 1/δt
                Nyquist Frequency
• The sampling theorem states that to reconstruct
  the frequency content of a signal accurately the
  sample rate fs must be more than twice the
  highest frequency content of the signal.
• The highest frequency of a signal that can be
  sampled at any particular sampling rate is called
  the Nyquist frequency .
• Let fm represent this maximum frequency.
• fs > 2 fm substituting for fs we get 1/δt > 2 fm
• Inverting the equation reverses the > sign:
                            δt < 1/2fm
   Effect of sample rate




Lost amplitude content   Lost frequency content
              Alias Frequencies
• When the signal is sampled at a sample rate that
  is less the 2fm the higher frequency assumes the
  identity of a lower frequency.
• An example of this is when the wagon wheel on a
  covered wagon in the early days of television
  would turn backwards.
• The folding point of the aliasing phenomenon
  seen in the next slide is at the Nyquist frequency:
                    fN = fs/2 = 1/2δt
         Frequency aliasing diagram




Frequency values seen will increase until the signal frequency reaches the Nyquist
frequency then they will begin to decrease and can even go back to zero until they
start to increase again.
          Amplitude Ambiguity
• As long as the sampling rate is twice the sampled
  signal’s highest frequency the frequency content
  remains valid for the digital signal but this rule
  doesn’t apply to amplitude fidelity.
• Consider N an integer. If for some value of N,
  N*δt equals the time period, Ts, of the frequency
  of the sampled signal then the DFT (the digitized
  signal) will contain the correct values for the
  amplitude of the signal.
• This can be seen in time diagrams and frequency
  spectrum diagrams of a signal on the next slide.
                                   This phenomenon is
                                   known as signal leakage
                                   (to adjacent frequencies)




The truncated wave creates a
modulated signal thus creating
sidebands that suck the power
from the correct amplitude value
shown at the sampled frequency
           Sampling summary
• The number of data points and the sample
  rate should be chosen to satisfy the criteria of
                    fs > 2 fm

                δf = 1/Nδt = fm/N

             In general, set fs ≥ 5 fm
Digital to Analog Converter
DAC schematic




 This is a design you can
 build yourself. The
 output bits control the
 switching of the source
 voltage to each resistor.
      Analog to Digital Conversion
• An ADC converts a voltage into a binary value by a
  process known as quantization.
• The analog side of the ADC is characterized by its full
  scale voltage range, VFSR.
• Resolution, Q, is the smallest voltage increment that
  will cause the LSB to change. Q = VFSR/2M
• Quantization error is caused by a voltage value falling
  between two different digital values. It behaves as
  noise imposed on the digital signal.
• The error value can vary between the LSB step value
  and ½ LSB. The span of the quantization error is the
  same as the resolution.
 Quantization, Bias, & Saturation




Making the quantization symmetric about the input shifts the dotted line, the
presumed linear input voltage thus changing the bias constant v in the linear
equation y=mx + b. The bias only needs to be moved a negative ½ LSB voltage.
             Conversion errors
• Sources of errors in the output voltage can be
  resolved into hysteresis, linearity, sensitivity,
  zero and repeatability errors . These were
  covered in Chapter 2.
• The extent of the errors depend on the
  particular method of ADC.
• Other factors include settling time, signal
  noise during sampling, leakage, and
  temperature.
    Successive Approximation Converters
• In a SAC ADC a comparator compares the input voltage to a
  reference voltage derived from a DAC value based upon the binary
  evaluation using the MSB first and setting each subsequent bit
  higher (1) or lower (0) until all the bits in the ADC register are filled.
• In this scheme the starting reference value is ½ FSR or 0x80 for an
  eight bit converter.
• If the input voltage is higher(lower), the MSB of the register is set at
  1(0) and then the next bit is evaluated after the register value is
  converted into the reference voltage by the DAC converter.
• The time of conversion is Mbits*bit conversion speed. This
  conversion can be effect by noise at the instance of conversion
  giving weighted value to small noise.
• The converter requires the input voltage be held constant during the
  conversion process so a Sample and Hold is required on the input.
A Successive Approximation Converter
    Ramp (integrating) Converters
• Low-level voltage measurement use this converter for
  its low-noise features.
• The main components are a comparator, a voltage
  generator that increases linearly that the name ramp,
  and the counter and the M-bit register.
• The basic principle of this converter is that as a
  reference voltage is increased on a timed basis, the
  comparator flips thus stopping the comparison. The
  counter is then used to determine the input voltage
  based on time taken by the ramp generator to cross
  the input voltage.
    Ramp Converter block diagram




