Too often taken for granted, signal conditioning is one of the most important components of any data acquisition system. It is the interface between real-world analog signals and the rest of the system. If the signal conditioner and the interconnecting cables are not specified properly, the data acquisition system most likely will output meaningless information.
A signal conditioner supplies essential circuit elements between the sensor and the data acquisition system. It provides a specific interface to a particular sensor, which includes proper excitation, scaling, and buffering. As an example of common sensor excitations, resistance temperature detectors (RTDs) require a constant current source, strain gauges need a constant voltage source, and linear variable differential transformers (LVDTs) must have an AC source.
Thermocouples put out millivolts due to the thermoelectric effect and do not require an external source. They do, however, need compensation for unintentional thermoelectric (cold) junctions.
Two common features of signal conditioning are low-cost customizing and isolation. For custom applications, signal conditioners can be calibrated to nonstandard input/output ranges such as a 10- to 50-mA output for a -12°C to +56°C input. Isolated versions also are available.
According to Jim Borton, product manager at Keithley, “Mandatory signal conditioning includes voltage isolation, noise-reduction techniques, and elimination of ground loops. When high levels of isolation are required, conditioning can’t always be accomplished inside the PC.”
Front-End Bandwidth Limitations
In process-monitoring applications, sensor response time usually dictates system bandwidth. Since most sensor outputs are low level, prone to noise, and do not change rapidly, circuit emphasis is on DC accuracy rather than bandwidth. In most temperature-monitoring applications, a bandwidth of 10 Hz is adequate.
The deliberate limitation of bandwidth reduces broadband noise and susceptibility to outside disturbances, making greater DC accuracy possible. It also may eliminate the need for anti-aliasing, low-pass filtering prior to analog-to-digital (A/D) conversion. If you need isolation but do not need to interface with a sensor, bandwidths to 30 kHz are possible with isolation amplifiers.
A signal with an acceptable signal-to-noise ratio (SNR) has few intrinsic bandwidth limitations. “If you can bring the data in raw, and there is enough power in your computer to analyze the data fast enough, you’re better off,” said Bill Marcotte, marketing associate at Gage Applied Sciences. “Once you have the raw data digitized, you can analyze it with sophisticated numerical techniques, and it can be undone afterwards.”
In the software realm, noise reduction can be achieved at the expense of bandwidth by averaging multiple readings. This will reduce the effects of evenly distributed random noise.
Reducing Noise With Proper Cabling
The cable from the sensor to the signal conditioner must be as short as possible, but long enough to avoid exposing signal conditioning electronics to harsh environments (Figure 1). The sensor is very susceptible to noise because of its small-signal/high-impedance output.
If this output were used to drive a load directly, the SNR would be very poor. With amplification, the signal conditioner can increase the signal level and provide a low-impedance output for greatly reduced noise susceptibility when driving cables.
In making connections to ground, avoid creating ground loops. Figure 2 shows how to connect ground wires so that they do not normally draw current. Grounding is intended to provide a voltage reference so the system isn’t floating at a dangerously high potential.
A cable is both a transmission line and an antenna. As an antenna, it is capable of unintentionally transmitting and receiving high-frequency signals that may cause interference. Attention to fixed-geometry shielding and ground planes gives better and more predictable results. Shielding and grounding should present a low impedance to ground for internal or external interference sources.
In considering the cable as a transmission line, a coaxial cable is designed for a certain characteristic impedance, such as 50 W or 75 W . As a result, line drivers and terminators must match this impedance to minimize line reflections at high frequencies. A twisted pair of wires presents a balanced distributed impedance (capacitance) to ground for outside disturbances for better common-mode rejection by the amplifier in the signal conditioner.
Do not exceed the practical maximum lengths for cables used to transmit certain types of signals. The transmit distance for an analog DC current source is up to 5,000 ft, an analog DC voltage source up to 300 ft, a digital TTL up to 100 ft, RS-232 up to 1,000 ft, and RS-422/485 up to 4,000 ft.
