PC-based instruments have made tremendous gains in the last few years, and now is a good time to take advantage of them. Not only is measurement performance better, but also more software is available. And when compared to conventional bench instruments, today’s PC-based units are much smaller in size as well as lower in cost.
But where do you start? When sorting through brochures, catalogs, data sheets and other information from instrument manufacturers, it helps to know what questions to ask. Here are some that could help save you time and money in your next PC-based instrument purchase.
1. How are the inputs and outputs protected against overvoltages?
The many techniques used to protect the external connections (inputs and outputs) of an instrument from overvoltages and current surges vary in the amount of protection they offer. Some instruments and data acquisition products do not have any overvoltage protection circuits and are susceptible to damage from overvoltages and static discharge. Many instruments protect their external signal I/O circuits by connecting diode clamps to an internal power supply. This type of protection is dangerous since an input voltage surge could “pull up” the internal power rail of the PC and substantial damage could occur. Also, this type of protection circuit does not work when the power is off. A better protection scheme is to reference the protection circuits to ground. This will protect the instrument whether the power is on or off, and it directs the current from the overvoltage condition to chassis ground.
The ground connection is also important: it has to be able to handle a large current surge and protect the instrument and the computer. Ask the manufacturer if the external connections are grounded directly to the rear panel bracket. This is easily accomplished by having a direct connection from the input connector’s ground to the chassis of the computer. Some instruments use the circuit board to provide current surge protection, which means the ground connection of the input is not directly connected to the rear panel bracket. The problem with this is that the current surge flows through the circuit board traces, the bus connector, the motherboard, and finally into the return of the power supply before it reaches chassis ground. A final question to ask is how the connections are protected against static discharge.
2. How does the instrument eliminate noise from the PC?
The two major sources of noise in a personal computer are the switching power supply and the digital logic (including the CPU) on the motherboard. The switching power supply generates noise with frequencies from about 10 kHz to 1 MHz, and the digital logic’s noise can have frequency components up to hundreds of megahertz. Make sure the manufacturer has taken the proper precautions to eliminate noise from the sources that are within the measurement (or source) bandwidth of the instrument you intend to buy. For example, an instrument with a measurement bandwidth of 1 MHz needs to be able to reject the noise generated by the switching power supply, while an instrument with a bandwidth of 100 MHz also needs to be able to reject the high-frequency noise generated by the digital logic within the PC. Ask the manufacturer what techniques are used. Standard techniques for rejecting power supply noise include on-board voltage regulators and good bypass capacitors. The high-frequency noise is rejected by using shields, multilayer circuit boards with external power planes, and special sampling techniques that reject any noise that is asynchronous to the triggering event.
3. How do I install the instrument?
Installing instruments into a PC involves the hardware installation and the software installation. Usually, a PC-based instrument will include a software package that helps you install your hardware. This installation program might automatically determine the base address of the board, it might show a picture of the DIP switch settings you should use, or it might even have a hardware verification routine that performs a self-test on the instrument. Ask your vendor how the installation is done. If you intend to have only a few instruments in your PC, installation is usually easy. If you plan to have many instruments (say, six to ten), then the installation is trickier since you’ll need to check for address and interrupt conflicts.
Installing instrument software is similar to installing any other PC software. Some vendors will automatically update all the necessary files (e.g., the autoexec.bat file and the config.sys file) and create all the necessary directories, while others will give you full control of the process. A good installation package will give you a choice between automatic installation and custom installation. Ask your vendor what files will be affected by the installation process and what control you have over the process.
4. Will the instrument work within my software environment?
If you plan on using DOS and/or Windows, this question is probably moot with most hardware vendors. If you use OS/2 or UNIX, or a network, you’ll need to find out what kind of software support the vendor provides for your software environment. Ask if other customers are using a similar software environment, and give them a call to see if they encountered many problems.
5. What are the required computer resources?
Questions concerning the computer resources can be divided into three groups: interface, memory and power. Interface questions include whether or not the instrument uses DMA, interrupts, I/O address space and/or memory-mapped address space. Questions concerning memory usage include the amount of RAM needed to run the software and how much disk space is required to store the software.
When asking about the power usage, don’t simply ask how much power the instrument uses; ask for a breakdown of the current draw on the +5 V, +12 V, -5 V, and -12 V power rails. Be sure your power supply can handle the current load of the instrument (most PC power supplies have a sticker that indicates the amount of current available on each rail). A good data sheet should answer all these questions, but give the vendor a call for more detailed information.
