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Louis Y. Ungar, Editor, The BestTest Newsletter
Data acquisition (DAQ) products are usually less expensive than other test equipment you may be using, but that is not a reflection of their simplicity. As with other test and measuring equipment the parameters you measure are of greater value than the equipment (or should be) and it is from the perspective of the unit under test (UUT) that you should evaluate the Best DAQ for you.
National Instruments in a glossary format
has done a very nice job of defining many of the terminology we use in specifying DAQs. We used that document to highlight the points we make here and readers are urged to consult that document (or our own
BestTest Dictionary) to get a better handle on some of the technical terms.
Data Acquisition usually collects analog inputs (sometimes digital inputs as well) and converts them to digital data that can be readily processed by a computer or controller, which is often a part of an automatic test equipment (ATE) set up. (DAQs with analog outputs also exist.)
Input Signal Characteristics
With analog input you will need to specify the number of analog channels you will need, whether they are single-ended, differential or pseudodifferential (with common grounds that are not connected to the computer ground.)
You should select the type of analog to digital converter (ADC) that is appropriate for your signals – successive approximation, flash, half-flash, integrating or delta-sigma modulation. You need to pay special attention to its resolution, its maximum sampling rate, input signal ranges, input coupling, maximum voltages, and its overvoltage protection.
Once the ADC converts the signal, handling the data is also crucial. Usually a buffer is used to collect the signals and the size of this buffer determines the maximum sampling rate. Data transfer can be accomplished in a number of ways – direct memory access (DMA), interrupts, and programmed I/O. The last two require CPU intervention and compromise speed. With DMA you generally maximize transfer rates, but there are still several modes to choose from. Single-mode transfers involve one data value transfer per request and may not be sufficiently fast. Block and demand transfer modes increase transfer rates. For block mode transfers the DMA controller performs the entire DMA sequence at the fastest possible rate in response to a single DMA request. For demand mode transfers, the DMA controller performs DMA transfers as fast as possible as long as the DMA request is asserted.
The ADC process is inherently compromised by inaccuracies and errors that should be specified for the DAQ you are considering. Relative accuracy measures the nonlinear errors associated with a DAQ system in terms of least significant bits (LSBs). Nonlinearity measures the percent of full scale range of the worst-case deviation from the ideal transfer function, a straight line. There is also a specification for one or more types of offset errors. There is also gain error to consider, in which the error increases as the gain increases. Since the ADC contains amplifiers, the amplifier characteristics should also be considered. These include, input and output impedance, bias and offset current and common mode rejection ratio (CMRR).
Consider the dynamic characteristics as well. For example, the bandwidth frequency range and the –3 dB cut-off are extremely important to match with the signal you expect to acquire. The dynamic range, the total harmonic distortion, amplitude flatness, phase linearity, interchannel phase, intermodulation distortion overload recovery time and crosstalk need to match your requirements.
Filter types indicate the purpose the filter serves. For example, simple single-pole RC filters are for noise rejection, while more sophisticated hardware and digital FIR filters are suitable for antialiasing filters. You should take into account that the cut-off frequency (-3 dB) attenuates power, and stationary filters such as RC filters express cut-off frequency in Hz while digital antialiasing filters that move with the sampling rate will express cut-off frequency in terms of the sample rate. You should look for the following specifications: Maximum external clock frequency, passband ripple, stopband attenuation, attenuation rate, normal mode rejection (NMR) and signal delay.
The ADC also has sample-and-hold (S/H) characteristics. The acquisition time is the time required for the S/H amplifiers to track to the correct voltage level of the input when coming out of the hold mode. This requires that you consider the hold-mode settling time, the droop rate, the interchannel and intermodule skew, the aperture delay time and jitter, and the hold step.
As temperature and other environmental consideration may come into play, you may need to take into account stability specifications. These include, recommended warm-up time, offset temperature coefficient, gain temperature coefficient, long-term stability and of course, calibration references.
Output Signal Characteristics
Many of the same characteristics apply to analog outputs as it does to analog inputs.
For Digital I/O some other characteristics need to be considered. Compatibility (TTL or CMOS), power-on state (logic low vs. logic high), digital logic levels (what voltage or current constitutes a low or a high), Darlington Drive Output Current, Common- Mode Isolation, Common –Mode Transient Rejection, transfer rate, propagation delay, and handshaking protocols.
Digital Relay Characteristics
DAQs use relays to switch the various channels. Relay types include single-pole double-throw (SPDT), latching or nonlatching. SPDT relays, also called Form C, can switch one channel from a common (COM) terminal between a normally open (NO) and normally closed (NC) terminal. Latching relays maintain their latest state even when powered down, while nonlatching relays return to their normally closed state at power down.
Specifications to look for include maximum input voltage, maximum switching voltage, maximum and minimum switching capacity, ON resistance, output capacitance, leakage current, contact material, expected life, thermal offset, maximum operating speed, relay set and reset times, as well as relay operate and release times.
Timing I/O Characteristics
Many of the parameters for Timing I/O are similar to those for Digital I/O, such as logic levels for input and output voltages and currents. There are some parameters, however, that are more specific to timing considerations.
Base clock (on-board counter timer) availability and accuracy should be considered. Maximum source frequency, minimum source pulse duration and minimum gate pulse duration parameters should be specified.
Also, as in Digital I/Os the data transfer mechanisms, including the availability of DMA transfer is an important consideration.
Triggers
You should understand both analog and digital triggers when you select a DAQ. For analog, the trigger source, its level, its slope, and its resolution should be quantified. You should know its hysteresis, bandwidth, coupling and the type of protection method it uses.
For digital triggering you need to consider the digital voltage level compatibility, responses to an edge or to a voltage level and the pulse width that can activate the digital trigger.
Excitation Channels
The number of excitation channels that a DAQ uses is an important selection criterion. The bridge type sensors that can be used (quarter, half or full) and the bridge completion (resistors) required is a factor to consider. The voltage level of excitation channels, their current drive, voltage drift, current level, maximum load resistance and current drift should be specified. There should also be cold-junction reference for output voltage, accuracy and repeatability.
Digital Signal Processor (DSP)
The digital signal processor on board the DAQ is an important criterion. Is the DSP chip computation performance optimized for math operations common to spectral analysis, such as Fast Fourier Transform or convolution? Do they feature parallel multiply operations and can they handle DMA transfers and interrupt requests? The DSP clock speed, instruction rate and floating point rate should be considered.
The size of the memory located in the DSP as well as its ability to perform dual-ported and dual access operations is also a factor.
DMA controller access, maximum transfer rate and number of channels should be specified. What types of interrupts does it use? What is the time required for transfer of 1 Kwords of data?
DAQ in System Integration
National Instruments uses the Real Time System Integration (RTSI) bus, but other system interfaces can be used for DAQ from other vendors. The system integrators will want to know a number of specifications to get the DAQ into a test system.
Electrical specification should provide for trigger lines, clock skew, DMA channels, serial links and their transfer rates, bus interfaces, and power requirements. The physical specifications should include dimensions, I/O connector types, PCMCIA card types if used. There should also be environmental specifications, including operating temperature, storage temperature and relative humidity.
Conclusion
For a number of choices of data acquisition, you can visit The BestTest Products and Services
Directory. Low price and high performance are desirable traits of DAQs. As a buyer, however, you have to be certain that it serves the purpose of testing your UUT. To make sure it does, you need to ask the right questions and this article alerts you to the specifications you should be looking for.
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