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USB data-acq modules synchronously sample multiple analog signals

Posted: 28 Feb 2006 ?? ?Print Version ?Bookmark and Share

Keywords:DT9832? data acquisition? Data Translation? Alex Mendelsohn?

USB-based data-acq module

This product clearly shows what can be done within the (generous) constraints of USB 2.0's 480Mbps signaling rate. The practical data-rate limit on a USB-based data-acq module is 10.9MBps, and Data Translation's USB-based DT9832 series of simultaneous-input data-acq modules leverage that nicely.

These modules should find favor with anyone needing multiple channels of data conversion. What's more, they can work with both USB 2.0 and the slower predecessor USB 1.1.

Obviously, hanging one of these systems on a USB 1.1 port will limit performance. On USB 2.0, however, Data Translation's wares support accurate simultaneous data acquisition with up to 5MHz aggregate speed, correlating measurements from channel to channel with minimum phase errors.

The simultaneous data-acq design ensures built-in accuracy, too. A maximum aperture delay of 35ns (the time that it takes one of the system's A/Ds to switch from track to hold) is matched at 5ns across all of the system's track-and-hold circuits. That essentially eliminates any channel-to-channel skew that you might get with a MUXed front-end.

Pay your money, take your choice
For the same price you can choose either DT9832 series or DT9832A series wares. The former provide four single-ended input channels with 16bit resolution conversion, at 1.25MHz/channel speeds. Analog signal bandwidth (at the -3dB points) is greater than 6.25MHz.

The DT9832A series gives you a pair of single-ended input channels, also with 16bit resolution, but operating at 2MHz/channel. Signal bandwidth is greater than 10MHz.

For all of these systems, the analog input-impedance is a high-Z 100 Mohms, so they won't load down your sensors and conditioning circuits. The modules also provide software-selectable input ranges of -10V to 10V or -5V to 5V.

Board or box
Modules are available in two configurations. You can choose BNC-connector box types, or OEM embedded versions. The BNC versions are housed in metal boxes with standard BNC connectors for signal I/O, as well as D-subminiature I/O connectors.

The OEM embedded configurations are in circuit-board form-factors. Whether you choose a BNC box or a board-level implementation, you'll enjoy the system's 500V of galvanic isolation.

Independent data converters
Regardless of which configuration you choose, each system's analog channel packs its own ADC, giving the system the ability to sample all inputs at exactly the same time. This supports the ability to measure high-bandwidth components that have multiple test points.

Synchronous operation lets all I/O data be processed and correlated for all inputs and outputs, so that you can determine response across a DUT's (device-under-test) nodes at the same instant. Data can then be streamed synchronously to host memory. This can be done via external triggering or using an internal clock.

To ensure that capability, front-end input amplifiers exhibit bandwidth specs that are ten times higher than the Nyquist limit. This bandwidth also minimizes signal roll-off and phase errors. A maximum aperture uncertainty of 1ns (jitter, or variance in aperture delay) essentially eliminates phase noise as a factor in your captured data.

As well, the input-impedance of each channel is closely matched to that of the other channels (Data Translation notes that the slightest mismatch would result in DC and AC errors in measurement when trying to correlate readings at any instant in time). The system's 12-layer circuit board also shields all signal paths from any high-speed digital transition noise.

The payoff is that the AC dynamic performance at high switching speeds for all channels shows accuracy better than 13.6bits. Indeed, Data Translation claims this level of performance is well beyond that of competitive data-acq systems.

All functions of the data-acq modules (whether they're A/D types, DACs, quadrature decoders, digital I/O, or counter/timers) can be simultaneously triggered internally or externally. You can trigger and clock the system's analog output sub-system (more on it in a moment) synchronously with, or independent of, the analog inputs.

In addition to the internal and external clocks, you can invoke software triggering, analog threshold triggering, and external digital triggers.

DAC outputs
Mentioned briefly in the list of features in Data Translation's press release is the DT9832 series' analog output feature. There are two simultaneous 16bit analog outputs. These use separate de-glitched DACs.

This sub-system generates output waveforms at throughputs of 500kHz each. You can use the DAC sub-system to generate sine, triangle, and square waves. They're produced by loading output memory and triggering synchronously or separately.

You can also update the system's analog outputs as you're acquiring analog input data. That nifty capability ensures gap-free simultaneous stimulus and response testing, for example. In addition, you can update digital output lines with the analog output channels at the analog output rate.

Output modes
You can also output a single value from a single analog output channel, or multiple values from multiple analog output channels. An output channel list lets you update only the analog output channels that you want, or you can update digital output lines with specified analog output channels at the DAC's clock rate.

As for those LVTTL (low-voltage TTL) I/O lines, you get 32 of them, dedicated as 16 in or 16 out. The first eight input lines can also be used for interrupt-on-change purposes.

You can also read all the digital inputs simultaneously with the analog input channels at the A/D clock rate. The digital input lines can also be clocked separately as the only channel in the channel-gain list at up to 1.25MHz on the DT9832, and up to 2MHz on the DT9832A.

Again, you can also update all the digital output lines with the analog output channels at the DAC's output clock rate.

The executables
Of course, software is what makes this system play, and Data Translation more than adequately supports this product with a variety of products. As the press release indicates, DT9832 series devices ship with a CD-ROM chock full of device drivers. These are for Windows 2000 and Windows XP systems.

You get evaluation versions of the company's data-acq executables, and the Omni CD provides canned applications that can get you up-and-running quickly. The Omni CD also includes freebie software tools for programmers using C#, C++, VisualBASIC and VB.NET.

You also get a trial version of the company's popular DT Measure Foundry program. DT Measure Foundry is a drag-and-drop Windows application builder that lets you compare, correlate, and analyze signals. Displays connect to the hardware using so-called property pages, so there's no coding or wiring required.

Menu-driven application
Similarly, Data Translation's Quick DataAcq is a menu-driven application, replete with source code. It lets you verify the operation of one of these boards, collect A/D data, display data on-screen, and save it to disk. Thanks to having the source code, you can readily modify and customize your programming of a Quick DataAcq application, using VisualBASIC.

You also get DAQ Adaptor for the popular Mathworks MATLAB environment. DAQ Adaptor is a software interface tool that gives you direct access to analog and digital I/O data from within MATLAB. When used with MATLAB's Data Acquisition Toolbox, you're working in an integrated environment that supports lots of data acquisition and signal analysis.

Similarly, Data Translation's DT-LV Link is a high-level data-acq link to National Instruments LabVIEW. DT-LV Link provides a collection of VIs (virtual instruments) that let you program using LabVIEW.

- Alex Mendelsohn

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