There is no substitute for proper signal conditioning and cabling to reduce the effects of line resistance and noise, but other methods should be used to further minimize it. The differential amplifier of the signal conditioner is a powerful tool that eliminates ground loops and rejects common-mode noise. The figure of merit for a differential amplifier is the common-mode rejection ratio (CMRR).
A CMRR of 120 dB or more is recommended by Larry Copeland, marketing manager at Datel. For proper operation, each amplifier input must be tied to ground through a fixed resistor, and the inputs of unused amplifiers must be grounded. This prevents amplifier inputs from floating and giving unpredictable results.
Another way to overcome noise is to drive the cable with a differential output amplifier. This type of output contains both the original signal and its inverse, which is redundant information for accurate interpretation by the differential amplifier at the receiving end.
Analog low-pass filtering is a must for anti-aliasing in data acquisition systems. The purpose of the low-pass filter is to eliminate frequencies above one half the sampling rate and this is the Nyquist theorem. By narrowing the bandwidth, low-pass filtering also reduces the effect of broadband (Johnson) noise. If 50/60 Hz power line-related noise is a problem, use notch filtering to eliminate it.
A multistage analog filter has a sharper roll-off than one or two stages. “Six- or eight-pole filters are required for waveform analysis at higher scan rates,” said Bob Cleary, CEO at KineticSystems. “It is important that frequencies above Nyquist are sufficiently attenuated.”
The limiting factors in the use of analog filters are space and cost. Low-end systems typically do not offer six- or eight-pole filters as standard.
The amplifier/filter combination also is important. “When using amplification and filtering together for signal conditioning,” pointed out Robert Yablonski, vice president of engineering at Alligator Technologies, “it is best to amplify first, then filter. Filters can introduce low-level noise, so amplifying first will increase the SNR.”
However, there is a drawback. According to Douglas Firth, marketing manager at Precision Filters, this technique may create a problem if out-of-band signals are larger than the data. This would limit the amount of gain that can be applied before clipping occurs.
Newer analog-to-digital converters (ADCs) include features that can make the signal conditioning job easier. “Some sigma-delta ADCs have built-in anti-aliasing functions,” stated Travis Ferguson, signal conditioning product marketing manager at National Instruments, “and, with 24-bit sensitivity, they can reduce the need for amplification.”
For best noise-reduction performance, some vendors advocate converting the signal to digital as soon as possible for formatting and communicating to a central computer. Digital signals consist of high and low levels that are separated by several volts, giving them greater noise immunity than analog signals, which may be only a few millivolts in amplitude. The best way to reduce noise is to place signal conditioning and ADC capabilities as close to the signal source as possible, said Rob Winkler, product marketing engineer at Intelligent Instrumentation. This leaves less room for the analog signals to pick up unwanted frequencies. Once digitized, signals are much less susceptible to the effects of noise.
According to Walt Maclay, president of Strawberry Tree, noise can be a big problem, especially for high-frequency or low-level signals. He indicated that multifunctional data acquisition front ends, which include signal conditioning, keep signals from being degraded in long cables. Keeping the cable from the signal source short and converting the signal to digital reduce noise, loss of accuracy, and high-frequency attenuation.
Selecting System Features
Clock rate and bit resolution are two important features of a data acquisition system, but its architecture also is a key to overall system performance. In the ultimate data acquisition system, each input has its own amplifier, low-pass filter, sample-and-hold amplifier, and ADC. The sample-and-
hold amplifier holds the sampled signal level constant for level stability at the onset of the A/D conversion process.
To save space and reduce cost, most systems multiplex the analog inputs into one ADC, which divides throughput by the number of channels and introduces switching noise. According to National Instruments’ Travis Ferguson, amplifier settling-time errors may occur, and this generally is cured by reducing the scan rate or having an amplifier on every channel. If phase relationship between signals is important, a simultaneous sample-and-hold amplifier should be used.
