Configuring a data acquisition system (DAS) is becoming easier every day, with more suppliers eager to provide you with all the information you need. But with the awesome variety of hardware and software now available, making an optimum choice between competing approaches is not easy.
Acceptable input voltage levels, maximum sampling rates and achievable accuracies are usually supplied, and it is relatively easy to determine whether these characteristics will satisfy your needs. But the kind or extent of signal-conditioning required and the appropriate equipment form factors are very much dependent on the type and number of signals to be acquired, sensor characteristics and the installation environment.
For most DAS applications, sensors or transducers translate physical events into electrical signals. The nature of these signals can differ:
Analog or digital.
Linearly or non-linearly related to the physical quantity measured.
Smaller or larger than the acceptable input range of the DAS.
To ensure a proper match between the sensor output and the DAS processing characteristics, signals are often preconditioned.
Diverse Forms and Purposes
Signal conditioning encompasses—but is not limited to—signal scaling, buffering, isolation and filtering and provides sensor-excitation power where necessary. “Essentially, signal conditioning covers everything needed to prepare the sensor output so that it can be applied to the digitizer,” said Rick Lagrand, vice president sales and marketing at ACCES I/O Products.
Typical signal-conditioning functions include:
Sensor excitation—
Some analog sensors, such as strain gages, require an excitation source to provide a measurable electrical output. Ordinarily this is a DC voltage or current. Constant voltage sources are most common but current sources often serve as an excitation source for quarter- or half-bridge strain gages and provide power for piezoelectric sensors with built-in or in-line electronics.
Other sensors, linear variable differential transformers for example, require AC excitation. “For these units, our LVDT-8 signal conditioner/multiplexer provides AC excitation for up to eight LVDTs, demodulates their outputs to DC, and multiplexes the outputs for application to an analog-to-digital (A/D) converter,” explained Mr. Lagrand.
Signal scaling—
Low-level output sensors such as thermocouples generate signals in the millivolt range, and amplification is mandatory. If the sensor-to-DAS distance is relatively short and the environment is not too noisy, the amplifier may be located in the DAS. Otherwise, a separate signal-conditioning unit, located near the sensor, must furnish the necessary gain to achieve a suitable signal-to-noise ratio and to fully use the entire dynamic range of the A/D converter.Isolation—
Isolation is mandatory in cases where the sensor output contains an offset voltage that exceeds levels acceptable to your DAS or where high-voltage transients may be picked up by your sensor or interconnecting wiring. Optocouplers or isolation amplifiers are used most often for this purpose.Buffering—
Unfortunately, many DAS multiplexers today impose performance penalties when signals are generated by high-impedance sources. “This is especially true as sample rates increase and the AC impedance of the multiplexers decreases,” commented Roger Lockhart, vice president at DATAQ Instruments.
Excessive channel crosstalk is symptomatic of this situation. The impedance mismatch is usually remedied by applying a buffer amplifier between the sensor and the DAS or, if possible, substituting low-source-impedance sensors.
Filtering—
Two types of filtering requirements must be considered. The first addresses signal contamination caused by a noisy environment and the second is applied to prevent aliasing.
Signal contamination may be prevented by using twisted-pair or shielded leads and by avoiding ground loops. But if these measures are inadequate, filters with cutoff frequencies targeted to minimize or eliminate the specific interferences may be required. Some A/D converters, specifically the integrating type, innately filter out high-frequency noise.
Aliasing, an effect that adds apparent (originally non-existent) lower-frequency components to the processed signal, is an inherent potential problem for any sampled data system. A sampling rate that is at least twice the highest-frequency component of the applied signal prevents this. Consequently, aliasing may be avoided by placing a low-pass filter with a cutoff frequency of less than half (usually one-third to one-fifth) of the sampling rate between the sensor signal and the A/D converter.
For many physical-measurement applications, sampling rates are so much greater than the highest-frequency signal components that anti-aliasing filters are not required. As long as this is the case or where appropriate anti-aliasing filters have been used, special signal filtering can be performed after A/D conversion to highlight certain data characteristics. Several digital filtering options are provided for this purpose by most analysis, data or display processing software packages.
