Out in Front: September 12, 1996

Despite its name, GW Instruments' new Instrunet data-acquisition system has nothing to do with the Internet or even with LANs. The "net" in the name derives from GW's having moved the A/D converter from the noisy computer backplane into a "network device." This device is a separate enclosure that replaces the external signal-termination panels that ISA-bus data-acquisition boards nearly always require. By placing the A/D converter closer to the sensors--only inches from the screw terminals--GW avoids the long ribbon cables that carry analog signals from the termination panel to the data-acquisition board. In Instrunet, digitized signals travel between the network device and a proprietary interface board in the host PC or Macintosh via a standard cable that contains 12 twisted pairs.

When routing groups of analog signals for more than a few feet and digitizing the signals to a precision of 12 bits or more, you should use cables comprising separately shielded, twisted pairs and a data-acquisition board that provides differential inputs. The reason for this ap-proach is that, in a 12-bit system, even with ±10V signals and wiring runs only a few feet long, the capacitance between a ribbon cable's unshielded wires can cause several counts of crosstalk. This crosstalk occurs especially if the receiving device is a single-ended amplifier or an analog multiplexer.

GW offers a report comparing Instrunet noise performance with that of conventional data-acquisition boards. The report is available in print or from GW's Web site. GW also specifies the sum of the errors from all sources for Instrunet. Separately specifying errors attributable to different sources makes it difficult to predict system accuracy. For example, without signal averaging, which substantially slows a data-acquisition system, four counts of crosstalk reduce a 12-bit system to a 10-bit system.

A $790 Instrunet device includes eight 8-bit, ±5V analog outputs, eight digital I/O points, and eight 14-bit differential analog inputs that you can also use as 16 single-ended inputs. The analog-input ranges are ±5, ±0.6, ±0.08, and ±0.01V. A similar network device, which adds a pair of BNCs for each differential input, costs $950. Both units draw power from the host computer, which can be a PC or a Macinstosh. You also need a network-controller card, which can host as many as 32 network devices. Cards for the PC ISA bus, the PC or Macintosh PCI bus, and the Macintosh NuBus cost $590 each. Instrunet is compatible with a variety of industry-standard, 32-bit, data-acquisition software packages, including the Macintosh and Windows 95 versions of LabView. You can also buy drivers for use with C or Visual Basic programs that you compile with a 32-bit compiler.

—by Dan Strassberg

GW Instruments Inc, Somerville, MA. (617) 625-4096, fax (617) 625-1322, info@gwinst.com, http://www.gwinst.com.