Using a capacitor of known fixed value charged by a fixed current source the time to
charge can be calculated. The coulombs charge C, is related to the current flow
(q=It) divided by the voltage (C=q/V). Since the charge is known, the rate of current
flow is know, substituting in It solves for V with the value of the count known (the
count stops when the comparator flips determining the time, t): V=It/C
              Parallel Converters
• Parallel are fast as one
  comparator is used for
  each bit.
• A highly stable reference
  voltage divides the voltage
  through a precision
  resistor network with the
  MSB resistor dropping the
  voltage by ½ VFSR and so
  on down the line by ½ at
  each branch.
• Time for conversion is only
  the time for one
  comparator to switch.
• Very fast but expensive.
           Delta Sigma Conversion

• The sigma-delta (Σ-Δ) ADC is the converter of choice
  for modern voiceband, audio, and high-resolution
  precision industrial measurement applications. The
  highly digital architecture is ideally suited for modern
  fine-line CMOS processes, thereby allowing easy
  addition of digital functionality without significantly
  increasing the cost. Because of its widespread use, it is
  important to understand the fundamental principles
  behind this converter architecture. *


   *http://www.analog.com/static/imported-files/tutorials/MT-022.pdf
      One-bit comparator output




Source http://www.analog.com/static/imported-files/tutorials/MT-022.pdf




         Source http://en.wikipedia.org/wiki/Sigma-delta_converter
    Oversampling filter effects




Source http://www.analog.com/static/imported-files/tutorials/MT-022.pdf
      Signal to Noise Ratio in ADC
• The SNR is improved by higher resolution as seen
  in the table below.
• Digital SNR is defined as the ratio of power in the
  signal to the noise power thus the larger the
  number the smaller the noise and the clearer the
  signal: SNR[dB] = 20 log 2M
   Note: If there is an amplifier in
   the ADC circuit then the
   resolution in Table 7.2 modifies
   the formulas to 20 log(Gain*2M)
   and Q = VFSR/(Gain*2M)
       Data Acquisition Systems
• This is a systematic look at the usage for an
  ADC converter after the value is sensed.
• A DAS is that portion of a measurement
  system that quantifies/store/displays/controls
  the value.
             Block Diagram of a DAS




INPUTS→MULTIPLEXER→ADC→CONTROLLER→OUTPUT:STORAGE/DISPLAY/DATA-COMM I/O
              DAS Components
• A voltage divider (lower right) is a very handy and
  simple two that is used for two different purposes.
• One—converts a current source sensor into a voltage
  source for reading by an ADC. R1 becomes a current
  limiting resistor.
• Two—converts a high voltage into a lower voltage
  readable by an ADC.
• A shunt circuit (lower left) also converts a current
  source into a voltage source if the current is “low”.
                    Nulling Circuit
• In order for an ADC to work correctly the signal has to
  have extraneous voltages removed.
• The nulling circuit sets a sensor to zero if it’s output
  should be zero. Its use is like the adjustment knob on
  a bathroom scale when no one is standing on the scale
  except the scale mechanism.
 This circuit is essential a
 Wheatstone bridge circuit
 that has a potentiometer
 across three terminals. It
 is used to balance the
 potential between CH+
 and CH- (equal to zero).
  Single and Double Ended sources
• A single-ended connection has one signal line (+High)
  measured relative to ground. Single ended are the
  usual connections we use; not obvious when using
  local grounds is all the devices share a common
  ground.
• A double-ended connection uses two signal lines due
  to different ground potential at either end. A signal
  high is paired with a signal return to isolate the signal
  from the local grounds (prevent ground loops).
        Pull-up/Pull-down Resistor
• In circuits where an input level is being detected,
  have a pull-up resistor can save the battery power.
• In the accompanying diagram of a pull-up resistor,
  having the switch open allows the logic gate to
  sense the input voltage level without having to
  supply the voltage. When the switch is closed, the
  ground can be seen by the logic gate because of
  the voltage drop across the resistor.
• Switching the position of the switch and the
  resistor will change what value the logic gate will
  sense when the switch is in the open position. In
  this case that would be a low so that is called a
  pull-down resistor.
• In actual circuits the switch is replaced with a
  CMOS device resulting in an open-drain terminal.
• This arrangement is very useful in 12C circuits.
          Data Communications
• Once the data has been acquired and digitized it is
  moved to either: data memory or to immediate data
  display or to data communication output.
• Modern devices require the capability for all three
  destinations.
• The ADC value goes onto an internal data bus inside a
  device.
• Software controls distribution nowadays rather than
  hardware as seen in Figure 7-14 and the following
  schematic of a 1990’s style ADC/DAC board from NI.
• Depending on the type of bus communication selected
  different controllers are used to host the bus.
Data acquisition plug-in board