Should Filtering Follow Amplification or Vice Versa?
Anti-aliasing filters must be placed ahead of the A/D converter. But if amplification is needed, should that take place before or after filtering?
“We recommend that you amplify the signal before filtering, instead of using the amplifier on an A/D board,” said Robert Galter, president of Alligator Technologies. “Doing so produces a higher signal-to-noise ratio.
“If the amplifier is on the A/D board (post-filter gain), it amplifies not only the signal, but also any noise added by the common measurement-system environment, such as the A/D input circuit or the computer interconnections. On the other hand, pre-filter amplification can make an originally low-level signal override the measurement-system noise, providing a dramatic improvement in signal-to-noise ratio. Using pre-filter amplification also eliminates the restrictions and delays associated with changing a multiplexed A/D board gain between samples,” Mr. Galter concluded.
But since amplification on the A/D board is almost always available and usually performed after multiplexing, a single programmable gain amplifier on the A/D card may be shared by all channels, providing a potential cost advantage. The correct choice
depends on the signal characteristics, required accuracies and resolution, and the noise environment of the application.
Sensor Options Influence System Contents
Sensor selection influences signal-conditioning requirements and both affect system implementation. Larry Copeland, marketing manager at DATEL provided an example of how the many choices can influence the system design even for a single type of application, such as temperature sensing.
“Look at all the ways temperature can be measured,” Mr. Copeland said. “There are thermocouples, resistance-temperature detectors (RTDs), thermistors, bolometers, radiometers, IR sensors and more. Each of these requires special signal-conditioning circuits. RTDs need a precision bridge and TCs need sensor linearization and cold- junction compensation.
“The TC linearization can be performed three ways: full lookup tables, piecewise linear extrapolated tables and the polynomial equation method. Each of these trades off memory size and CPU time. The equation method requires floating-point and transcendentals-equation-solving capabilities which can be provided by a math chip or by algorithm. And that is just for temperature sensing,” Mr. Copeland concluded.
Keep in mind that signal conditioning must be tuned for the application and that sensor, conditioning and data processing capabilities mutually affect each other. Since most DASs must have the capabilities to acquire, measure and process data from a variety of sensors, signal conditioning must be implementable in a flexible manner.
Physical Implementations
Signal-conditioning circuits may be located on an assembly situated within a dedicated enclosure or in a self-contained DAS, reside on an A/D board, or be placed on a separate PC-plug-in board. Optimum placement depends on the number of sensors to be accommodated by the DAS, sensor-to-DAS distance, sensor signal levels, the noise characteristics of the environment and, if the DAS is PC-based, the space available within the PC.
In the latter case, when more than 16 inputs require conditioning, perform the conditioning outside the PC, suggested Don Di Rocco, president of ADAC. “But for applications with a small number of inputs that require conditioning (eight thermocouple inputs, for example), some data acquisition cards offer a unique solution,” Mr. Di Rocco said. “These cards contain the A/D conversion function and signal-conditioning circuitry on a single plug-in ISA card.
“This solution places the signal conditioning inside the PC and saves money since there is no need to buy external signal-conditioning accessories. Some DAS vendors claim that, due to the noisy PC environment, you must perform signal conditioning outside the PC. However, careful attention to shielding and board design
proves this theory is completely false,” Mr. Di Rocco concluded.
To provide flexibility, some companies place the signal-conditioning circuits on daughterboards or similar replaceable modules. “For instance, the DCP5B modules interface to a specific sensor and are selected on a per-channel basis,” said Scott Walsh, applications engineer at Analogic. “Each module contains circuitry that addresses all three basic signal-conditioning requirements; that is, amplify, filter and isolate.”
For many applications, minimizing the sensor-to-conditioner distance is still preferred, which normally requires an additional enclosure external to the DAS. However, a new implementation scheme combines the signal conditioning and the A/D conversion circuits into an assembly small enough so that it may be easily located adjacent to a group of sensors. The resulting digitized signals are transmitted to a PC or workstation through dedicated or network facilities.
