Multifunction DAQ Modules Integrate and Diversify

At first glance, a multifunction data acquisition module may only look like a compact way to obtain the variety of functionality needed to address a particular application. Of course, it is. Being compact and economical are two major advantages of such a module. But today’s products also are much more advanced than those from just a few years ago.

A typical multifunction module application was described by Vineet Aggarwal, data acquisition product manager at National Instruments (NI), “A group at Wyeth Neuroscience used an NI PCI-6229 multifunction data acquisition (DAQ) board to implement the Synaptic Explorer system that observes brain slices for neuroscience drug discovery. The PCI-6229 analog output channels stimulate the brain slice with the analog input channels recording the resulting response. The digital lines are used to control the pinch valves for various drug solutions.”

Most PCI/PXI multifunction modules combine analog input, analog output, digital input, and digital output. Data Translation’s Model DT3016 is a typical example, having 32 multiplexed, single-ended input channels; 16-b resolution; a 250-kS/s aggregate sampling rate; two 16-b analog output channels; 16 bidirectional digital I/O lines in two 8-b ports; and four counter/timers.

On some modules, the digital channels may provide additional functionality such as pulse width modulation, quadrature inputs, or more recently, serial bus protocols. Some analog inputs include signal conditioning for thermocouples or strain gages. A wide range of models is available with different mixes of input/output, analog/digital, and special functions.

Several trends have developed as better and lower cost components continue to become available. Older designs were constrained by the high cost of a good ADC, so analog input channels were multiplexed. This approach allowed a high aggregate sampling rate to be claimed, but the achievable rate per channel was much lower because the ADC was shared among the channels in use. And, if sample-and-hold circuits were used to align the channel sampling, further signal distortion could occur. A separate ADC per channel enables all inputs to be simultaneously sampled at full speed.

Because data acquisition applications involve physical quantities such as temperature, pressure, and mechanical strain, a wide variety of input configurations is needed. Instead of requiring separate signal conditioning ahead of the data acquisition system, today’s modules often integrate conditioning. In addition to variable filtering, offset, and gain, more specialized bridge completion and cold-junction compensation may be included.

Input sensitivity typically ranges from a few millivolts to 5 or 10 V maximum. Signals very much smaller or larger can be accommodated with separate signal conditioning. Differential signals require differential inputs, and many modules allow either single-ended or differential input configuration. Most often, the entire group of inputs must be treated one way or the other.

Analog inputs are isolated to provide equipment and personnel safety and improve measurement accuracy, especially in electrically noisy industrial applications. Sometimes called galvanic isolation, neither the signal nor its associated ground return directly connects to the data acquisition front-end circuitry. Instead, through optical, capacitive, or inductive techniques, an isolation barrier is created. Isolation also can be accomplished after the signal has been digitized and generally is less expensive than analog isolation largely because signal fidelity isn’t an issue.

Simply using a high-impedance probe does not provide isolation. Although the signal current can be reduced to a miniscule level, a single-ended probe connects the measuring system to the DUT ground. Galvanic isolation breaks both the signal and ground current paths.

To summarize some of the analog input-related aspects of multifunction modules, the key points are the following:
•?Separate per-channel ADC
•?Simultaneous sampling
•?Integrated signal conditioning
•?Input characteristics to match signal type
•?Isolation

Analog outputs can provide anything from a variable DC level to a high-speed arbitrary waveform. An analog output is developed by a DAC, so this device controls the resolution and noise performance. The clock rate and output filtering determine the bandwidth. Some modules offer enhanced analog output capabilities that include video and audio, both synchronized to the other I/O signals.

Although digital I/O signals by definition only have two states, there are many more varieties of protocols associated with them than there are types of analog inputs. In that sense, digital I/O can be more complex than analog, especially when synchronization is included. For example, digital inputs are received from encoders, tachometers, and basic on/off controls, and digital outputs are used to drive stepper motors, serial buses, and solenoids.

Just as ADCs have become better and more affordable over time, so too have FPGAs. These devices allow manufacturers to create very complex digital circuits at low cost and provide most of the digital logic in multifunction modules. According to Hank Lin, product-marketing manager at ADLINK Technology, “Today, we use an FPGA to combine all of the logic required into a single chip, including PCI or PCI Express (PCIe) bridge functions. Originally, we used an off-the-shelf ASIC to interface with PCI and on-board logic.

“We would use one PCI bridge with multiple complex programmable logic devices (CPLDs) or FPGAs to implement the required timing and flexible settings and coordinate the analog I/O data transfer with the PCI bridge direct memory access (DMA) engines.” He continued, “Now, with our investment in FPGA coding and our own IP, we significantly save cost, and with the improved functionality of today’s FPGAs, we also integrate additional functions such as stepper motor control, encoder inputs, and pulse generators as well as digital I/O.”

Nevertheless, in spite of many technical advances, there simply are too many types of signals and sources for a single module design to address them all. Instead, what appears to be happening is that the required number of test system modules is becoming smaller because each one has greater capabilities.

