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Chapter IV – Computerized Data-acquisition Systems
CHAPTER IV
Computerized Data-acquisition Systems
The signal outputting a sensor is usually an analog signal. Post-processing methods
involve, however, complex mathematical formulations. The data recorded by the sensor
have, therefore, to be digitalized before post-processing. This is the role of the
computerized data-acquisition (DAQ) system.
1. Characteristics of DAQ systems:
- The number of bits used to represent the signal: The higher is the number of bits, the
higher is the accuracy of the digital representation of the analog input.
- Input range: How many (max, min) volts the DAQ will accept (it can be unipolar: 0 to
5 V or bipolar:  5V).
- Conversion speed: the speed it takes to convert the analog input into a digital output.
- Unipolar single-slope integrating convert
Figure 4.1. Ramp A/D converter process.
Figure 4.2. Single-slope integrating A/D converter circuit.
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Chapter IV – Computerized Data-acquisition Systems
Example
A 8-bit single-slope integrating analog-to-digital converter has an input rage of 0 to 5 V.
Determine the digital output for an analog input of 1 V.
If an input is out of range. The analog-to-digital converter is said to be saturated. It is the
responsibility of the experimentalist to ensure that the value of the input is lower than the
limits.
Example
A 8-bit single-slope integrating analog-to-digital converter has an input rage of 0 to 5 V.
Determine the digital output for an analog input of 1 V, using the above formulas.
Example
A 12-bit A/D converter with 2’s-complement output has an input range of -10 to +10 V.
Find the output codes when the input is -11 V; -5 V; 0 V; 6.115 V and 12 V.
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Chapter IV – Computerized Data-acquisition Systems
- A/D converters errors:
The typical errors associated with A/D converters are:
-
Linearity.
Zero error.
Sensitivity.
Furthermore, since conversion from analog to digital signal requires converting a
continuous function to a discrete one. A resolution error, called quantizing error has to be
considered:
V V
input resolution error  0.5 ru N rl
2
Example
A 8-bit A/D converter has an input range of -5 to +5 V. Find the input resolution error.
Note: if the inlet signal input has the order of magnitude of the input resolution error, it
would be better to amplify the input signal before conversion.
If data change rapidly in time, the A/D converter can cause an error, since it will not have
enough time to convert the analog input into digital output. To solve this problem a
sample-and-hold device is usually inserted before the A/D converter. The conversion can
be performed every 1.5 s or less (normal value: 10 to 25 s).
- Practical Analog-to-Digital converters
The most common used A/D converter used in DAQ systems is a successive
approximation converter. It is based on interval halving technique
Figure 4.3. Successive approximations method (for 4-bit A/D converter)
Example
A 8-bit unipolar successive A/D converter has an input range of 0 to +5 V. An analog
voltage of 3.15 V is applied to the input. Find the analog output.
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Chapter IV – Computerized Data-acquisition Systems
If high sampling rates are required, faster A/D converters are used: parallel or flash A/D
converters.
In this type of converters, all the conversion process is performed in one step using
several comparators and the conversion can be performed as fast as 10 ns. These A/D
converters are adequate for 8-bit resolution. Half-flash A/D converter are also commonly
used (2 flash converters and need two steps for a complete conversion). They perform a
complete conversion in about 100 ns.
Another type of very accurate, but slow, A/D converter is the dual-slope integrating
converter.
Figure 4.4. Dual slope integrating A/D. (a) block diagram; (b) integration process.
Vi
; Vi: input unknown signal and Vr: reference signal.
Vr
The time for conversion is around 4 to 8 ms.
t1  t0
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Chapter IV – Computerized Data-acquisition Systems
- Digital-to-Analog converters
Computers (through DAQ systems) can be used to control instruments (run a pump, close
a valve …). In this case a conversion from digital to analog signal is required.
Figure 4.5. Digital to analog converter.
It uses a set of switches to convert the digital signal to an analog one. Usually the signal
produced is quite low and need therefore amplification.
- Simultaneous Sample-and-hold subsystems:
These subsystems are really critical went one has to ensure that there is a simultaneous
reading of the data.
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Chapter IV – Computerized Data-acquisition Systems
Types of Data Acquistion Systems
Wireless Data Aquisition Systems
Wireless data acquisition systems can eliminate costly and time
consuming field wiring of process sensors. These systems consist of
one or more wireless transmitters sending data back to a wireless
receiver connected to a remote computer. Wireless transmitters are
available for ambient temperature and relative humidity,
thermocouples, RTDs, pulse output sensors, 4 to 20 mA transmitters
and voltage output transducers. Receivers can be connected to the
USB or Ethernet port on the PC.
Serial Communication Data Acquistion Systems
Serial communcation data acquistion systems are a good choice
when the measurement needs to be made at a location which is
distant from the computer. There are several different
communication standards, RS232 is the most common but only
supports tranmission distances up to 50 feet. RS485 is superior to
RS485 and supports transmission distances to 5,000 feet.
USB Data Acquistion Systems
The Universal Serial Bus (USB) is a new standard for connecting
PCs to peripheral devices such as printers, monitors, modems and
data acquistion devices. USB offers several advantages over
conventional serial and parallel connections, including higher
bandwidth (up to 12 Mbits/s) and the ability to provide power to the
peripheral device. USB is ideal for data acquisition applications.
Since USB connections supply power, only one cable is required to
link the data acquisition device to the PC, which most likely has at
least one USB port.
Data Acquisition Plug-in Boards
Computer data acquisition boards plug directly into the computer
bus. Advantages of using boards are speed (because they are
connected directly to the bus) and cost (because the overhead of
packaging and power is provided by the computer). Boards offered
are primarily for IBM PC and compatible computers. Features
provided by the cards can vary due to number and type of inputs
(voltage, thermocouple, on/off), outputs, speed and other functions
provided. Each board installed in the computer is addressed at a
unique Input/Output map location. The I/O map in the computer
provides the address locations the processor uses to gain access to
the specific device as required by its program.
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