Customer SolutionsEvaluating High-Performance Digital-to-Analog Converters Using NI LabVIEW and Modular Instruments
Author(s):Michael Haag, National Instruments; Natasa Lingurovska, National Instruments
Industry:Electronics
Product:High-Speed Digital I/O, LabVIEW, Modular Instruments, PXI/CompactPCI
The Challenge:Developing a test solution to benchmark and analyze high-performance digital-to-analog converters with a low-voltage differential signaling interface.
The Solution:Using National Instruments LabVIEW and NI high-speed digital I/O instruments to automate custom digital-to-analog converter benchmark tests, including spurious-free dynamic range, noise floor, and total harmonic distortion analysis.
The Need for In-House DAC Characterization Whether in high-end instruments or standard consumer electronics, analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) are critical components of most digital devices. The types of data converters used today range from low-cost, generic devices to high-performance, very accurate converters. The majority of electrical devices, such as DVD players and PC audio cards, use inexpensive and low-quality converters. For high-precision devices such as advanced instruments and communication systems, converter quality is of greater importance because it determines the maximum level of quality that the digital system can attain. Data converter manufacturers rely on in-house test solutions to evaluate each converter, but each manufacturer uses different testing strategies and specifies different characteristics. Because of the discrepancy between various converters’ specifications, designers using these chips must also be able to perform their own characterizations. Engineers are turning to the accurate, low-cost PXI test platform to develop standard, in-house test strategies they can use to accurately compare different converters and verify their performance. The Challenges of Testing Next-Generation DACs While a number of standard test systems exist for common data converters, many current test systems are incapable of keeping up with the increased speeds and higher resolutions of new DACs. To support the need for higher performance, many new converters have switched to a low-voltage differential signaling (LVDS) interface. In addition, the dynamic tests for spurious-free dynamic range (SFDR), noise floor, pulse response, and total harmonic distortion (THD) measurements require the reduced noise available on a differential interface. This presents a challenge to engineers, because many traditional test instruments cannot accommodate new DAC requirements like LVDS and high data rates. Engineers typically try to overcome this challenge by evaluating data converters with a predefined evaluation board and software from the manufacturer. For new, high-performance converters, however, it is often difficult and cumbersome to obtain an evaluation board, which they then can only use for a limited period of time. Even with the evaluation equipment, engineers must run many specific tests through custom software with complex, ad hoc test systems. Performing custom, detailed evaluation thus becomes a time-consuming, manual process. These challenges exemplify the need for a flexible, in-house test solution with minimal restrictions. The LVDS Interface – High Data Rates and Low Noise LVDS is an emerging high-speed digital interface for applications that demand low power consumption, high noise immunity, and high data rates. In contrast to traditional single-ended signals, differential signals use two complementary low-voltage lines instead of one line to transmit a signal; the two signals are generated of opposite polarity, and then the data transmission references the two signals to one another. This transmission scheme provides large common-mode rejection and noise immunity that a single-ended system referenced to ground cannot provide. These benefits make LVDS the preferred interface for new, high-performance digital systems such as PCI Express, HyperTransport, and next-generation DACs. High-performance data converter manufacturers, including Analog Devices and National Semiconductor, are beginning to use LVDS interfaces to meet the increasing demand for faster data rates. The LVDS interface can receive high-speed and wide-bandwidth data signals while maintaining low noise and distortion. Additionally, some manufacturers have been able to reduce the number of pins required to transmit data from one point to another by serializing the output bits onto an LVDS pair. This further improves accuracy by avoiding skew-related errors that occur when transmitting data in parallel. Testing Key DAC Specifications To address the challenges of developing a test solution that can communicate with an LVDS interface, engineers have used the National Instruments PXI-6562 high-speed digital I/O device to test next-generation DACs. The NI PXI-6562 offers 16 bidirectional LVDS channels per module with deep onboard memory, pattern sequencing, and a maximum data rate of 400 Mbits/second. Engineers can tightly synchronize the PXI-6562 with multiple boards to address high-channel-count devices. The speed and flexibility of the PXI-6562 make it ideal for engineers who need to perform in-house DAC benchmark testing. One of the main characteristics of the PXI-6562 is its easy synchronization with other modules – which makes it a great fit for DAC testing. With multidevice synchronization, engineers can test high-channel-count DACs and accurately correlate digital and analog measurements. The PXI-6562 offers both internal and external clock sources, as well as a highly accurate synchronization method called T-Clock (TClk), which features out-of-the-box synchronization in the sub-nanosecond range. One company recently used a system based almost entirely on the National Instruments PXI platform to perform dynamic testing and characterization of a new, high-speed DAC, including:
Four of the five instruments in the test system were based on the NI PXI platform. The remaining traditional instrument connected to the PXI system through external cabling. The engineers used National Instruments LabVIEW to control the instruments, automate tests, and perform custom data analysis. Using this compact test system, they were able to perform SFDR, noise floor, and THD DAC tests. The NI PXI-6562 and NI LabVIEW – a Flexible, Automated DAC Testing Solution An increasing number of data-converter manufacturers are relying on the high-noise immunity and fast data rates of LVDS for their next-generation DACs and ADCs. The fast data rates and flexibility of the PXI-6562 digital I/O device make it an ideal choice for testing and analyzing the growing number of high-speed DACs with LVDS interface. Along with the necessary hardware features, NI LabVIEW provides the flexibility to easily automate and customize the dynamic tests, greatly reducing development and test execution time. With the automation of DAC testing, engineers can enjoy easy upgrades and test system maintainability for present and future DAC test development. For more information, contact: Michael Haag National Instruments 11500 N. Mopac Expressway
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