Intelligent Instrumentation, for instance, now offers the Ethernet data acquisition system consisting of one or several boxes that can be placed next to the sensor. “The signals are conditioned, converted and transmitted to the PC over Ethernet,” explained Rob Winkler, product marketing engineer. “The Ethernet cable allows the DAS to be placed up to 100 meters away from the PC.”
Similarly, GW Instruments now offers the instruNet System that consists of a PC- or Mac-compatible network controller card and one or multiple daisy-chained network devices. Each network device accepts 16 analog inputs and digital I/O and contains signal-conditioning amplifiers for each channel as well as A/D conversion facilities.
One of the smallest implementations of this type is the Intelligent Link Module System from Keithley. It comes in various versions, each matching a particular sensor class, and provides a variety of network interface choices for its output.
“Signal conditioning in these units takes place internally after the multiplexer, which holds down costs since a separate signal conditioner is not required for each channel,” commented Frank Pereira, marketing manager at Keithley Instruments. “Some of the conditioning, such as scaling and filtering, takes place in the CPU after
A/D conversion. Doing it there provides a high level of flexibility and performance while maintaining low cost.”
Many choices exist, not only for selecting sensors and any required signal conditioning facilities, but also for the physical implementation. Start by selecting a sensor that provides the needed conversion accuracy and does not require
excessive amounts of support and processing resources.
The expected input data rate and the number of sensors to be accommodated determine the sampling rates. The DAS configuration is established in accordance with the data rates and accuracy. Sensor signal levels, the environment and sample rates determine the type and optimum location of the signal conditioners and the required A/D features.
Data Acquisition Products
Programmable Filter Card Meets
Need for 16-Bit A/D Resolution
The AAF-16 is a plug-in low-pass filter card designed for anti-aliasing and noise- elimination applications requiring A/D resolution to 16 bits. The card accommodates 16 channels, each of which provides 8-pole Cauer, Bessel, Butterworth or high-speed linear-phase characteristics with software-selectable cutoff frequencies between 2 Hz and 200 kHz and attenuation floors to 120 dB/octave. Input/output ranges extend to ± 10 V and stop-band rejection is >90 dB. Menu-driven software, LabVIEW drivers and libraries for DOS and Windows are provided. $3,995. Alligator Technologies, (714) 850-9984.
Digital I/O Board Has
50-MHz Clock Rate
The GT50-DIO Dynamic Digital I/O Card features 32 bidirectional I/O pins and a 50-MHz clock rate. Up to eight boards can be daisy-chained to provide a 256-bit data acquisition or test system. Memory behind each pin may be configured from 64-kbits to 1 Mbit. Boards are I/O mapped and can be programmed by reading/writing to on-board ports. DOS and Windows libraries are provided. Programming can also be accomplished with the company’s DIOEasy software. Geotest, (800) 330-9774.
Features Added to Integrated
Acquisition, Analysis Software
Snap-Master™ Version 3.1, an integrated ensemble of data acquisition and analysis software for Windows, features added analysis functions and a new sensor data base. The sensor function performs scaling, conversion, channel assignments and sensor data-base management. Enhanced analysis features include two-dimensional integration with respect to another channel, simplified time-domain analysis and an expanded industry-specific frequency-domain FFT. A new thermocouple linearization element replaces signal conditioning hardware. Data acquisition module: $995; waveform analyzer: $495; frequency analyzer: $495. HEM Data, (810) 559-5607.
Recorder Stores
Up to 102 MB of Data
The PANTHER Model EDR-4 Series Recorders are self-contained, user-programmable acceleration sensor and recorder units. The 5-lb system performs remote stand-alone shock and vibration measurements and recordings from several hours to many weeks. It features a built-in piezoresistive triaxial accelerometer; 12-bit, 15-kS/s digitization per channel; and storage to 102 MB. The battery-powered equipment has a real-time clock and a 110 k-baud serial and parallel-port data interface. The recorder acquires data on three or six channels simultaneously. Instrumented Sensor Technology, (517) 349-8487.
