This automobile lubrication application uses an SCXI chassis and LabVIEW running on an Apple Macintosh.

Application notes can help you understand all the specifications on DAQ products. While evaluating DAQ products, also consider the DNL (differential nonlinearity), relative accuracy, settling time of the instrumentation amplifier, and noise. You should do this because what you don't know about a DAQ board you're considering can hurt your measurements.

Straight Line Deviation
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Ideally, as you increase the level of voltage applied to a DAQ board, the digital codes from the A/D converter should also increase linearly. If you were to plot the voltage versus the output code from an ideal A/D, the plot would be a straight line. Deviations from this ideal straight line are specified as the nonlinearity.

DNL is a measure in LSB (least significant bits) of the worst-case deviation of code widths from their ideal value of 1 LSB. An ideal DAQ board has a DNL of 0 LSB. Practically speaking, a good DAQ board will have a DNL of ±0.5 LSB. There's no upper limit on how wide a code can be. Codes don't have widths of less than 0 LSB, so the DNL is never worse than -1 LSB.

A DAQ board with poor performance may have a code width equal to or very near zero, which indicates a missing code. No matter what voltage you input to the DAQ board with a missing code, the board will never quantize the voltage to the value represented by this code.

Sometimes DNL is specified by stating that a DAQ board has no missing codes. That means that the DNL is bounded below by -1 LSB but doesn't make any specifications about the upper boundaries. For example, all National Instruments' E Series boards are guaranteed to have no missing codes, and the specifications for them clearly state what the DNL is. That way you know the accuracy or the board's linearity.

If the DAQ board in the previous example, which had a code width of 1.5 microvolts, had a missing code slightly above 500 microvolts, then increasing the voltage to 502 microvolts wouldn't be detectable. Only when the voltage is increased another LSB, or in this example, beyond 503 microvolts, will the voltage change be detectable. As you can see, poor DNL reduces the resolution of the board.

Relative Accuracy
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Relative accuracy is a measure, in LSBs, of the worst-case deviation from the ideal DAQ board transfer function, which is a straight line. Relative accuracy is determined on a DAQ board by connecting a voltage at negative full scale, digitizing the voltage, increasing the voltage, and repeating the steps until the input range of the board has been covered.

When the digitized points are plotted, the result will be an apparent straight line, as shown in the figure below. Note the apparent straight-line plot generated by sweeping the input.

Note that you can also subtract actual straight-line values from the digitized values and plot these resulting points, as shown in the following figure. The maximum deviation from zero is the relative accuracy of the DAQ board. Note that by subtracting-out calculated straight-line values, the plot isn't straight.

The driver software for a DAQ board will translate the binary code value of the A/D converter to voltage by multiplying it by a constant. Good relative accuracy is important for a DAQ board because it ensures that the translation from the binary code of the A/D to the voltage value is accurate. Obtaining good relative accuracy requires that both the A/D and the surrounding analog circuitry be designed properly.

Sampling Multiple Channels
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Keep in mind that the instrumentation amplifier is most likely not to settle when you're sampling several channels at high gains and high rates. Under such conditions, the instrumentation amplifier has difficulty tracking large voltage differences that can occur as the multiplexer switches between input signals. Typically, the higher the gain and the faster the channel switching time, the less likely it is that the instrumentation amplifier will settle.

In fact, no off-the-shelf programmable-gain instrumentation amplifier can settle to 12-bit accuracy in less than 2 us when amplifying at a gain of 100. That's why National Instruments developed its NI-PGIA specifically for DAQ board applications (so the boards that use the NI-PGIA can consistently settle at high gains and sampling rates).

Now, let's consider noise. It's any unwanted signal that appears in the digitized signal of the DAQ board. Because the PC is inherently a noisy digital environment, acquiring data on a plug-in board takes a very careful layout using multilayer boards crafted by skilled analog designers. Simply placing an A/D converter, an instrumentation amplifier, and bus interface circuitry on a one- or two-layer board will most likely result in a very noisy DAQ board.