6. What software should I use?
There are usually three sources of software: the hardware manufacturer, a third-party vendor and yourself. Depending on your source of software, how will the hardware vendor support each of these? If you want to use the software provided by the hardware vendor, is the graphics user interface sufficient for your application?
Using a software package from a third party is a good idea especially if your test set has hardware from many different vendors. Does each vendor provide drivers for the software package you are going to use? How does the hardware interface to third-party packages such as LabVIEW and DADiSP? What is the cost of the drivers? Some manufacturers provide a lifetime free update service with their drivers. Find out the software upgrade policy of the hardware vendor.
If you are designing your own software, you’ll need to know what type of tools the vendor provides for your environment and if the vendor supports your compiler. Also, if you might be using a different software environment in the future, find out if the hardware vendor intends to provide support for your future requirements and how much it will cost.
7. Is the product complete?
Ask the hardware vendor what is included with the instrument. Does it include software drivers, example programs, graphical user interfaces or calibration routines? Does the vendor charge for applications support or software upgrades? Are connectors or probes included? Ask these questions before you make your decision so you won’t be surprised after you receive your instrument and find something missing.
8. What are the measurement parameters of the instrument?
This group of questions is independent of whether you are buying instruments-on-a-card or instruments-in-a-box. Ask the manufacturer about bandwidth, accuracy, voltage limits, noise, input impedance and any other measurement parameter that will be important to your application. Also, find out how fast the instrument can transfer information to the PC and how lively the user interface will be.
9. Does hardware performance depend on the CPU?
You can ask a manufacturer three questions to determine how independent the instrument is from the PC: Does the instrument have an on-board controller? Does the instrument have on-board memory? How is calibration performed?
An instrument with an on-board controller most likely will be able to function without assistance from the PC. For example, a measurement instrument with an on-board controller can continue to process data without help from the PC. In the case of an oscilloscope, it could be performing waveform averaging, or in the case of a DMM, it could be calculating the rms level. An instrument with on-board memory will be able to collect a full record of data without having to use the PC’s data bus to access the memory on the motherboard.
Three techniques are used to calibrate the data coming from a measurement instrument or the data going to a source instrument, and the first two of these do not involve the PC. The first technique is analog calibration, where analog circuit elements are adjusted (either with potentiometers or programmable devices such as D/A converters) so that gain and offset errors are eliminated from the waveform being sourced or generated. In the case of a measurement instrument, the amplifiers are calibrated prior to the ADC, and in the case of a source instrument, the output amplifiers are calibrated. This technique is optimal since it does not affect the throughput of the instrument and does not involve babysitting from the PC.
The second technique uses digital calculations, and utilizes the on-board controller (if the instrument has one) to calibrate the data after the acquisition has been completed (in the case of a measurement instrument) or during the waveform loading (in the case of a source instrument). This technique does not involve using the PC, but it does affect system throughput.
The third technique, and the worst, is to have the PC perform digital calibration of the data. This technique slows throughput of the system and requires that the PC manipulate the data mathematically. Ask the manufacturer which technique is used and how it will affect your application.
10. Does the product take advantage of the computer?
Many new instruments-on-a-card have capabilities that are hard to find in instruments-in-a-box. For example, a field service engineer can easily fax a waveform back to the factory from a PC-based oscilloscope. Some companies are using the remote control capability of PC-based instruments (via a modem) to make a service call more efficient. By using field-based test equipment and a modem, service engineers can run preliminary tests on the equipment before they travel to the field site, and therefore bring the necessary spare parts for the service call. And of course, archiving, documentation and analysis are all easy to do on a PC-based instrument. Ask your hardware vendor for a list of advantages for your particular application.
Conclusion
The measurement capabilities of PC-based instruments are growing in leaps and bounds, and more and more engineers are switching from boxes to cards. Whether you are purchasing your first instrument-on-a-card or are an experienced user, it makes sense to ask plenty of questions before you begin your project and commit your valuable time and money.
About the Author
Stuart Streiff is President of PC Instruments. He received his BS-ECE degree from the University of Wisconsin in 1978, and has spent his career in the design and marketing of instrumentation. PC Instruments Inc., 9261 Ravenna Rd., Building B11, Twinsburg, OH 44087, (216) 487-0220.
Copyright 1995 Nelson Publishing Inc.
February 1995