DAS Provides Remote
I/O Over Ethernet
The EDAS-1002E-1 Multifunction System, the newest member of the EDAS™ family of Ethernet-based data acquisition systems, provides 16 single-ended/8 differential analog inputs at 12-bit resolution, two 12-bit analog outputs and 16 digital I/O channels. Designed for remote I/O applications, it communicates over any Ethernet local area network or the Internet. The A/D converter samples waveforms up to 100 kHz. A programmable gain amplifier extends the input range with gains of 1, 10 and 100. $1,195. Intelligent Instrumentation, (800) 685-9911.
On-Board Intelligence Enables
Lab-Grade Measurements
Intelligent Link™ Network Measurement Modules perform measurements at remote locations with laboratory-grade quality. The modules feature on-board intelligence, provide control and signal conditioning, and enable transmission of processed signals to remote PCs or controllers. Intelligent Link accepts a class of analog input signals and matches a specific network/protocol interface at the output. Input classes include strain/force/weight bridges, precision RTD, thermistors, DCV/W and torque transducers. Output networks/protocols include Ethernet, RS-232, RS-422, RS-485, GPIB and other common interfaces. $550 to $2,200 per module. Keithley Instruments, (800) 552-1115.
Data Acquisition System
Can Multiplex 4,096 Channels
DAS-Scan is a rack-mounted data acquisition system that multiplexes up to 4,096 channels from analog and thermocouple measurement devices into a single PC expansion slot. Allowing expansion in 64-channel increments, DAS-Scan accommodates up to 896 channels in each full 19″ rack configuration and up to 448 channels in a half-rack. Multiple racks can be daisy-chained. The scan rate is 2,000 channels/s. ASO-SCAN software provides function call drivers supporting Windows 3.1/95/NT, C/C++ and Visual Basic. From $3,350. Keithley MetraByte, (800) 348-0033.
Signal-Conditioning Card
Offers Programmable Filters
The DBK4 is a two-channel dynamic signal-input card that conditions signals of two piezoelectric transducers such as accelerometers or microphones. Each DBK4 channel is equipped with a software-programmable gain amplifier and an anti-aliasing filter, a simultaneous sample-and-hold amplifier, and a constant-current source output. Eight cutoff frequencies (141.6 Hz to 18 kHz) are software selectable. The bandwidth can be extended to 40 kHz by bypassing the anti-aliasing filters. DBK4 is compatible with DaqBook parallel-port-based data acquisition systems, and the DaqBoard ISA-bus and Daq PCMCIA A/D cards. $795. IOtech, (216) 439-4091.
Network DAS Windows NT 32-Bit
Software Records 500 kS/s
PI660, the latest member of Pacific Instruments’ GRASP data acquisition and analysis software family, programs transducer signal conditioning, calibrates channels and displays real-time data while recording to disk. PI332 companion post-test data-analysis software displays multiple channels in engineering units in time and frequency domains, performs digital filtering and has a BASIC-like macro language for data manipulation. Network support facilitates multiuser programming and data distribution. Test setups are stored in Microsoft Access data-base files.
Pacific Instruments
, (510) 827-9010.Data-Analysis Package
Works With Microsoft Excel
DADiSP, the graphic spreadsheet for scientific data analysis, now works with Microsoft Excel to transfer data from Excel spreadsheets into DADiSP. From DADiSP, you can initialize Excel, retrieve and send data to Excel, transfer data automatically as cell updates, and execute commands. From Excel, you can retrieve analyzed results from, and send data to DADiSP, and activate commands. DADiSP/WIN 4.0: $1,895; DADiSP/DOS: $995. DSP Development, (617) 577-1133.
Portable High-Speed DAS Family
Connects to Parallel Port
The DATAshuttle-Express family compliments the existing DATAshuttle line of parallel-port data acquisition systems. It features acquisition speeds of up to 100 kHz, 16 differential analog inputs (12-bit), two analog outputs (12-bit), two counters/timers (16-bit) and a 24-line digital I/O parallel port interface. Up to 240 analog channels may be accommodated. The unit performs signal conditioning and has eight software- settable gain ranges. There is no need to set board addresses or adjust calibration. Starting at $1,795. Strawberry Tree, (800) 736-8810.