Shielding's Role
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You can use metal shielding on a DAQ board to help reduce noise. Proper shielding should not only be added around sensitive analog sections on a DAQ board, but must also be built into the layers of the DAQ board with ground planes. The figures here show the dc noise plot of two DAQ products, both of which use the same A/D converter. Two qualities of the DAQ board can be determined from the noise plots: the range of noise and the distribution.

The plot in the first (left-hand) figure, which is taken from NI's Model AT-MIO-16X board, has a high distribution of samples at zero and a very small number of points occurring at other codes. The distribution is Gaussian, which is what's expected from random noise.

From the plot, you can see that the peak noise level is within ý3 LSB. The plot in the second part of the figure (right-hand trace), made with a very noisy DAQ product from a different vendor, has a far different distribution. It has noise greater than 20 LSBs, with many samples occurring at points other than the expected value.

For the DAQ products in the figure below, the test was run with an input range of ±10 V and a gain of 10. Therefore, 1 LSB = 31 microvolts, so a noise level of 20 LSB is equivalent to 620 microvolts of noise. Note that the input to this instrumentation amplifier that's multiplexing 40 dc signals appears to be a high-frequency ac signal.

Evaluate the Specs
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With a sophisticated measuring device such as a plug-in DAQ board, you can get significantly different accuracies depending on whose board you're using. As an example, National Instruments goes to great lengths to make its boards extremely accurate--in many cases more accurate than stand-alone instruments. The company then publishes this accuracy in its specifications.

Be skeptical of boards that are inadequately specified, because the specification that's omitted may be the one that causes your measurements to be inaccurate. By evaluating more analog input specifications than simply the resolution of the A/D converter, you can make sure that you're getting a DAQ product that's accurate enough for your application.

Stimuli and Analog Outputs
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Analog output circuitry is often required to provide stimuli for a DAQ system. Several specifications for the digital-to-analog converter (DAC) determine the quality of the output signal produced. These specs include settling time, slew rate, and resolution.

Settling time and slew rate work together in determining how fast a DAC can change the level of the output signal. Settling time is the time required for the output to settle to the specified accuracy. The settling time is usually specified for a full-scale change in voltage.

The slew rate is the maximum rate of change that the DAC can produce on the output signal. Therefore, a DAC with a small settling time and a high slew rate can generate high-frequency signals, because little time is needed to accurately change the output to a new voltage level.

An example of an application that requires high performance in these parameters is the generation of audio signals. A DAC requires a high slew rate and small settling time to generate the high frequencies necessary to cover the audio range.

In contrast, an example of an application that doesn't require fast D/A conversion is a voltage source that controls a heater. Because the heater can't respond quickly to a voltage change, fast D/A conversion isn't necessary. So, the application will determine the DAC specifications.

Output Resolution
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Output resolution is similar to input resolution. It's the number of bits in the digital code that generates the analog output. A larger number of bits reduces the magnitude of each output voltage increment, thereby making it possible to generate smoothly changing signals. Applications requiring a wide dynamic range with small incremental voltage changes in the analog output signal may need high-resolution voltage outputs.

Triggers
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It's time to touch on both digital and analog triggering. Many DAQ applications need to start or stop a DAQ operation based on an external event. Digital triggers synchronize the acquisition and voltage generation to an external digital pulse.

Analog triggers, used primarily in analog input operations, start or stop a DAQ operation when an input signal reaches a specified analog voltage level and slope polarity.

National Instruments expertise in instrumentation led to the development of what's called Real-Time System Integration. The RTSI bus is included in NI's DAQ products. The RTSI bus uses a custom gate-array and a ribbon cable to route timing and trigger signals between multiple functions on one DAQ board, or between two or more boards.

With RTSI, you can synchronize A/D conversions, D/A conversions, digital inputs, digital outputs, and counter/timer operations. For example, with RTSI, two analog input boards can capture data simultaneously while a third board generates an output pattern synchronized to the sampling rate of the inputs.

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