**3. General principles of data acquisition systems**

Data acquisition systems are products and/or processes used to collect informations that can be processed or stored by a computer to document or analyze some phenomenon (Judd, 2008). Measurement systems are used in the data acquisition industry since 1981 and have supplied several million data acquisition channels on an international basis.

Data acquisition systems come in many different PC technology forms to offer flexibility when choosing a measurement system. The organization of information flow in the system is an important problem in designing and operating the measurement. Two aspects are essential for this organization: the kind of transmission in the system (serial, bit-by bit, parallel) and the mode of information exchange between system devices (Nawrocki, 2005).

The functional components of a basic data acquisition are:


More complicated data acquisition systems can be constructed in the hierarchical structure. On the lowest level are subsystems to collect data from physical quantities. The main controller of the system receives processed measuring data and sends commands relating to the execution of a measuring procedure or a set of commands to subsystems.


$$f\_s > 2f\_u \tag{1}$$

Otherwice, the reproduction of the discrete signal recorded yields a distorted analog signal, caused by a too low sampling frequency, phenomenon called aliasing. In order to eliminate aliasing, is utilised a lowpass input filter or an antialiasing filter.

 Quantizing: assigning to every sample a value from a set of N values into which the measurement range is divided. Figure 8 shows a LabVIEW™ application for the quantizing of voltage into an n=3 bit digital signal, the number of quants is 23=8.

The A/D converter is connected to an analog input signal; it measures the analog input and then provides the measurement in digital form suitable for use by a computer. The resolution of an A/D input channel describes the number or range of different possible measurements the system is capable of providing. This specification is almost universally provided in term of "bits". For example, 8-bit resolution corresponds to a resolution of one part in 28 – 1 or 255, 12 bit corresponds to one part in 216 -1 or 65,536. To determine the resolution in engineering units, simply divide the range of the input by the resolution. A16-bit input with a 0-10 Volt input range provides 10 V / 216 or 152.6 microvolts. Table 1 provides a comparison of the resolutions for the most commonly used converters in data acquisition systems.

Fig. 7. Signal sampling



More complicated data acquisition systems can be constructed in the hierarchical structure. On the lowest level are subsystems to collect data from physical quantities. The main controller of the system receives processed measuring data and sends commands relating to

 Sampling – after this operation signal is represented by a set of values {*x*(*kT*s)}, drawn at time periods *T*s – Figure 7. Sampling frequency should be high enough to provide a number of samples sufficient to reproduce the signal in the analog form. According to the Shannon theorem, the sampling frequency *f*s should not be lower than twice the

Otherwice, the reproduction of the discrete signal recorded yields a distorted analog signal, caused by a too low sampling frequency, phenomenon called aliasing. In order to eliminate

 Quantizing: assigning to every sample a value from a set of N values into which the measurement range is divided. Figure 8 shows a LabVIEW™ application for the

quantizing of voltage into an n=3 bit digital signal, the number of quants is 23=8. The A/D converter is connected to an analog input signal; it measures the analog input and then provides the measurement in digital form suitable for use by a computer. The resolution of an A/D input channel describes the number or range of different possible measurements the system is capable of providing. This specification is almost universally provided in term of "bits". For example, 8-bit resolution corresponds to a resolution of one part in 28 – 1 or 255, 12 bit corresponds to one part in 216 -1 or 65,536. To determine the resolution in engineering units, simply divide the range of the input by the resolution. A16-bit input with a 0-10 Volt input range provides 10 V / 216 or 152.6 microvolts. Table 1 provides a comparison of the

resolutions for the most commonly used converters in data acquisition systems.

2 *s u f f* (1)

The functional components of a basic data acquisition are:

input voltage of the analog-to digital converter (ADC);

instruments, oscilloscopes or a computer monitor); - Computer with dedicated software and memory resources.

upper frequency *f*u of the sampled signal spectrum:

aliasing, is utilised a lowpass input filter or an antialiasing filter.


the execution of a measuring procedure or a set of commands to subsystems. Analog-to-Digital (A/D) conversion of an analog signal involves two processes:


Fig. 7. Signal sampling

Fig. 8. Quantization of signal


Table 1. Common Analog-Digital-Converter Resolution

From the point of view of the method of conversion there is a variety of different types of A/D converters used in data acquisition. The most commonly used A/D converters in today's data acquisition products are divided into:


Most multi-channel data acquisition systems are based upon a single A/D converter. A multiplexer is then used between the input channels and the A/D converter. The multiplexer connects a particular input to the A/D, allowing it to sample that channel. Figure 9 depicts a typical, multiplexed input configuration.

The primary disadvantage of this system is that even if the switching and sampling are very fast, the samples are actually taken at different times. The ability to sample inputs at the same instant in time is typically referred to as simultaneous sampling.

There are two ways to achieve simultaneous sampling. The first is to place a separate A/D converter on each channel. They may all be triggered by the same signal and will thus sample the channels simultaneously. The second is to place a device called a sample & hold (S/H) on each input. When commanded to "hold", the S/H effectively freezes its output at that instant and maintains that output voltage until released back into sample mode. Once the inputs have been placed into hold mode, the multiplexed A/D system samples the desired channels. The signal to be sampled will all have been "held" at the same time and so the A/D readings will be of simultaneous samples. The simultaneous sampling configurations are shown in Figure 10.