Architecture and Software Choices
Unless you’re prepared to include software development as a significant part of the data acquisition project, you may prefer an existing hardware/software combination with minimal setup on the PC. Many packages are available, but there are other parameters worthy of attention.
Some considerations are power, packaging, speed, software, and networking. Choosing a bus will determine how the card connects to the PC or laptop, whether separate power is required, what software is available, and what speed or data transfer rates are possible.
References
1. The Handbook of Personal Computer Instrumentation, Intelligent Instrumentation, 1995, p. 8-3 – 8-29.
2. Data Acquisition and Control, KineticSystems, 1994, p. 18-42.
3. Signal Conditioning and PC Based Data Acquisition, IOtech, 1997, p. 72-75.
4. High-Performance Data Acquisition Catalog, Analogic, 1996, p. 8-13.
Signal Conditioning System
Features Strain Gauge Module
The SCS-800-Based Signal Conditioning System, with optional 12-VDC power, accommodates a 3U-sized SCS-824 four-channel strain gauge amplifier module. Featured in the SCS-824 are multiple bridge excitation voltages for full, half, or quarter bridges, Kelvin connections, excitation output current limit, excitation shutdown, cable shield drive, and programmable resistors for high- and low-side calibration. Additional parameters are autozero; selectable gain; AC/DC coupling; a tuneable, four-pole analog anti-aliasing filter; an output multiplex, and differential drive output. SCS-800: from $2,495. SCS-824: $1,795. Alligator Technologies, (714) 850-9984.
PCI Card Provides
10-MHz Data Acquisition
The CPCI-14-1 is a 14-bit, 3U-sized, dual-channel, analog input board for CompactPCI. At 5 MHz, the board achieves a 75-dB signal-to-noise ratio (SNR) and a spurious-free dynamic range (SFDR) of 90 dB. The highest possible SNRs and SFDRs are realized from 1 MHz to 10 MHz and over a 0°C to 70°C range. An optional port links to a SHARC®-based board for real-time DSP or data transfer. The analog input range is ±1.25V and 50 WAnalogic, (800) 446-8936. terminated. Software support for Windows NT is included. From $2,500.
PCI Card Accepts
32 Analog Inputs
With 32 single-ended or 16 differential inputs, the DT3016 features
16-bit resolution and a sample rate of 250 kS/s. The PCI card handles unipolar or bipolar inputs from 1.25 to 10 V and provides two analog outputs to ±10 V at 200 kHz. A custom PCI interface allows DMA transfers without CPU intervention. There are 16 digital I/O lines and four counters/timers. Optional software is HP VEE with DT VPI 5.0. DT3016: $1,495. HP VEE with DT VPI 5.0: $1,490. DT VPI only: $195. Data Translation, (800) 525-8528.
PCI Card Offers
Excitation for Sensors
The PCI-441 PCI Card provides multiple sensor excitation with adjustable, regulated, ±10 V at 100 mA to drive bridge circuits such as strain gauges and 500 µA to excite RTDs. There are 16 (A version) or 32 (B version) analog inputs and two 4 to 20 mA outputs (B version). Each analog input can accommodate optional signal conditioning modules. The 16-bit A/D sample rate is 200 kHz, and a FIFO stores 512 samples. The PCI interface supports DMA and plug-and-play. PCI-441WIN executable software for Windows is included, and the PCI-WINS source code version is optional. PC-441A: $895. PC-441B: $1,195. PCI-441WINS: $395. Datel, (800) 233-2765.
12-Bit PCI Card
Handles 100 MS/s
The Compuscope 12100 Data Acquisition Card for the PCI bus has 12-bit resolution and can store 4 MS. It runs at 100 MS/s for one channel or 50 MS/s with two channels. GageScope software, with drivers available for DOS, Windows 95, and Windows NT, operates the card like an oscilloscope without writing code. Data can be stored, analyzed, printed, and converted to ASCII for use in spreadsheets and mathematical software packages. $5,995; software drivers: $250 each. Gage Applied Sciences, (800) 567-4243.
