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Data Acquisition with Small Modules

Data Acquisition Solutions Stack Up

The 90 x 96 mm PC/104 form factor family, which includes the unifying factor of SUMIT-ISM, continues to offer expansion, flexibility, inclusion of legacy devices and ruggedness for an increasing range of data acquisition applications.

ROBERT A. BURCKLE, WINSYSTEMS

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Data Acquisition (DAQ) systems come in as many facets and form factors as application areas. This is because system designers need to consider a number of important issues such as size, performance, ruggedness, modularity, software support, connectors, packaging, operating temperature, long-term availability, sensor types and locations, network connectivity and power consumption as they decide upon their final system configuration. Data can be from the analog or digital domain or from remote sensors that have gathered information for further analysis and processing.

Yet whether for portable data collection, medical instrumentation, laboratory analysis, environmental and utility management, pipeline, remote environmental monitoring, transportation or automated test equipment (ATE) for semiconductor, military/aerospace and telecommunications, DAQ system designers are under pressure to reduce size and improve reliability while retaining application software compatibility. To that end, small form factor (SFF) computers and I/O cards continue to grow in popularity among OEMs who must interface to the real world for data acquisition, while ATX-type motherboards continue to give up ground as applications become smaller and more portable.

Form Factor Forensics

PC/104 stackable modules are known for their rugged design and small size. These self-stacking modules can serve as either a single mezzanine on top of an SBC or with multiple cards for even greater I/O requirements. During the past ten years, an abundance of new industry-standard and single-sourced form factors have posed challenges to the self-stacking 3.550” x 3.775” (90 x 96 mm) PC/104 single board computers (SBCs) and I/O modules. One worth comparing is the computer-on-module (COM) phenomenon, which has crept into this space for high-volume applications. Yet each COM requires a new carrier board for all of the unique application-specific I/O circuits and connections, which incurs more NRE and resources than most projects can afford. Currently, the original legacy-friendly ETX COM (with ISA bus and serial ports) is giving way to high-speed legacy-free COM Express (“COMe”), but this is not a compatible upgrade path (different connectors, expansion buses and I/O).

The smallest COMe outline measures 95 x 95 mm. Although similar to PC/104 in size, the requirement to design and manufacture a custom, application-specific carrier board pushes the minimum cost and overall space to be similar to or greater than a PC/104 SBC with one I/O card. Yet a mezzanine card is still required if the potential for additional I/O modules needs to be added. In fact, some companies make carrier boards that support a COM on one side and a PC/104 interface on the other, since they want to have the option to take advantage of off-the-shelf, proven PC/104 I/O modules. Reduced legacy support, multiple incompatible pinout definitions, and the requirement to provide high voltage +12 VDC input reduce the likelihood that COM Express can make significant inroads into DAQ applications. Figure 1 compares popular SBC and COM form factors.

Figure 1
Comparison of host computer form factors.

State of the Stack

Stackable data acquisition thrives in spite of the trend toward computer-on-module (COM) CPUs in non-acquisition markets. This is largely because precision A/D (analog-to-digital converter) expertise continues to reside with specialty PC/104 I/O module manufacturers, while COM vendors have partitioned the architecture to include the greatest processing circuitry subset that is common to all applications—and data acquisition is not part of that subset.

There are three types of PC/104 DAQ solutions already in widespread use in the market. The first is the pure I/O expansion module separate from the SBC, which allows a designer to mix and match from different manufacturers or even their own custom design. The second type of PC/104 DAQ consists of processor and analog I/O circuits on a single circuit board. Experts have diligently conquered the risk of noise coupling from high-speed digital sections to sensitive amplifier and sample-and-hold circuits, especially for 16-bit sampling.

The third DAQ type consists of small, stand-alone modules that are networked by either “tethering” (cabled) or wireless links back to the main board stack. This is popular because of increased isolation and signal conditioning options over the other two DAQ types, and because of the great deal of mounting flexibility in the field. Networked signal-conditioning modules often can provide some signal preprocessing, are small and can be located next to the signal source being measured, which allows the host computer system to be further away.

The key to each acquisition application is the ability to meet system requirements for a wide variety of sources and bandwidth necessary for sampling and processing the data. The higher the sample rate, the faster the system bus or interconnect required. However, this comes with a substantial power consumption penalty, which is a major setback for battery- or solar-powered DAQ devices.