Sometimes this means providing a larger number of channels such as in an ADLINK half-size PCI board with 128 isolated digital I/O and 32 TTL I/O channels. In other cases, signal conditioning is integrated with the data acquisition circuitry to create a more comprehensive solution. Both approaches eliminate modules and reduce cost.

NI’s Mr. Aggarwal said that, in addition to hardware advances, as the software experience for DAQ system developers has become more straightforward, users have come to expect both easy and powerful programming interfaces. Complex timing tasks that might once have required a DAQ expert now are possible with only a few function calls, and many low-level routing and configuration parameters have become abstracted by software wizards like NI’s DAQ Assistant.

Examples

Strain Gages
Fortunately, the Wheatstone Bridge caught on in the mid 1800s and remains the basis for many types of sensor and transducer measurements. In Figure 1, if the Ra/Rb ratio is equal to the R1/R2 ratio, the voltage between the midpoints is zero. But if an imbalance exists, increasing the voltage across the bridge also amplifies the voltage difference. Ideally, temperature affects all four elements equally so the element ratios don’t change.

Figure 1. Wheatstone Bridge Circuit Diagram

In a strain gage application, one, two, or all four bridge elements can be strain gages. Some configurations only bond one gage to the DUT but provide a second identical gage to complete one half of the bridge.

The other half uses precision resistors. This is a type of quarter bridge because only one active gage is used. It’s possible to balance the bridge with three resistors and only one gage, but the temperature performance won’t be as good as when a compensating gage is used to complete one side of the bridge.

A half-bridge configuration uses two strain gages, but both are bonded to the DUT. Depending on the gage positioning relative to each other and the DUT, certain directions of bending or twisting will unbalance the bridge.

Finally, all four elements can be strain gages, resulting in a full bridge configuration. Strain gages have been used to illustrate the operation of the Wheatstone bridge, but any sensor that changes resistance in response to a physical quantity is appropriate.

Steve Krebs, director of engineering at DynamicSignals, commented, “Our new KineticSystems single-slot CP246 6U CompactPCI/PXI Module offers eight differential channels with independent 16-b ADCs, built-in bridge signal conditioning, and 16 multifunction digital I/O channels. It saves customers both space in the chassis and money. Typically, these functionalities would be found in three different modules.”

Integrating eight channels of bridge signal conditioning may not seem like a big deal, but it’s not simple. The CP246 supports 10-wire transducer connections. This means that Kelvin four-wire sensing is used to ensure accurate excitation voltage. There also is provision for internal and front-panel calibration as well as shunt calibration using a known-value precision resistance. As the data sheet succinctly states, “The CP246…contains fully programmable gain, shunt calibration, bridge balance, excitation, and filtering on a per-channel basis.”

This is a good example of the scale of integration that’s available in today’s modules, but it also illustrates the continuing need for a range of multifunction products. The CP246 handles just about any aspect of a resistive bridge application. It doesn’t have cold-junction compensation for thermocouples although it does support RTD temperature measurement. You can’t connect 4-kV electric locomotive drive pulses to it, nor can you deal with more than 16 digital signals. The CP246 is only a bridge signal-conditioning and acquisition module, but it does the job completely.

Isolation
A range of isolation modules is available from Microstar Laboratories. The Signal Interface (SI) Series comprises several separate models with isolated analog inputs, analog outputs, 4-20 mA outputs, 4-20 mA inputs, and digital I/O. According to the company’s Isolated Data Acquisition technical note, isolation is needed to eliminate ground loops between the measurement system and the signals you want to measure.

These interfaces connect to one of the company’s data acquisition processor (DAP) boards and share a common ground with that board. However, the SI module inputs are isolated from the DAP ground. Only digital signals cross the isolation barrier from an on-board FPGA to an isolated DAC or ADC, depending on the SI model.

Similarly, NI’s SCXI and SCC Modules provide signal conditioning and isolation separate from the data acquisition hardware. The SCXI Isolated Universal Input Modules are programmable, handle four or more channels, and include cold-junction compensation and transducer conditioning. The SCC Modules are much smaller than an SCXI, cost less, and deal with only one or two channels, often with fixed gain. For example, the Model SCC-AO10 provides one isolated analog voltage output channel.

In contrast to having separate signal conditioning, isolation, and data acquisition, many newer designs have integrated conditioning and acquisition, sometimes also with isolation. The iCoupler® technology developed by Analog Devices has enabled some of these higher density designs.

It provides multiple transformer-isolated digital channels as well as isolated power, all from an IC-size device. As used in NI’s Industrial M- Series Multifunction DAQ Devices, the iCoupler supports 60-V DC continuous isolation and 1,400-V rms or 1,900-V DC channel-to-bus isolation withstand for 5 s on multiple analog and digital channels.