Signal Conditioning Board
Plugs Into VXI Module
The HP E1503A is a signal conditioning plug-in board for use with one of HP’s VXI-based scanning ADC modules. The board provides a programmable low-pass filter and an input amplifier for each of eight analog channels. The two-pole low-pass filters have cutoff frequency settings of 2, 10, and 100 Hz, and a filter-off mode allows a pass through of up to 1.5 kHz. The input amplifier range settings are ±0.25 V, ±2 V, and ±16 V. $1,150. Hewlett-Packard, (800) 452-4844.
Data Acquisition System
Communicates Via Internet
The EDAS™-1031E Data Acquisition System communicates over an Ethernet LAN or the internet and uses TCP/IP protocol. It accepts 16 single-ended or eight differential analog inputs, has a 16-bit ADC that samples at 100 kHz, and provides two 12-bit analog outputs. Features include a 16-bit counter at 250 kHz; 16 digital I/O lines; an amplifier with programmable gains of 1, 10, and 100; and an RS-232 or optional RS-485 port. Visual Designer software and support for many other software packages also are available. $1,295. Intelligent Instrumentation, (800) 685-9911.
Networking Instrument Logs
Six Isolated Thermocouples
The KNM-TC42 Thermocouple Signal Conditioner has 1,500-V input/output and 800-V channel-to-channel isolation and provides current isolation to eliminate ground loops. Each of the six inputs accepts a type J, K, R, E, S, T, N, or B thermocouple. A 20-bit ADC delivers up to 15 readings per second, and the memory stores 10,000 8-bit or 5,000 16-bit readings. There is one solid- state relay output. Communications selections are RS-232, RS-485, and Ethernet. The unit requires 9.5 to 34 VDC power. NetAcq stand-alone software for Windows is provided. $1,095. Keithley Instruments, (800) 552-1115.
Plug-In Card Forms
Bridge Signal Conditioner
The SC20 is a 3U-size, two-channel, bridge signal conditioner intended to work with an ADC that includes gain and filtering such as the V200 or V213 VXI modules. Features are per-channel excitation, bridge balancing shunt calibration, and full programmability of the bridge. A transformer option provides high-frequency common-mode rejection for electrically noisy environments. The package incorporates the board, chassis, power supply, and serial interface. From $1,000. Kinetic Systems, (800) 328-2669.
Self-Contained Data System
Has 20-Bit Resolution
The Model 203 Data Acquisition System offers one differential analog input with 100-GW input impedance. This input is protected to ±60 V and has an input range of ±5 V with options to ±0.1 V. Other parameters are a programmable data rate of 0.5 to 120 Hz for the 20-bit ADC, one digital input, eight latched digital outputs, and two analog outputs. An RS-232 interface is optically isolated. DC power of 11 to 16 V regulated or 15 to 22 V unregulated with a preregulator option is required. $250; Preregulator option: $20. Lawson Labs, (800) 321-5355.
PCI Card Streams Data
To Disk at 2.5 MB/s
The DAP 4200a/526 PCI Bus Data Acquisition Card has its own on-board intelligence implemented as a multitasking real-time operating system. When run with any Windows program, the 14-bit system dynamically buffers data at a maximum rate for up to 5 s with on-board RAM to protect an application from random operating system or network delays. The board samples 16 analog inputs simultaneously at 769 kS/s and provides two 12-bit analog outputs at 400k updates/s. Software for Windows logs data to disk at 2.5 MB/s. $4,095. Microstar Labs (888) 678-2752.
PXI Data Acquisition Card
Has 16 Inputs at 16 Bits
Featuring 16-bit resolution at 100 kS/s, the PXI-6030E PXI Data Acquisition Card offers 16 input channels. There also are two 16-bit analog outputs; eight digital I/O lines; and two 24-bit, 20-MHz counters/timers. A programmable gain amplifier delivers 16-bit accuracy even when scanning multiple inputs at high gains and fast rates. The card includes a trigger bus and reference clock for multiboard synchronization. PXI is interoperable with CompactPCI. $2,195. National Instruments, (800) 258-7022.
