World’s Most Accurate Multifunction Data Acquisition Device

Measurement accuracy is one of the most important specifications of any data acquisition (DAQ) system. The following five technologies show why the new high-accuracy National Instruments M Series devices are the world's most accurate multifunction data acquisition devices. Review the real experimental data in this paper to compare the accuracy of the 18-bit NI PCI-6289 to that of the 16-bit NI PCI-6229 M Series devices.

The World’s Most Accurate Multifunction Data Acquisition Device

When you need a data acquisition system, you must make many important decisions before ever acquiring the first sample. How many channels? What type of acquisition rates? What type of I/O? The more you develop and define the requirements of your system, the more questions you must answer. One of the most important questions relates to the integrity of the data itself – how accurate will it need to be? The acquired data only can be as good as the data acquisition system, and accuracy is a system specification that should not be compromised. In the past, device accuracy was compromised for faster sampling rates. For this reason, National Instruments introduced the high-accuracy M Series data acquisition (DAQ) devices to set a new precedent for data acquisition device accuracy. The following five technologies show why the new high-accuracy M Series devices are the world's most accurate multifunction data acquisition devices. Review the real experimental data in this paper to compare the accuracy of the 18-bit NI PCI-6289 to that of the 16-bit NI PCI-6229 M Series devices.

1. Make Measurements with 4X Improvement in Analog Resolution

The number of bits in an analog-to-digital converter (ADC) determines the theoretical resolution and limits the accuracy of a data acquisition device. A higher number of bits results in better device resolution. Prior to the launch of NI M Series DAQ, most multifunction data acquisition devices used a 12- or 16-bit ADC, which can represent an analog signal in 4,096 and 65,536 discrete levels, respectively. This means that the theoretical resolution (the smallest resolvable voltage) for a voltage range of ±10 V is 4.88 mV for a noiseless 12-bit ADC and 305 μV for a noiseless 16-bit ADC. Without averaging data at a fixed-range setting, you cannot improve the data acquisition device resolution, which limits your device measurement capabilities. To improve it, you must use a device with a higher-resolution ADC.

High-accuracy National Instruments 628x M Series DAQ devices increase measurement resolution by using the AD7674 18-bit ADC from Analog Devices. By making use of the 18-bit converter, National Instruments can attain higher resolution and more accurate measurements without sacrificing sampling speeds. The AD7674 is a high-resolution, fast-throughput, and high-accuracy 18-bit ADC making it ideal for data acquisition applications. Additionally, the AD7674 design provides a fully balanced signal path to minimize common-mode noise. The 18-bit ADC offers a 64X resolution increase over 12-bit ADCs and a 4X increase over 16-bit ADCs. The 18-bit ADC provides 262,144 discrete levels to resolve voltage in comparison to the 4,096 and 65,536 levels obtained with lower-resolution ADCs. The analog inputs of the NI 628x accurately provide more than 5½ digits of resolution for DC measurements. For the same ±10 V, the high-accuracy M Series devices measure much smaller voltage levels and have a theoretical resolution of 76 μV. Figure 1 demonstrates both the 16-bit PCI-6229 and the 18-bit PCI-6289 measuring a ±600 μV, 1 kHz sine wave on the ±10 V range by acquiring 500 samples at 250 kS/s. The PCI-6289 measures and digitizes the sine wave much more accurately to the original signal.
The substantial increase in device resolution available with the NI 628x DAQ devices increases measurement capabilities because it can measure voltages that other multifunction data acquisition devices could never detect. Though ADC resolution theoretically determines the smallest resolvable voltage, it does not tell the whole story on a device’s measurement capabilities.

2. Minimize Noise with Onboard Lowpass Filter

The NI 628x M Series devices are the first multifunction DAQ devices from National Instruments to include an onboard programmable lowpass filter. The lowpass filter blocks signals above a set frequency, improving accuracy by eliminating high-frequency noise components. The lowpass filter greatly contributes to the accuracy improvement over other data acquisition devices, and provides a 14 to 77 percent increase in measurement sensitivity depending on the range. National Instruments defines measurement sensitivity as the smallest voltage change in the input signal that can be detected, assuming 100 point averaging, at a statistical confidence of 95 percent. The smallest resolvable voltage according to the ADC resolution is 76 μV at the ±10 V range. However, the measurement sensitivity of NI 628x devices at that same range is actually 24 μV – over three times smaller than what the ADC resolution theoretically allows. In fact, the high-accuracy devices capably detect a voltage change as low as 0.8 μV at the ±100 mV range. Figure 2 shows the experimental data of acquiring a ±500 μV sine wave on the ±10 V range. The left graph shows data from the PCI-6229 and the right graph from the PCI-6289 with the lowpass filter enabled. The 18-bit ADC and enabled lowpass filter of the high-accuracy M Series device allows detection of the sine wave.
The filter is a second-order lowpass filter with a fixed cutoff frequency of 40 kHz. When enabled, the 40 kHz cutoff frequency eliminates any unwanted high frequencies and wideband noise that would otherwise be present within the 750 kHz device bandwidth. The lowpass filter has a fully balanced design just like the 18-bit ADC and the low-noise, fast-settling NI-PGIA 2 amplifier. The high-accuracy M Series devices thereby contain a fully balanced signal path – from input amplifier to ADC. The fully balanced design ensures maximum noise rejection along the entire signal path.