Multipurpose PCMCIA Card
Supports 100-kHz Sampling Rate
The DAQP-208 is a Type II PCMCIA data acquisition card with eight single-ended or four differential 12-bit multiplexed A/D input channels (expandable to 128). The maximum sampling rate is 100 kHz and programmable gains are 1, 2, 4 or 8. The DAQP-208 also provides two 12-bit D/A output channels with direct or synchronized updates and four digital I/O channels. A programmable scan list provides up to 512 entries for gain and scan speed selection. DAQDrive software is included. $695. Quatech, (800) 553-1170.
Lab-Based DAS Features
RAMDISK Data Transfer
The new lab-based AC-powered MEGADAC Data Acquisition and Signal Conditioning Systems are more compact and modular than earlier MEGADAC systems, and offer features including RAMDISK data transfer. The size has been reduced by housing the CPU, memory and analog control circuitry on a slide-in module of the same height as the MEGADAC input modules. The MEGADAC 5414AC handles data rates up to 250,000 S/s; the MEGADAC 3415AC accommodates selectable rates to 25,000 S/s. From $8,000. OPTIM Electronics, (800) 234-5430.
Configurable Platform Offered
With More Than 30 Modules
The PRESYS 1000 System combines the functions of a data acquisition system and a data-conversion system in a reconfigurable instrumentation platform. More than 30 interchangeable modules may be installed including A/D converters with up to 16-bit resolution and conversion speeds to 1 MHz, D/A converters, multiplexers, programmable amplifiers and filters, and simultaneous sample-and-hold amplifiers with ± 0.003% accuracy. Interfaces are available for more than 15 types of computers, including PC compatibles and workstations. Self-test and diagnostic functions are provided. From $9,900. Preston Scientific, (714) 632-3700.
Sensor-to-Computer Interface
Pods Offer Alternative I/O
REMOTE ACCES™ Intelligent I/O Pods interface remote analog and digital devices with any host computer. Housed in a rugged, stackable NEMA4 enclosure, the pods combine solid-state optically isolated I/O with stand-alone multichannel operation. Up to 31 pods may be linked on a single two-wire multidrop RS-485 network, using half-duplex serial communications protocol. The series consists of five models, including a two-channel, 24-bit analog input pod; an eight-channel, programmable A/D converter/controller, and a 54-bit digital input pod. From $325. ACCES I/O Products, (619) 693-9005.
Data Acquisition System
Uses Parallel Processing
System 7 is a parallel-processing, multichannel, high-speed data acquisition system including an embedded 486 computer. Multiple 68000 processors facilitate wideband measurements, simultaneous real-time computing, storage, plotting, display and control. Signal conditioners are available for any standard signal source. A user- programmable test sequencer provides internal/external test control. Additional features include digital filtering, limit checking, automatic channel calibration, extensive post-processing facilities and multiple real-time displays. A separate linear power supply prevents signal contamination. From $9,500. S. Himmelstein and Co., (847) 843-3300.
Network Link Enhances Remote
Data Acquisition/Conditioning
Two new entries add remote communications capabilities to the SCXI signal conditioning and I/O interfacing product line. The SCXI-2000 chassis is a rugged shielded enclosure accepting four modules. The SCXI-2400 module adds an RS-485/RS-232 interface to SCXI systems for long-distance communications. The RS-232 interface connects a SCXI chassis to the PC serial COM port; the RS-485 interface is appropriate when longer-distance communications or more chassis (up to 31 on a network) are required. SCXI-2000: $995; SCXI-2400: $795. National Instruments, (800) 433-3488.
Data Acquisition Boards Offer
Extended Temperature Option
An extended temperature option is now available for the AIM Series of high- speed, high-resolution PC/104 data acquisition boards. The option guarantees full specification compliance over a temperature range of -40° C to +85° C. The AIM16-1/104T is a 16-bit 100-kHz analog input board that provides 85 dB of spurious-free dynamic range. The AIM12-1/104T is a 12-bit, 100-kHz analog input board. Both products feature 16 analog inputs and 16 lines of digital I/O, DMA and interrupt operation. AIM16-1/104: $699; AIM12-1/104T: $499. Analogic, (800) 446-8936.
