Multifunction Data Acquisition: Multiple Instruments on a Single Device

Standard benchtop instruments offer many features for specific types of measurements, defined by the instrument vendor to meet a wide range of performance levels and measurement needs. However, there are many applications that do not require the full performance of a dedicated instrument and would easily benefit from lower-cost alternatives that are more optimized to the job at hand. Testing a particular electrical design, for example, may not require all 50MHz of analog bandwidth on an oscilloscope or 5.5 digits of precision for a digital multimeter (DMM). It all depends on the signals that need to be measured, and for many applications, a single multifunction data acquisition (DAQ) device can provide the right level of performance, taking the place of multiple instruments.

PC-based DAQ devices are available from multiple vendors, and provide direct analog, digital and counter I/O for laptop and desktop computers. With user-defined software, the computer itself can function as a DMM, oscilloscope, function generator, logic analyzer and frequency counter all at the same time. Beyond the cost savings, one of the biggest benefits of turning a general-purpose computer into an instrument is the ability to automate measurement tasks like analysis, logging and reporting. This is the same reason that many benchtop instruments also include PC connectivity options over instrument control buses like GPIB, Serial or USB. Let’s take a look at how DAQ devices measure up to traditional instrumentation when it comes to test and measurement applications.

OSCILLOSCOPES
When trying to characterize and validate periodic waveforms, a good oscilloscope is a great tool to have. For lower-frequency waveforms that don’t require a lot of analog front-end bandwidth, the analog input channels on a DAQ device provide a good low-cost alternative to using standard benchtop oscilloscopes. It’s common for a DAQ device to have at least 8 analog inputs, and these could be multiplexed or simultaneous. Similar to any digital oscilloscope, DAQ devices use an analog-to-digital converter (ADC) and a sample clock to continuously measure a voltage signal and display the resulting waveform for processing and analysis. The primary difference is that a benchtop oscilloscope retains all samples in internal memory (the record length) where as a DAQ device streams the samples to RAM on your PC. The number samples displayed on an oscilloscope is limited to the maximum record length per channel, where as a DAQ device can continuously stream samples until the available PC RAM is used up. With the gigabytes of RAM offered PCs these days, analyzing large waveforms with millions of samples is no problem at all. The biggest trade-off with using this PC-based approach, however, is the maximum sampling rate available on most multifunction DAQ devices. For similar costs to a low-end oscilloscope, a multifunction DAQ device will have sampling rates of up to 2MS/s, with 1MHz analog front-end bandwidth. This is still good enough for many measurement applications, including electrical power measurements and interfacing with sensors and transducers, because many transducers don’t physically respond faster than that. If your voltage signals don’t require more than 1MHz of bandwidth, a DAQ device can turn your PC into an oscilloscope with user-defined math and analysis capabilities.


Figure 1: Oscilloscope application measuring two waveforms in LabVIEW.
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Figure 1 shows an example of a LabVIEW application that is used for oscilloscope measurements. The two channels are sampled and then data is passed to analysis functions perform an Fast Fourier Transform (FFT), calculate RMS values and take peak-to-peak measurements.

DIGITAL MULTIMETERS
Digital multimeters (DMMs) are the most commonly used electrical measurement devices in the world. They provide quick and easy snapshots of voltage, current, resistance and other electrical properties. Many applications, however, require measurements to be taken repeatedly, and measurement values to be recorded for further analysis or reporting. When DMM measurements need to be recorded, they are usually either manually written down and then typed into a spreadsheet program by hand, or logged locally and then electronically transferred to a PC afterwards. Using a multifunction DAQ device, the PC can directly take voltage measurements and perform the required processing, analysis and logging, which offers a low-cost way to automate DMM measurements. Plotting values on a graph, point-by-point as they are acquired, is particularly easy with PC-based I/O. Once the data is in PC memory, applying averaging, digital filtering or calculating true RMS values are also simple and straight forward using software. Figure 2 shows an example of typical DMM measurements using LabVIEW.


Figure 2: DMM application that plots point-by-point data and logs measurement values.
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When comparing DMMs to multifunction DAQ devices, one specification that might seem a little unclear is how the digits of precision on a DMM correspond to bits of resolution on a DAQ device. A 4.5-digit DMM, for example, might be able to represent 50,000 different numbers, depending on how the vendor implements the “half” digit. If compared to bits of resolution, this would be similar to somewhere between a 15-bit and 16-bit analog-to-digital converter (ADC).

(2^15 < 50,000 < 2^16)

16-bit multifunction DAQ devices on the USB bus start at around INR 40,000, which is similar to the entry point for many low-end benchtop DMMs. On the down side, most DAQ devices can only measure voltages up to 10V and don’t have built-in options for current and resistance measurements. On the other hand, DMMs only have a single channel where as DAQ devices have at least 8 analog input channels, and adding shunt resistors to make current measurements with a DAQ device is not difficult at all. Therefore if you have multiple channels, with measurements less than 10V, using your PC to automate your DMM measurements may be a good option for you.

