NI High-Speed Digitizers: Optimized for Automated Test

The gigahertz bandwidth barrier was broken decades ago — but up until this point, high-bandwidth digitizers and oscilloscopes have had features and functionality best suited for a benchtop use model. These instruments center on the interactive use case, focusing on features such as time to first measurement, waveform display update rate and visualization, and connectivity to a wide array of probes. One area that is frequently overlooked in this high-bandwidth space is the automated test use model, where measurement throughput and test system footprint can dramatically reduce the overall cost-of-test of a shipping product.

National Instruments offers a family of high-speed digitizers optimized for automated test. National Instruments high-speed digitizers range in performance from the 300 MHz, 2 GS/s PXI-5152 up to the 5 GHz, 12.5 GS/s PXIe-5186.  This paper discusses the distinguishing features that make NI digitizers ideal for automated test applications.       

Overview of Modular Instrumentation

A modular instrumentation system achieves flexibility by using commercial technologies. In a modular instrumentation system, many of the core components — such as the chassis and power supply — are shared across instrument modules. These instrument modules can include different types of hardware, such as digitizers/oscilloscopes, function generators, digital I/O, and RF. For PXI-based instruments, an embedded or remote controller is the computer that controls instruments, executes measurements, and performs post-processing/analysis. Years ago, it could be argued that a modular platform sacrificed measurement performance. Today, however, instruments designed with a modular instrumentation approach include some of the highest performance instruments available, including industry’s highest-resolution digitizer, the NI PXI-5922, and industry’s most accurate 7½-digit digital multimeter, the NI PXI-4071. 

Modular instruments — NI high-speed digitizers in particular — share three characteristics that make them optimized for automated test: high data throughput, tight synchronization between channels, and ease of integration with other types of instrumentation.

High Data Throughput

Companies incur a variety of expenses related to efficiently and accurately testing products. In addition to the initial test equipment expenditures, factors such as test time greatly influence your cost of test. While every application is unique and has specific needs, automated test systems always have some commonality. Bandwidth and latency are two of the most important considerations for an automated test system, because the combination of the two dictates the overall speed of your measurement system.

Figure 1.  Bus bandwidth and latency of popular instrumentation busses

Latency describes the amount of time it takes for an instrument to respond to a remote command, like a measurement query. Bandwidth refers primarily to the data throughput capacity of data bus that connects the measurement instrument with the host PC or controller. A high-bandwidth bus results in lower test times, no matter if the application is to transfer a large data record one time or transfer small data records multiple times in a row. The PXI platform — upon which NI high-speed digitizers are built — provides high speed for a wide range of applications with both high bandwidth and low latency via the PCI and PCI Express buses. PXI Express, a version of PXI that uses the high-speed PCI Express bus, achieves system throughput rates of several GB/s, depending on the chassis and controller. Both PXI and PXI Express data throughput rates are significantly faster than that of GPIB, USB, or LAN — other popular buses for automating test instrumentation (Table 1).

Table 1.  Data transfer rates of the NI PXIe-5185/86 versus an LXI compliant oscilloscope controlled over Gigabit Ethernet

Synchronization for Mixed-Signal or High-Channel-Count Systems

Through synchronization, you can match the timebase and triggering of several instruments of the same type for channel expansion purposes, or tightly correlate the inputs and outputs of different instruments. There are different methods of synchronizing instruments, but the main considerations are the sample clock rates and the phase differences of those clocks among multiple devices. If the required level of synchronization is not considered when a test system is being designed, the resulting measurement differences may cause problems with product quality and yields.

The key to perfect synchronization is the combination of sample clock alignment with triggering. The NI SMC (Synchronization and Memory Core) technology, which is foundational to all NI high-speed digitizers, employs a patented scheme whereby a separate clock domain signal is used to enable the driving and receiving of triggers. This signal, called the Trigger Clock (TClk), enables synchronization in the 10s of picoseconds among channels of NI digitizers and other instruments, resulting in significantly better performance than locking the sample clock to the 10 MHz reference clock only.

Figure 3. The PXI Express backplane simplifies timing and synchronization

Integration with Other Instruments

Test systems are typically composed of many types of instruments — including signal sources, measurement devices, and switches.  The PXI platform has unparalleled breadth, with modules for analog and digital I/O, high-speed instrumentation, vision, motion, and numerous bus interfaces. In fact, more than 1,500 PXI modules are available from the more than 70 vendors. With a PXI system and NI modular instrumentation, you can not only build a comprehensive test system in a single chassis, but also ensure measurement quality and repeatability because modules in the chassis can be synchronized with picosecond-level accuracy.

Figure 3. A wide variety of modular instruments are available in the PXI Platform

Additional Resources

Learn more about NI digitizers and oscilloscopes

Explore the advantages of the NI PXI platform

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