Signal Conditioners
Have 1,800-V Isolation
The DRN and DRX Series of Signal Conditioners mount on a DIN rail and accommodate thermocouple, RTD, strain, frequency, pulse, AC voltage, AC current, or process voltage inputs. Up to 1,800 VDC three-way isolation is provided. The DRX Series has an RS-485 ouput; the DRN Series supplies a field-selectable output of 0 to 10 VDC, 4 to 20 mA, or 0 to 20 mA. DRN modules are set up through optional software and operate independently. Another 24-VDC module supplies the required power. DRX: $250 to $300; DRN: $295 to $355. Omega Engineering, (203) 359-1660.
Turnkey System
Networks Data
The Series 6000 is a turnkey data acquisition system for the PC with selectable enclosures that hold three or 16 modules, expandable to 256 by adding slave enclosures. The plug-in boards handle thermocouple, RTD, LVDT, strain gauge, frequency, voltage, current, and digital inputs along with analog and digital outputs. On a Pentium computer with PCI interface, more than 800 KS/s may be written to disk, and a history buffer saves the last 2 million samples. Programmable data-selection scans and sample rates also are offered. The data and control interface is IEEE 488, and Windows-based software is available in single user or client/server editions. Call company for price. Pacific Instruments, (925) 827-9010.
Integrated Enclosure Offers
Universal Signal Conditioning
The 27000A is a series of mainframes, modules, and software that works with available controllers or the PC, via RS-232, to provide universal signal conditioning solutions. The channel count is 1,024 per controller and 16 modules with up to 256 channels per chassis. Boards interface with transducers, frequency, RTDs, thermocouples, potentiometers, strain gauges, charge, voltage, and piezoelectric. Software for Windows or LabWindows/CVI™ provides a graphical user interface to set up the signal conditioners. Call company for price. Precision Filters, (607) 277- 3550.
Data Measurement Solution
Accommodates 256 Inputs
The PreSys 1000mV Data Measuring System holds up to 16 modules, each with 16 inputs. The programmable analog input signal conditioning module has 16-bit resolution with a maximum sensitivity of 5 mV per step. The 3-dB, high-frequency cutoff of the four-pole filter is adjustable between 10 Hz and 1 kHz. Windows-based visual basic software guides you through test parameters. The entry-level system contains 64 analog input channels, a 16-bit ADC, a 128k FIFO, an optically isolated analog and digital bus, an SCSI-2 interface, and software. Starts at $20,000. Preston Scientific, (714) 632-3700.
Adapter Houses Choice
Of Data Acquisition Modules
In the DAQ-1201 Desktop Data Acquisition Adapter, you have a choice of 3U-sized QTC Signal Conditioning Modules. The adapter is the housing, DC power supply, and connection to the PC. QTC modules interface with thermocouples, RTDs, strain gauges, accelerometers, voltage, and current. Windows-based software and 32-bit universal drivers that support application development are included. DAQ-1201 adapter: $695; QTC modules: $450 to $895. Quatech, (800) 553-1170.
125-MHz Waveform Digitizer
Is PCI-Compatible
The PDA12A Board for PCI offers 12-bit resolution and a full-scale voltage input range of 100-mV p-p to 3.0-V p-p. With two channels, the signals may be sampled simultaneously at 62.5 MS/s or interleaved to achieve the single-input, 125-MS/s rate. Memory size is 512k; 2M is optional. Data transfer rates of 250 MB/s are accomplished via a proprietary bus. One master and three slave PDA12As may be connected with a 20-conductor ribbon cable. Source codes providing C functions and device drivers for Windows 95/NT are included. $5,900. Signatec, (909) 734-3001.