Synchronism
At the most basic level, synchronism among analog input channels is necessary to ensure correct interpretation of cause and effect during data analysis. Modules that provide a separate ADC per channel eliminate many of the timing constraints associated with multiplexed systems by having truly simultaneous sampling.

According to Grant Smith, Dewetron president, “One of the most important issues for our customers is synchronicity between analog inputs and digital inputs including simple on/off inputs; counter, encoder, and tachometer inputs; and even CANbus interface channels. To solve this problem, we put all four of these input types onto our ORION Series ADC Cards. These input types are synchronized within nanoseconds on the card. All Dewetron A/D boards are simultaneously sampled to eliminate phase differences among channels.”

In NI’s M-Series data acquisition modules, a custom-designed system-timing controller ASIC (NI-STC-2) provides a number of benefits including synchronization at several levels: six DMA channels, clocked digital I/O lines up to 10 MHz, 32-b counter/timers with encoder compatibility, generation and routing of real-time system integration (RTSI) bus signals for multidevice synchronization, generation and routing of internal and external timing signals, PLL for clock synchronization, and an integrated PCI bus interface.

At high data rates, acquisition module performance can be limited by an inability to transfer the data to PC memory in a timely manner. In the NI-STC-2, two separate FIFO buffers interface input and output data to 32 hardware-timed digital I/O lines, and each FIFO has its own DMA channel. Also, analog I/O and counters can be synchronized with the digital I/O lines by providing a single clock source for the related signals.

M-Series Modules derive internal timing as well as a 10-MHz reference from an 80-MHz clock. The PLL included in the NI-STC-2 device allows each M-Series device in a system to synchronize its own 80-MHz time base to the master 10-MHz clock. The end result is that 8x greater timing resolution is possible on each module, but all modules still are synchronized to the same master clock.

Many manufacturers have adopted the much faster PCIe bus, so this is one more aspect to consider when selecting a module. This technology provides dedicated rather than shared bandwidth for high-throughput processes as well as low-latency I/O for control.

Even for seemingly routine streaming applications, the required DAQ-to-PC memory bus speed can easily approach the practical PCI limit of about 100 MB/s. For example, 80 channels of 10-MHz digital I/O are equivalent to a 100-MB/s data rate. PCIe-based modules can transfer data in real time fast enough that a huge amount of on-board acquisition memory isn’t needed.

Summary

Although there always will be odd sensors that require special, and therefore separate, signal conditioning, the input characteristics available on today’s multifunction DAQ modules deal with most common types of signals. However, the usual multifunction module analog I/O and digital I/O definition isn’t sufficiently broad to include the diverse ways that the new products save space and money. Carrier boards with plug-in modules are an approach that accomplishes both objectives in a very flexible manner.

North Atlantic Industries (NAI) has a range of carriers such as the Model cPCI/PXI 78C1 Multifunction I/O Board that allows you to mix and match up to six function modules mounted as mezzanine boards. Two of the latest capabilities available are MIL-STD-1553 and CANBus.

The N7 and N8 Modules provide two redundant MIL-STD-1553B Notice 2 interface channels, and each can be configured as a bus controller, remote terminal, or monitor. The P6 Module supports four independent, isolated channels of both CAN A and B links conforming to ISO 11898 and implemented based on the Bosch FPGA core.

C&H Technologies provides a wide range of M-Modules as well as carrier boards. The 3U Model AMi3002 Carrier accepts one or two M-Modules to create a customized combination of functions within a single cPCI/PXI module.

According to the KineticSystems data sheet, “The CP387 base board supports 128 channels of TTL I/O. Four mezzanine card sites can be populated with other forms of digital I/O including isolated input, isolated output, relay output, AC switch output, or differential I/O. The mezzanine card concept allows multiple digital I/O types to be configured within a single module to match the application requirements.”

It’s clear that multifunction modules are developing in several directions at once. The same FPGA, PCIe, and simultaneous sampling ADC technology may be involved, but rather than constraining modules to be similar, advanced technology supports even greater diversity.

With a large number of modules to choose from, most application requirements can be satisfied. In fact, for less stringent data acquisition jobs, several modules will be suitable, and the final choice may be based on price, manufacturer’s service, and delivery schedule as much as on technical specifications. More comprehensive modules such as KineticSystems Model CP246 and NAI’s Model 78C1 are not as plentiful, and the 6U cPCI/PXI form factor of these solutions may be incompatible with the rest of your test system.

FOR MORE INFORMATION Click below
ADLINK Technology PCI-6202 Multifunction PCI Board Click here
Analog Devices iCoupler Technology Click here
C&H Technologies AMi3002 Carrier Board Click here
Data Translation DT3016 16-b DAQ Board Click here
Dewetron ORION Series ADC Cards Click here
KineticSystems CP246 Thermocouple Bridge Module Click here
Microstar Laboratories SI Series Isolation Click here
National Instruments PCI-6229 Multifunction DAQ Board Click here
North Atlantic Industries 78C1 6-Module cPCI/PXI Carrier Click here

July 2009

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