FUNCTION GENERATORS OR ARBITRARY WAVEFORM GENERATORS
Up until now, we’ve mostly focused on the analog input capabilities of a DAQ device. In Test and Measurement applications, it’s also common to use an analog output instrument such as a function generator or arbitrary waveform generator. These are often used to evaluate the functionality of a device or circuit by sending in some known voltage waveform and observing the resulting response. The waveform shape might be standard (sine, square, triangle, saw tooth, etc.) or completely arbitrary and defined by the user. The analog output channels of a DAQ device can be used to provide a DC reference, or generate a continuous waveform to serve as a low-cost option for signal generation. Using onboard memory, a waveform can be downloaded and configured to continuously cycle through samples without any further PC involvement. If a waveform is too large to fit within the onboard memory, a circular buffer can be created within PC RAM and samples can be continuously streamed across a PC bus, such as USB, PCI and PCI Express. This gives you very flexible options for both periodic signals and signals that might be constantly changing. Figure 3 shows an example of a simple LabVIEW application that continuously generates sine and triangle waveforms with a data acquisition device.


Figure 3: Simple signal generation application in LabVIEW.
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In stimulus-response test applications, it’s important to correlate the stimulus output signals with the input signals being measured. By having analog input and analog output channels on the same device, you can easily synchronize the two by internally sharing sample clocks and start triggers. Achieving this same level of synchronization with two separate benchtop instruments can be cumbersome, requiring external cables to route timing signals.

As with the previous comparisons, there are definitely drawbacks to using a DAQ device as a function generator or arbitrary waveform generator. The biggest limitation is the maximum update rate, which is around 3MS/s for DAQ devices sold today. To use an example, if generating a sine wave with a minimum of 100 samples per cycle, maximum achievable frequency based on that update rate would be 30kHz. For applications that don’t require higher speed waveforms, such as test systems that simulate sensor or transducer output signals, the analog output channels on a multifunction DAQ device may meet your requirements just fine.

LOGIC ANALYZERS AND PATTERN GENERATORS
Digital logic analyzers are yet another example of instrumentation that can be served by a general purpose multifunction DAQ device. All DAQ devices offer digital input and output lines that operate on 5V or 3.3V TTL logic. For many of these devices, the digital lines have built-in timing engines, where digital inputs and outputs can be strobed or clocked to particular timing source. This provides a very low-cost option for creating a PC-based logic analyzer or digital pattern generator. With maximum clocking frequency of 10MHz, a DAQ device may not offer the full performance of a dedicated digital instrument, but it’s good enough for many applications like lower-speed digital protocol emulation, or interfacing with microcontrollers. In many embedded systems, common protocols like I2C and SPI are often used at rates less than 1MHz. Trying to test or emulate these buses with a PC provides the added flexibility of complete software-defined testing, with custom test patterns and inline analysis. In addition, synchronization with analog I/O also becomes much easier, because all clocks and triggers can be shared among different subsystems on a single device. When automating measurements, digital I/O can also be used to interface with switches and relays to control different parts of the test system and decrease overall test times.


Figure 4: Digital pattern generation in LabVIEW.
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FREQUENCY COUNTERS/TIMERS
Counters are extremely flexible, and can be used for digital event counting, pulse measurements, frequency and period measurements, interfacing with pulse-width modulation (PWM) or encoder signals, trigger generation, dividing down clocks and many other digital logic operations. All multifunction DAQ devices have onboard general purpose counters just for these types of needs, and with up to four counters per device, you get many options for generating or measuring digital pulses. A dedicated counter instrument is typically designed to address higher performance test requirements, with higher performance oscillators and selectable input ranges, but if your application can suffice with up to 100MHz clocking and 5V or 3.3V TTL logic, the built-in counters of a DAQ device can save you quite a bit of money. Beyond having the flexibility of software-defined systems, one of the unique benefits of using a PC-based counter is that you can continuously stream counter measurements across the PC bus, which is good for very sensitive applications where you’d like to know the exact time that certain measurements or events occur. Two 32-bit counters can also be cascaded to create a single 64-bit counter, where you can count to very high values without any fear of the count register rolling over. Using the onboard counters of a multifunction DAQ device, can provide you with a cost-effective PC-based solution for both simple and complex counter applications.

COST-EFFECTIVE, PC-BASED INSTRUMENTATION
Thanks to software-defined functionality, a general purpose Multifuntion DAQ device can take on different personalities to turn your existing computer into several different types of instruments. None of these DAQ-based instruments will ever have the performance of a benchtop instrument, dedicated to a certain type of measurement, but for applications that don’t need the highest bandwidth or maximum number of digits, a single device can take on the responsibilities of several instruments simultaneously, making a lower overall impact to your budget. DAQ devices come in quite a large range of options for price and performance, with the lowest-cost starting at around INR 10K and going up to several lakhs for higher performance.

Click here for more information on multifunction DAQ devices from National Instruments.

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