Eight-Input Module
Has Variable Filtering
The TD-GP/AAF is a drawer for the DATAshuttle Express Enclosure. It contains eight inputs, each with a five-pole filter and software-selectable cutoff frequencies from 12 Hz to 40 kHz. Attenuation is 30 dB per octave, and phase match is 2°. The Input voltage ranges are ±2.5 mv to ±10 V for thermocouples, strain gauges, and a plug-in for RTDs. The sampling rate is 100 kS/s. Software for Windows automatically configures I/O and includes messages, math functions, alarms, and datalogging. $2,500. Strawberry Tree, (800) 736-8810.
Board Has Eight
24-Bit ADCs
The PAR24B is an eight-input ADC board with a separate 24-bit ADC for each channel. No noise, crosstalk, or phase errors are introduced into the measurement since there is no multiplexing. True 22-bit accuracy occurs at a 20-Hz sample rate, with a maximum rate of 1 kHz. Gain, offset, and autocalibration are programmable for each input. The jumper-selectable input range is ±2.5 V or ±10 V, and the PC interface is via an EPP/BPP parallel port. Application software for Windows, source code, and circuit diagrams are included. $800. Symmetric Research, (702) 341-9325.
PCI Board Offers
12-Bit Resolution
The PD-MF-16-330/12L PCI Card has 16 inputs with 12-bit resolution at 330 kHz. There are two 12-bit analog outputs at 200 kHz, eight digital inputs, eight digital outputs, and three 16-bit counters/timers. Gains are programmable from 1 to 1,000. Software for Windows 95/98/NT supports visual programming, and has drivers for third parties. $895. United Electronic Industries, (800) 829-4632.
Remote Data Acquisition System
Comes in NEMA4 Enclosure
The Remote Intelligent Data Acquisition and Control System (RIDACS) embodied in a 4″ or 6″ deep, watertight, NEMA4 enclosure occupies one of up to 256 locations in a two-wire, RS-485 network. Inside is a DC power supply and two optically isolated modules: the RAG128 12-bit ADC with programmable gain and eight single-ended multiplexed inputs and a choice of the AIM-16P or LVDT-8 Signal Conditioning/Multiplexer Modules. The software supports high-level languages that use ASCII strings. Firmware can be updated over RS-485 or provided as custom. A development kit is available. $1,114 to $1,593. Acces, (800) 326-1649.
Data Collector Expands
To 300 Inputs, Scans Fast
The DC100 data collector offers a multiline display, an editing keypad, a floppy disk drive, 1-MB nonvolatile RAM, and slots for four I/O modules in a 10-lb package. The wide range of plug-in modules allows tailoring from 10 to 40 channels, expandable to 300 channels. All channels are scanned in 0.5 s. Captured data is convertible to ASCII for import into standard spreadsheet programs with no intermediate software, and available RS-232/422/485 and IEEE 488 interfaces facilitate data transfer or networking via a PC. From $3,010. Yokogawa Corp. of America, (800) 258-2552, ext. 577.
System Provides 32 Channels
Of Continuous Data
The SPS6132 Signal Processing System provides 32 fully conditioned
±10-VDC analog outputs with up to 10 kHz-per-channel bandwidth. Typically accurate to within ±0.02% of full scale following calibration, these continuous, anti-aliased outputs serve as inputs for conventional analog-to-digital conversion. All required signal conditioning is performed, including excitation, scaling, and active low-pass filtering. Logic I/O implements control parameters, and optional analog modules supply specific functions. System configuration and calibration are accomplished via a serial PC link and software. Front-panel display is optional. $1,495. Daytronic, (937) 866-3300.
Two New Cards Control
Digital I/O for PCI Bus
With 16 digital inputs and 16 digital outputs, all rated at 48 VDC and having 2,500 VDC isolation, the PCI-1750 supports wet and dry contact connections for easy interface with other devices. For TTL interface, the PCI-1751 emulates mode 0 of the 8255 chip, and provides 48 TTL input or output lines. The user can set any port to output with a jumper, and this avoids being software configured as inputs by default. The board also includes two 16-bit timers and one counter. Advantech, (408) 245-6678.
Copyright 1998 Nelson Publishing Inc.
November 1998
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