When implementing a data acquisition project, the first issue to address is the base architecture of the hardware. The choices encompass a wide range, including build it yourself from ICs and discrete components; assemble a PC-based system from ISA or PCI bus modules; interconnect GPIB instruments; assemble a VXI system, or purchase a stand-alone data acquisition system.
If you are not in the all-too-common position of using the equipment in the stockroom from the last project, two conflicting parameters—price and capability—dictate the choice of equipment. Both of these parameters are more complicated than they appear at first glance.
Evaluating Price and Performance
If we are to consider the price parameter accurately, the cost of the time required for system assembly, checkout, and debug must be included. This is the area of consideration where the higher-priced hardware often can be justified. The actual system cost is the equipment cost plus the cost of the time that is required to do everything from unpacking to installing and checking out the system.
Performance requirements dictate the data-handling capability in terms of the number of data channels and bits of resolution. The noise susceptibility, the intermodule data-transfer rate, thermal considerations, size, and expansion capability also must be addressed to assure successful system performance. Flexibility also should be considered, for two reasons. First, at the end of the project, the equipment will be better suited for the next project. Secondly, what project hasn’t changed in scope during its construction? A little margin included at the system specification stage will spare a good deal of grief later in the project.
Often in the case of industrial data acquisition projects where time to implement is the driving factor, hardware cost is of less concern than rapid, successful implementation of the system. In this environment, an excellent choice of system architecture with fewer potential pitfalls is a VXI system.
The VXI concept simplifies the system design with physical and electrical interface commonality that can be depended on regardless of who manufactured each module. The specification-compliant design of the VXI boards already has taken care of the EMI, RFI, and crosstalk issues. Each module is shielded electrically and physically in its own protective metal case, and the whole system is powered by and protected in the VXI mainframe.
The VXI system designer must select modules that accommodate the required number of inputs and provide the needed functions. The VXI implementation provides TTL triggering at 10 MHz and ECL triggering at >50 MHz, and its local bus allows transfer rates approaching 100 MB/s. The capability to move this much data enables this configuration to accept relatively large number of input channels at high input rates.
Input Channel Capacity
The small front panel of a VXI module is a limiting factor when it comes to the number of inputs that can be handled. However, high-density connectors can overcome this problem. Since VXI can cost more than other hardware configurations, manufacturers try to offset the cost by accommodating more inputs. This strategy makes the VXI products more competitive if the price per input channel is considered.
Paul Dhillon, executive vice president of VXI Technology, addressed this point: “Density becomes an issue with VXI. The cost of entry into VXI is higher than PC approaches, so to offer a VXI card with eight to 16 data acquisition channels only makes sense if the customer has many other products in the VXI mainframe.”
Unfortunately, this could relegate VXI to applications requiring massive numbers of input channels if it were not for the other advantages of VXI. These include small size, ease of integration, system integrity, and interference immunity.
Data Acquisition Board Trade-Offs
While the issues of speed and board configuration are similar to those faced with PC plug-in cards, the high VXI data transfer rate raises the ceiling on channels and acquisition rates. For very high bandwidth data applications, a configuration with a dedicated analog-to-digital converter (ADC) per input accommodates the needed high sample rate. For low-frequency applications, sharing the ADC among many input lines with a multiplexer saves cost, components, and PCB real estate.
The disadvantages of multiplexing the ADCs were addressed by Mike Wadzita, VXI product marketing manager at Tektronix. “There are several drawbacks to scanning ADCs, including increased noise due to switching, crosstalk between channels, and settling time when switching from one channel to the next,” he said.
In applications where high-resolution, 16-bit or more ADCs are being used, this can become an important consideration. One interesting solution to this trade-off is the availability of independent multiplexer and ADC cards which may be used alone to provide independent conversions. The multiplexer cards can be added as required to expand the number of inputs. This arrangement also provides flexibility even after the system is complete. “To serve the largest number of applications, many VXI vendors implement a scalable strategy regarding the number of I/O channels per slot,” added James Kimery, strategic marketing manager of test and measurement products at National Instruments. “For example, we use one base analog input module and expand the number of I/O channels by adding signal-conditioning multiplexer modules in adjacent slots. In this way, the base I/O module can handle the small applications when used separately, and the user can add signal-conditioner/multiplexer modules as their needs expand.”
On-board processing is another option to be considered as inputs increase to the point of taxing the VXI bus capability. With on-board processing, some mathematical operations are performed on the acquisition card.