                            Note: not
                            shown in this
                            plug-in board
                            is the UART
                            for serial I/O
                                  UART
• A Universal Asynchronous Receiver/Transmitter, abbreviated UART (
  /ˈjuːɑrt/), is a type of "asynchronous receiver/transmitter", a piece of
  computer hardware that translates data between parallel and serial forms.
  The universal designation indicates that the data format and transmission
  speeds are configurable and that the actual electric signaling levels and
  methods (such as differential signaling etc.) typically are handled by a
  special driver circuit external to the UART.*

• A UART is usually an individual (or part of an) integrated circuit used for
  serial communications over a computer or peripheral device serial port.
  UARTs are now commonly included in microcontrollers. A dual UART, or
  DUART, combines two UARTs into a single chip. Many modern ICs now
  come with a UART that can also communicate synchronously; these
  devices are called USARTs (universal synchronous/asynchronous
  receiver/transmitter).*
• A USI is Universal Serial Interface is a stripped-down UART that specializes
  in hosting two wire communication protocols: SPI, I2C, UNI/O, and 1-Wire.



     *Source: http://en.wikipedia.org/wiki/UART
                RS232 protocol
• One of the first major data communication
  buses was RS-232C protocol for serial digital
  communication over telephone lines.
• There is still a huge number of installed devices
  that utilize this serial protocol characterized by a
  DB9 connection on the back of units. This is a
  slow connection (< 9600 bps).

      There is a plethora of
      assistance on this connector
      and RS232 on the internet.
        Universal Serial Interface
• SPI—Synchronous Peripheral Interface handles a three-
  wire communication protocol using separate
  transmitter, receive, and clock lines to a dedicated
  device
• I2C—Inter-Integrated Circuits is a two-wire
  communication protocol utilizing a data line and a
  clock line. Control signals are passed on the bus when
  data is not being transmitted for up to 8 different
  devices of 16 different types (7 bit addresses).
• SMBus and PMBus are derivatives of 12C bus.
• No license is required for I2C so it will be around for
  quite some time. The dropping resistors on the bus
  mean that small microcontrollers operating on
  batteries do not have to power the bus.
An I2C data communication sequence
             High speed data links
• The Universal Serial Bus can connect to 128 devices at transfer
  rates 1.5-12 Mbs on USBv1.0 and up to 450 Mbs on version 2.
  Hot swapping (aka, plug and play) is allowed.
• Other high speed protocols include Firewire which can be
  used for video links.
• Ethernet is a serial data communication link when it connects
  to a LAN for control from a HMI software package such as NI
  LabView.
• Serial communication using double-ended connections using
  twisted wire for Direct Memory Access using Low Voltage
  Differential Signaling (LVDS) is now commonplace. The
  devices don’t even wait for the voltage rise to complete, they
  just look for transitional voltage swings, low to high…
         Parallel communications
• GPIB—General Purpose Interface Bus is a high speed
  parallel bus is still used and has a large installed base of
  equipment. This bus is the IEEE 488 standard that used
  8-bit parallel, a 3-wire handshake bus (Data Valid, Not
  Ready for Data, and Data Accepted), and 5 lines for bus
  management (Service Request, Interface Clear,
  Attention, End Of Transmission, Remote Enable).
• While the bus is more expensive than serial, multiple
  instruments share the cost. Most scientific devices rely
  on software drivers controlled by software programs.
Modern devices—Texas Instrument
  ADS41B298 runs @250MSPS
                   Summary
•   Sampling principles and calculations
•   Analogue converter basics: ADC and DAC
•   Data acquisition system overview
•   Selected data acquisition components
•   Types of data communications protocols
•   I2C data communication sequence, view demo
•   More information is available in the text and
    on the internet.

				
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