PCI Board Optimized for Signal
Processing, Data Recording
The PCI-416 Analog Input Board Family consists of several data acquisition boards based on the 32-bit PCI bus architecture. Exploiting a “banked” first-in, first-out architecture, the PCI-416 moves two A/D words in each 32-bit PCI transfer. Pluggable analog options offer up to 16 input channels; multiple input ranges; sample rates to 10 MHz; 12-, 14- or 16-bit resolution, and various simultaneous sampling choices. The PCI-416SET software implements a menu-driven, no-programming fast data recording system to memory and disk. DATEL, (508) 339-3000.
Data Acquisition Boards
Stress Low-Cost
The Value-Line Data Acquisition Board family includes four models featuring ± 10-V inputs with programmable gain. The eight-channel, 12-bit A/D multifunction 5500MF provides 25-kHz throughput plus 16 digital I/O. The high-speed 5508LC features 100-kHz throughput plus DMA. The multifunction 5516DMA offers 12-bit A/D inputs and 60-kHz throughput. The high-resolution 5500HR has a 16-bit A/D and 15- kHz throughput. The latter two models feature 16-channels, DMA and eight digital I/O channels. 5500MF: $195; 5508LC: $245; 5516DMA: $295; 5500HR: $595. ADAC, 617-935-3200.
32-Channel Digital Input Card
Provides 2,500 V Isolation
The PCL-733 Card features 32 isolated, bidirectional digital input channels which provide 2,500 V of protection. The PCL-733 offers two PC interrupts with jumper-selectable interrupt request levels. The input voltage range extends from 5 to 24 VDC and the maximum throughput is 10 kHz. Each input channel corresponds to a bit in a PC I/O port, making the unit easy to program. The board is equipped with a D-type connector. Advantech, (408) 245-6678.
16-Channel A/D Card
Designed for PC 104 Systems
The 150-kHz PC104-DAS16/JR Card conforms to the PC104 form factor. Available in a 12-bit or 16-bit version, the card accepts unipolar/bipolar inputs and features software-selectable gains. Sampling of A/D signals is switch selectable for 16 single-ended or eight differential channels. A 512-sample first-in, first-out buffer ensures the transfer of large blocks of data without missing samples. Triggering can be performed through software, internally paced or externally triggered. Three 16-bit counters and eight digital I/O lines are provided. 12-bit: $399; 16-bit: $499. ComputerBoards, (508) 261-1123.
Single-Box PC-Based DAS Offers
Built-In Signal Conditioning
The DI-500 Series consists of a line of 10 data acquisition products designed to simplify industrial measurements where signal conditioning is typically required. The DI-500 comes in a portable or a desktop configuration and connects to the parallel port of any PC. The backplane accepts up to 32 industry-standard 5B Series plug-in signal conditioners. Sixty different types of 5B modules are offered, all with built-in 1,000-V isolation. The system is supported by WinDaq recording, playback and analysis software for Windows. From $3,995. DATAQ Instruments, (800) 553-9006.
ISA Bus Card Features 12-Bit
A/D Conversion at 100-MS/S Rate
CompuScope 8012 is an IBM PC/AT-compatible ISA bus card featuring 12-bit A/D conversion at real-time rates up to 100 MS/s in the single-channel mode and 50 MS/s in the dual-channel mode. The unit provides a bandwidth of 40 MHz, a 60-dB dynamic range, up to 4 MS of on-board memory, programmable input gain/offset and coupling, and scope-like internal/external trigger capability. A multiple-record mode “stacks” data from successive triggers. Gage Applied Sciences, (800) 567-GAGE (4243).
Networked DAS Locates Analog
Circuits Adjacent to Sensors
The instruNet® Data Acquisition Hardware/Software Family consists of network devices, a controller and interconnecting cables. Each network device accepts 16 analog inputs and provides eight analog outputs and eight digital I/O bits. Digital electronics, including a 32-bit microprocessor and memory, are located on a Macintosh or a Windows 95 x86 PC plug-in board. Network devices may be daisy-chained to one controller to form a networked data acquisition system. The instruNet World application program manages, monitors and operates the network. Controller: $590; Network Device: $790. GW Instruments, (617) 625-4096.
Copyright 1996 Nelson Publishing Inc.
November 1996
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