When asked about the pros and cons of this technique, Ted Miller, marketing manager at Racal Instruments, said, “For low- to medium-volume test scenarios, data processing is best done off-board because software environments, such as LabVIEW and LabWindows/CVI, have built-in features that make data manipulation very easy. Ideally, the test software should have fast access to the analog data so that it can be processed into test results quickly.
“For high-volume, high-speed test scenarios, the test software may slow down system throughput, making on-board processing more attractive. Users point out that they can’t always trust the mathematical reliability of on-board data-processing utilities because they don’t have the source code and therefore can’t prove that the algorithms are valid,” he concluded.
Even more definite about separating the data acquisition and post-processing functions was Robert Cleary, CEO at KineticSystems. “We have chosen to limit the signal processing on the ADC cards to providing functions such as limit checking,” he explained. We perform signal processing and analysis in a separate DSP module or in the host computer software. This gives the greatest degree of versatility and provides the easiest way to make changes when needed.”
The CX2401 VXI C-size Analog-to-Digital Converter samples a single input at a speed of 1 billion S/s or two inputs at half that rate with an 8-bit resolution per sample. The card includes triggering and synchronization circuitry, dynamic memory of up to 256 MB, and a high-speed local bus with up to an 80-MB/s data transfer rate. Plug-and-play software and LabVIEW instrument drivers are included. From $15,900. MicroSignal Technology, (630) 325-8173.
Modular Design Offers
Flexible, High-Density I/O
The 6700 Series VXI I/O Modules allow a mix and match of up to eight submodules on one Model 6753 Motherboard. Channel densities of 192 A/D, 128 D/A, or 384 digital I/O in a single slot can be achieved. The submodules provide 48 channels of digital I/O, 24 channels of 16-bit A/D, or 16 channels of 16-bit D/A each. An autonomous data acquisition mode accommodates a complete data scan without software intervention. VXIplug&play, LabVIEW, and LabWindows/CVI drivers are included. Starts at $1,695. Racal Instruments, (800) 722-2528.
16-Channel Digitizer Offers
Many Configuration Options
Up to three Model VM2616 Digitizer Cards can be installed in one VM9000 VXI C-size base unit to provide up to 48 independent A/D channels with true differential input, 16 bits of resolution, and 100-kS/s speed. Data is acquired when a trigger is applied via the external front panel, an input channel, a word command, or the VXI TTL trigger bus. The on-board memory can be allocated within four channel blocks and is expandable to 512k. VXIplug&play drivers are included. Starts at $5,500. VXI Technology, (714) 955-1894.
C-Size Modules Create
1-GHz Virtual Scope
The TVS600A Series of C-size VXI are available in four models with either two or four channels, 250 MHz or 1 GHz bandwidth and sampling rates of 1 or 5 GS/s. A soft-panel display, a 10-mV to 100-V full-scale range, and switchable 50-W or 1-MW input impedance combined with signal analysis and waveform template testing capabilities are incorporated along with all standard oscilloscope control functions. Input BNC connectors accept all standard scope probes. $6,800 to $25,900. Tektronix, (800) 426-2200, press 3, code 1031.
20-MS/s ADC Provides
Filter and FIFO Functions
The E1437A is a C-size ultra-linear digitizer that allows characterization of signals in the time and frequency domains with 23 bits of raw amplitude resolution. A high-speed local bus provides an 80-MB/s data transfer rate. The unit features 8 MB of FIFO memory for seamless acquisition, a switchable antialiasing filter, and picosecond time-base resolution. Twenty-four built-in real time digital filters reduce spurs and ringing. VXIplug&play software is included.$16,320. Hewlett-Packard, (800) 452-4844.
VXI Controller Simplifies
Upgrade With Daughter Board
The VXIpc-850/200 Controller is a 200-MHz Pentium version of the company’s embedded VXI controller. The C-sized card uses an integrated local PCI bus structure and a MITEÔ ASIC to provide high-performance DMA and VME 64 compatibility. The controller is a full-function PC in a VXI form factor aimed at high-speed, real-time VXI applications. Starts at $9,595. National Instruments, (800) 258-7022.
C-Size Module Offers
The V213 is a single-width, C-size register-based VXI module that digitizes as many as 64 analog voltage channels. The 16-bit ADC scans all channels at a user-selected rate. The V213 also has programmable gain that can be set on a channel-by-channel basis. On-board calibration is available on each channel for end-to-end calibration. Starts at $3,895. KineticSystems, (815) 838-0005.
Copyright 1998 Nelson Publishing Inc.