Innovation and technology bring performance to instrumentation in small packages
A combination of vendor innovation and silicon component technology is reducing the size of industrial and measurement instrumentation while increasing its performance.
Two decades ago, instrumentation primarily involved dedicated, mainframe 19" rack-mount or even cart-based instruments. Today's instrumentation still includes those high-end platforms but now also incorporates PC-based and smaller stand-alone devices. Brett looks at the key trends driving this transition.
For most electronic devices, size and speed are the measures of improvement, while small and fast serve as the overriding goals. Even with televisions, where bigger screens are better, the objective for all other components on the set is to disappear, hence the flat-panel sets consumers use today.
This mantra is not at all lost on the measurement and automation industry. A combination of vendor innovation and silicon component technology is reducing the size of industrial and measurement instrumentation while increasing its performance.
Virtual instrumentation and multicore processing streamline test systems
Instrumentation vendors are taking advantage of the large, competitive commercial PC market to drive down equipment size and cost. More than 20 years ago, the concept of basing instrumentation on the widely available PC platform began to take shape. This movement, known as virtual instrumentation, removed many computer components such as memory, processors, and disk storage from the instrumentation and substituted an actual PC.
With the virtual instrumentation concept, test systems that once needed multiple racks to house tons of equipment were reduced to channel-dense modules packed into PCs or PC extension buses such as PXI. Removing components such as displays, controls, processors, and storage from each instrument instantly reduces the space required, but basing instruments on PC technology offers more benefits than just eliminating duplicate items.
Along with the addition of virtual instrumentation functions, the absolute size of PC and PC-based instrumentation has decreased. Desktop towers have shrunk to mid-towers, then laptops, and now the Ultra-Mobile PC (UMPC) and other high-powered handhelds.
Recently, with the advent of multicore processors, test systems built with a PC backbone are beginning to utilize parallel processing to increase throughput. In a virtual instrument, measurement hardware and user-defined test software are isolated from PC components; therefore, transitioning to multicore technology is more efficient than using instrumentation built on traditional system architectures.
ADCs provide a study in silicon design advancements
At an even lower level, advancements in silicon design have enabled smaller, faster components at more economical prices for pure instrument components such as Analog-to-Digital Converters (ADCs). The ADC is one of the main components that has increased in performance, resolution, and sampling rate while decreasing in price, allowing for more prolific distribution in today's test equipment.
A flashback to older, digital multimeter-based data acquisition systems shows that, partially because of high ADC prices, these systems had a single ADC and a switch network of relays to provide multiple channel inputs. But as Figure 1 illustrates, ADC prices are trending sharply lower.
For a similar price to the previous generation with one expensive ADC and a switching front end, designers can now find devices that are up to a third of the size and feature multiple ADCs. More ADCs means a higher aggregate sampling rate and lower phase offset between channels.
For a slightly increased price, designers can use a single ADC per channel, which all but eliminates phase offset and further increases throughput. By comparison, some of these older systems have a sample rate five orders of magnitude less than the multichannel, multi-ADC architectures of today.
On the other end of the spectrum, devices with a single ADC have become extremely small, even pocket-sized, such as the National Instruments USB-6008 shown in Figure 2. Some devices on the market are smaller than a deck of playing cards yet still perform multiple functions, such as analog input, analog generation, and digital input/output. These devices, which are often USB-powered, sample at speeds in the 10-50 KHz range and at 14-bit resolutions. If the performance-to-size ratio does not provide enough evidence for the ADC evolution, consider these multifunction devices' market price of less than $150.
Smaller yet are devices that have an ADC and are not multifunction. These devices, such as small-scale temperature or humidity loggers, are battery-operated and about the size of a lipstick case. With low-power requirements, these devices can log data for months or years in some cases without needing a fresh battery.
Digital isolation offers increased bandwidth for high-speed equipment
Analog-to-digital is just one area that has enabled smaller instrumentation; digital isolation, or more specifically, the move from analog to digital isolation, is another. The initial perception of isolation for instrumentation is that it acts like a surge protector by protecting the unit under test, test instrumentation, and human operators.
Besides offering this function, switching to isolation provides other advantages, such as eliminating ground loops. Ground loops can cause errors in measurement readings by forcing the instrumentation to read not only the test voltage level but also the voltage difference between the multiple grounds in a system. An isolated design breaks these ground loops, rejects the common-mode voltage between all channels, and in doing so, provides a more accurate system that is easier to set up.
Isolated instrumentation's expense and design complexity previously relegated its implementation to must-have situations, and it used to be considered more of a luxury for any other purpose. These older designs predominantly used an analog isolation architecture, meaning the signal being isolated is an analog signal. The isolation took place on the analog end of the circuitry to protect the costly ADC, but this involved special components that needed more space on the PCB and more power from the system.
With the shift to digital technology, galvanic isolation takes place after the analog-to-digital conversion, preserving the front-end analog design and eliminating nonlinearities introduced by analog isolation components. Though digital isolation technology has been around for a while, such as the board depicted in Figure 3, recent developments have increased bandwidth, making it viable for high-speed measurement equipment.
Whatever the method of digital isolation-optical, inductive, or capacitive vendors that sell these digital chips are touting the benefits of reduced board space, required components, and power consumption as well as increased data throughput. These isolator chips, such as the iCoupler from Analog Devices, are about the size of a fingernail and can provide transient isolation of up to 5,000 V. The multi-ADC board, module, or instrument can now have both an ADC and an isolator chip for each channel. As little as five years ago, the bandwidth would have been too restrictive and the cost too prohibitive.
Virtual instruments perform faster at smaller sizes
Customers and vendors alike are cashing in on virtual instrumentation. A quick snapshot of products on the market today reveals that customer demands and testing requirements are taking advantage of advancements in chip technology by requiring smaller, more cost-competitive devices. Many devices also have simultaneous ADCs or isolation, and in some cases, both.
An example of a device that integrates the concept of virtual instrumentation, new ADCs, and digital isolator chips is the NI CompactDAQ data acquisition system (pictured at the beginning of this article). This chassis-based system is modular and designed to be controlled by a PC running application-level software developed in one of many languages, such as LabVIEW, Visual Studio, or ANSI C. A simplified block diagram is shown in Figure 4.
Some of the modules, which measure approximately 9 cm x 9 cm, contain not only multiple 24-bit ADCs but also multiple digital isolators with one pair for each channel. This parallel design and compact size is made possible in part by these silicon components' decreasing cost and size. And with an aggregate sampling rate of more than 5 MSps streaming back to a multicore PC, the bar has been raised significantly over the relay-based systems of years ago.
As for small, lower-cost devices like the USB-6008, several companies offer data acquisition and data logging products with power requirements low enough to be run directly from a USB port. These products combine with a laptop to make a portable virtual instrument and would not have as low of prices or as high of performance levels without the latest generation of chip technology. Digital isolators require significantly less power than their optical counterparts, making them an ideal choice for bringing high-bandwidth isolation to portable data acquisition devices.
Advancements in virtual instrumentation by way of innovation in chip and PC technology will continue to drive down measurement and computing components size and power consumption while maintaining performance. This trend will collide with what seems to be the next growing wave of customer demand: wireless connectivity. Future devices will bring desktop performance to remote locations as the performance of today is achieved with less power and space.