There is also a tradeoff between sample resolution (8/12/16 bits) and bandwidth. Sensitive low-noise input circuitry needs more settling time for 16 bits, which extends the sample period (inverse of the frequency). Most embedded applications reside somewhere on this spectrum. Many DAQ environments require isolation and signal conditioning, as will be explored later. Finally, there are business issues such as time-to-market, reconfigurability, multiple sources of supply and configuration, flexibility, and extended temperature operation that are benefits gained with stackable DAQ modules.

Getting the Drift

In order to achieve good accuracy and resolution in a typical system, a 16-bit A/D is desirable. However, it takes specially designed circuits to mitigate noise, drift and matching- and leakage-related inaccuracies over temperature. The challenge to board designers is to shrink all of this circuitry into a space-saving size while improving operation and cost of ownership over the long haul.

New boards should not require user calibration to maintain data integrity. Older technology boards with trim pots (potentiometers) were prone to time- and temperature-related drift. Unpredictable and untraceable errors render data questionable and perhaps unusable. In addition, there is down time and the costs of a technician required to measure and adjust the system. To minimize the effects of drift error, analog experts are now approaching calibration and drift from the ground up through careful design and component selection.  

The single most important component in any analog converter design is the analog voltage reference. Any drift of the reference voltage directly affects analog conversion accuracy, expressed as full-scale (gain) error. The reference voltage tends to vary over temperature as well, although well-matched and compensated ICs keep the variations to a minimum. Over the years, circuits have evolved from manual calibration to auto-calibration with the entire analog section rolled into one chip.

One example that targets small form factor boards such as the PC/104 board is the Linear Technologies LTC1859 data acquisition “system-on-chip.” It lends itself to 8-channel applications with 16-bit conversions at fast sample rates. The device includes input multiplexer, range select, sample and hold, analog-to-digital converter voltage reference and associated control logic. Each high-resolution, high input voltage range ADC in this family has an on-chip, temperature compensated, curvature corrected, 2.50V factory-trimmed band gap reference. The use of precision, laser-trimmed thin-film resistors eliminates the need for user calibration. The PC/104 board shown in Figure 2 does not require any calibration for the A/D or D/A circuits but will still operate from -40° to +85°C within specification limits.

Figure 2
This PC/104 DAQ module requires no calibration for the A/Ds and D/As.

Depending on the signal source, it may be necessary to isolate or provide other protection for the inputs and outputs so that the measurement circuit doesn’t load down the source, and also to eliminate ground loops due to common-mode voltages between the host computer and the monitor/control nodes. Figure 3 shows a digital I/O termination board that handles voltages from 3 to 28 VAC or DC, while relay outputs switch up to 1A @ 24VDC (60 VDC max.) or 0.5A @ 125 VAC while offering 1000 VAC or DC isolation for a wide range of industrial applications. 

Figure 3
This isolation and termination module fits directly on a PC/104 stack or can be mounted closer to the field wiring or signal sources.

Faster, Better Data Acquisition

In the realm of the tiny 90 x 96 mm “PC/104” size I/O industry-standard modules, new applications are developing due to a third generation expansion interface called SUMIT. This connector provides support for the latest generation of ultra-low-power mobile processors.  SUMIT is a stackable bus architecture from the Small Form Factor Special Interest Group (SFF-SIG), which is the only stackable PCI Express standard that allows low-cost, low-power PC/104 I/O modules to plug directly into new host SBCs. Data acquisition can now be interfaced through a myriad of options from PC/104 ISA bus to two-wire SPI and I2C, and scaling up to high-speed PCI Express (“PCIe”). SUMIT offers solution methods for high-speed I/O that were not previously possible, plus it bridges existing data acquisition systems with the variety of SUMIT host SBCs on the market.

The ISM (Industry Standard Module) form factor specification was created by the SFF-SIG to carry the 90 x 96 mm size and mounting holes forward regardless of what expansion bus exists or whether one is needed for a given module. This pure form factor is barely more than a board outline drawing, yet is flexible enough to allow compliant boards like the signal conditioning product shown in Figure 3. To get the SFF-SIG’s latest specifications including SUMIT and ISM, visit www.sff-sig.org. 

Figure 3
This isolation and termination module fits directly on a PC/104 stack or can be mounted closer to the field wiring or signal sources.

Wired-in and Wireless

Traditionally, DAQ endpoint devices such as sensors, actuators, motor controllers, current loops and transducers were controlled by PLCs. Existing Modbus deployments are upgraded with Ethernet gateways to couple data acquisition directly to factory networks. USB DAQ modules are available that are capable of tens to several hundred samples per second—even at 16-bit resolution—powered through the USB connection for devices close to a host PC. Ethernet cable runs are used for broader deployments.