Fig. 9. Typical Multiplexer/ADC DAQ System

a. using individual ADCs on each input b. using Sample-Hold, MUX, and a single ADC

Fig. 10. Simultaneous Sampling Techniques

A criterion applied to classify data acquisition systems is the transmission type of digital announcements (data, addresses, commands): serial transmission or parallel transmission.

The interface system assures equipment and adjustments of devices attached to the bus. The original PC-bus, ISA-bus, is a simple, robust, and inexpensive interface that has certainly stood the test of time. However, most of today's plug-in board business is based on the PCI bus (or variants such as cPCI and PXI). The external box vendors now have Ethernet and USB standards to work with as well as some less used, but very viable, interfaces such as Firewire, CAN, and perhaps the oldest standard in computing, RS-232.

Software has progressed from the original DOS-based version to hyghly complex applications such as MATLAB and LabVIEW™ that are both easy to use and able to take advantage of today's powerful computers.

 a. using individual ADCs on each input b. using Sample-Hold, MUX, and a single ADC

A criterion applied to classify data acquisition systems is the transmission type of digital announcements (data, addresses, commands): serial transmission or parallel transmission. The interface system assures equipment and adjustments of devices attached to the bus. The original PC-bus, ISA-bus, is a simple, robust, and inexpensive interface that has certainly stood the test of time. However, most of today's plug-in board business is based on the PCI bus (or variants such as cPCI and PXI). The external box vendors now have Ethernet and USB standards to work with as well as some less used, but very viable, interfaces such as

Software has progressed from the original DOS-based version to hyghly complex applications such as MATLAB and LabVIEW™ that are both easy to use and able to take

Firewire, CAN, and perhaps the oldest standard in computing, RS-232.

Fig. 9. Typical Multiplexer/ADC DAQ System

Fig. 10. Simultaneous Sampling Techniques

advantage of today's powerful computers.

Currently, most data acquisition companies provide hardware either at board level or external box and software. PC-based DAQ systems are available with a wide variety of interfaces. Ethernet, PCI, USB, PXI, PCI Express, Firewire, Compact Flash and even the venerable GPIB, RS-232/485, and ISA bus are all popular (Manea & Cepisca, 2007).

RS-232 is by far the simplest and least expensive interface. Every PC shipped today can communicate over an RS-232 line. Only a compatible cable and a terminal program are required. RS-232 is limited to about 50 feet, but it can operate over huge distances with a little help from a modem. Only one device can be connected to a single RS-232 port. This interface is typically not electrically isolated from the host computer. For example, if a power line were inadvertently dropped across the RS-232 line, the host computer most likely would be destroyed. As a result, RS-232 may be appropriate for situations in which very few remote I/O systems are needed and where the likelihood of electrical transients is low.

RS-485 allows multiple devices (up to 32) to communicate at half-duplex on a single pair of wires, plus a ground wire (more on that later), at distances up to 1200 meters (4000 feet). Both the length of the network and the number of nodes can easily be extended using a variety of repeater products on the market. Many RS-422/485 protocols were developed in the 1980s for such applications. Profibus, Interbus and CAN are a few. These were designed with proprietary interests in mind, and each subsequently received backing from several vendors in an attempt to develop an open international standard with widespread support.

Ethernet has been around for more than 20 years and has become a commodity in modern business environments. Ethernet also has many characteristics that make it suitable for industrial networked and remote-sensor I/O applications. It combines the low cost of RS-232 with the multidrop capability of RS-422/485 and provides a clear standard for communications. Ethernet has become the medium of choice to communicate management data throughout the enterprise. Interoperability of Ethernet based data acquisition devices from multiple vendors has not always been stellar. However, most Ethernet based data acquisition systems are single vendor and this has not been a major issue in the acquisition space. The LXI Consortium has developed a specification that ensures simple and seamless multi-vendor interoperability.

Gigabit Ethernet is a version of Ethernet that supports 1 Gigabit per second data transfer rates. Boasting the same speed capability as standard Ethernet, the fiber optic implementation extends the range of the system to 2 kilometers. Fiber also provides virtually absolute electrical isolation and has immunity to electrical and magnetic interference.

Firewire is a high speed serial interface. However, at approximately the same time Firewire was being promoted, USB interface was coming on line. At this time, there are a wide variety of data acquisition vendors and products actively promoting USB devices, Firewire success has been confined to the original target market of audio and video.

USB's simple plug-and-play installation, combined with its 480 Mbps data transfer rate, makes it an ideal interface for many data acquisition applications. Also, the popularity of USB in the consumer market has made USB components very inexpensive. USB's 5-meter range is perhaps its largest detraction as it limits the ability to implement remote and distributed I/O systems based on USB.

Today, board level solutions offering 24-bit resolution are now available as, e.g. 6.5 digit DMM boards. On the box side, USB 2.0 is capable of delivering 30 million 16-bit conversions per second and Gigabit Ethernet will handle more than twice that. The internal plug-in slot data transfer rates have increased 10 fold in recent years.

The market of data acquisition equipment is very large, different companies propose new solutions. National Instruments offers several hardware platforms for data acquisition. The most readily available platform is the desktop computer, with PCI DAQ boards that plug into any desktop computer. For distributed measurements, the Compact FieldPoint platform delivers modular I/O, embedded operation, and Ethernet communication. For portable or handheld measurements, National Instruments DAQ devices for USB and PCMCIA work with laptops or Windows Mobile PDAs. In addition, National Instruments has launched DAQ devices for PCI Express, the next-generation PC I/O bus, and for PXI Express, the high-performance PXI bus.