Cable runs are expensive, impractical and sometimes unsafe in environments ranging from factories to oil fields. Two approaches to wireless DAQ include “Wi-Fi,” similar to consumer networks for laptop PCs, and ZigBee.

The IEEE 802.11 standard divides the unlicensed 2.4 GHz spectrum into 13 overlapping channels 22 MHz wide but only 5 MHz apart. The large ecosystem of 802.11b/g access points and radios (consumer adapters) has contributed to broader usage in embedded applications. Data rates vary from 1 Mbit/s to 54 Mbit/s depending on noise levels (interference) and number of endpoints, with ranges up to 100 ft (indoor) or 300 ft (outdoor). Directional antennas can extend outdoor range to a mile or more, with transmit power governed by FCC Part 15 local regulations. The recent 802.11n addition increases data rate and/or range by occupying multiple channels and is typically overkill for embedded DAQ applications. For portable embedded DAQ devices, 802.11 consumes significant power and reduces battery life between charges.

Although it is being positioned for the high-volume smart energy and home automation markets by its namesake alliance, the ZigBee standard is also gaining popularity in embedded markets such as industrial automation. Optimized for power conservation and short range on the IEEE 802.15.4 physical radio specification for 2.4 GHz, 900 MHz and 868 MHz unlicensed bands worldwide, ZigBee networks can be used with sensors, control nodes, actuators and process control (Figure 4).

Figure 4
PC/104 card with ZigBee for remote sensor network support.

ZigBee solves the Wi-Fi battery life problem due to its low transmit duty cycle and extremely low 30ms wake-up latency. Up to 65,000 nodes can exist in a network, with 128-bit AES encryption for secure connections. Decentralized “mesh” networking brings reliability to wireless communications by re-routing data away from failed links using a pack-based protocol. Reliability is further bolstered by means of re-tries, acknowledgements and collision avoidance techniques. Each local sensor or control point gets its own ZigBee wireless modem in a dual in-line (through-hole) package the size of a quarter. The host end of the network can consist of an Ethernet gateway or an embedded board stack with its own RS-232 serial port ZigBee adapter. ZigBee is an affordable wireless alternative to Wi-Fi where large numbers of DAQ endpoints exist, and yet long range is preserved due to the low transmit data rate and duty cycle.

Radios are also available as stand-alone components to be used with any processor or microcontroller. ZigBee chip vendors like Freescale, TI, STMicro, Atmel and Samsung often sell integrated radios and microcontrollers with between 60 Kbyte and 256 Kbyte flash memory. The ZigBee software stack is available from the chip vendors or third parties. Visit www.zigbee.org to learn more.

The SUMIT interface is the ideal multi-module board-level architecture for either of the wireless standards, since PCIe x1 interfaces to off-the-shelf PCI Express Mini Card Wi-Fi adapters and because LPC, SPI and I2C are easy to interface to low-speed ZigBee modems. Off-the-shelf LPC quad UART chips cost only a few dollars.

The 90 x 96 mm PC/104 stackable board market benefits from the aforementioned new developments in performance, flexibility, connectivity and data acquisition options while retaining its ruggedness, reliability, industrial temperature range and long-term availability.  SUMIT-ISM is the only form factor that allows PCI Express, USB, LPC, SPI, I2C and ISA (PC/104 bus) as expansion interfaces off a single host computer, which minimizes the size, cost and power consumption of new DAQ deployments. Legacy I/O support is a common requirement in existing systems. Many applications require additional I/O as well, so PC/104 is the low-power, off-the-shelf choice compared to COMs with custom carriers. As a result, SUMIT-ISM is the only viable architecture for these types of data acquisition environments.

Using state-of-the-art, low noise 16-bit A/D and D/A converters with no calibration required, gives a shot in the arm to the PC/104 DAQ market. This clean, simple design yields smaller size, lower cost and much better accuracy by avoiding error-prone manual calibrations. The PC/104 bus platform provides a power-efficient and long-lifecycle 16-bit data path for these converters. And now SUMIT-ISM DAQ cards allow even higher data acquisition in the same, stackable, small form factor. This is attractive to both integrators and OEMs alike who need to integrate the PC and the DAQ circuitry together for their next designs, regardless of whether the host connection is wired or wireless.

WinSystems
Arlington, TX. 
(817) 274-7553. 
[www.winsystems.com].

Linear Technology
Milpitas, CA. 
(408) 432-1900. 
[www.linear.com].