NI-DAQmx driver and measurement services software provides the seamless integration necessary to easily obtain the added accuracy in your analog measurements from using the onboard lowpass filters. As shown in Figure 3, enabling or disabling the onboard lowpass filter is as simple as writing a true or false to a NI-DAQmx Channel Property Node.
Another measure of multifunction data acquisition device quality is the multiple-channel scanning ability. NI 6284 and NI 6289 devices provide 32 channels of analog measurements at 18-bit resolution. ADC settling time can significantly impact accuracy when you are scanning multiple channels at fast rates. Settling time is the amount of time required for a signal you are amplifying with the onboard programmable gain amplifier to reach a specified level of accuracy. If the programmable gain amplifier used on the data acquisition device does not have fast enough settling time, you may digitize the measured signal inaccurately. For any given level of resolution (or accuracy), shorter settling times are desirable because they facilitate faster sampling rates without sacrificing accuracy.

To ensure accurate measurements, National Instruments designed the M Series with a custom programmable gain instrumentation amplifier (PGIA) called the NI-PGIA 2. The NI-PGIA 2 custom amplifier design incorporates a fully balanced signal path paired to the lowpass filters and 18-bit ADC. NI-PGIA 2 technology improves accuracy by minimizing settling time and maintaining the specified resolution even at the maximum sampling rate of the device. The NI 628x DAQ devices settle to within 15 ppm within 2 μs following a full-scale step (typical). The NI-PGIA 2 is optimized for different performance criteria based on the M Series family -- low-cost, high-speed, or high-accuracy. Figure 4 demonstrates the settling time capabilities of the NI-PGIA 2 on the PCI-6289, optimized for 18-bit fast settling, low noise, and high linearity, versus the NI-PGIA 2 on the PCI-6229. In this acquisition, each device scans two channels at 100 kS/s on the ±10 V range. One channel acquires a full-scale 1 kHz sine wave connected, while the other is grounded. The plots show the acquired signal on the grounded. The NI-PGIA 2 optimized for the high-accuracy device clearly demonstrates superior settling time and noise suppression.
Even with the improved accuracy of the M Series analog front-end design shown in Figure 5, you must correct inherent error caused by the system component non-idealities appropriately. In many data acquisition devices, you implement system error correction with a two-point calibration technique that handles both offset and gain error. Unfortunately, this common self-calibration device technique cannot correct for nonlinearity error inherent in all ADCs. NI-MCal calibration, a new calibration technology introduced as a feature on National Instruments M Series devices, lets you take a unique approach to device self-calibration. The new calibration technology uses new techniques in hardware and software to characterize and correct offset, gain, and nonlinearity error.

NI-MCal calibration technology has three primary benefits that contribute to generating the most accurate measurements possible. First, at the heart of the technology is an algorithm that determines a set of third-order polynomial coefficients to accurately translate the digital code of an ADC into voltage data. In addition to correcting offset and gain errors, the third-order polynomial provides compensation for nonlinearity error -- an error not previously corrected by traditional two-point calibration techniques.

Next, NI-MCal provides self-calibration at all input ranges by dynamically loading the channel-calibration parameters while keeping up with high-speed, multichannel acquisition. Self-calibration at all input ranges means accurate measurements on all channels of a multichannel scan list, regardless of the individual channel gain settings.

Finally, high-accuracy M Series devices employ an ultrastable 7 V voltage reference that provides extremely low drift to guarantee accurate measurements between external device calibrations. The combination of the 7 V reference and NI-MCal improve the external device calibration interval to a full two years – an improvement over the one-year interval standard with other data acquisition devices. The low-cost M Series devices use a different voltage reference for calibration purposes. To demonstrate the impact of using a different reference, Figure 6 shows four different plots with the left plots representing the PCI-6229 and the right plots representing the PCI-6289. The top plots demonstrate the drift of the onboard voltage references over a three-day period in units of ppm. The bottom plots display the corresponding room temperature over the three days. The voltage reference of the PCI-6229 is much more susceptible to a change in room temperature, whereas the PCI-6289 stays consistent around 0 ppm regardless of a change in temperature, thus guaranteeing maximum accuracy. For more information on the NI-MCal calibration technique, refer to the NI-MCal white paper.In addition to the unique analog measurement features, high-accuracy M Series devices also provide improved waveform generation. In particular, NI 6281 and NI 6289 devices, with two and four 16-bit analog output channels, respectively, can offer user-defined offset and range parameters. With a user-defined, programmable offset, you have many choices for the analog output offset including the onboard 0 V, 5 V, a generated voltage from a different onboard analog output channel, or an externally provided signal. With this programmable reference, you can target the analog output with a range close to the desired voltage you need to generate, thus maximizing the resolution of the generated signal.

Some applications require small changes in analog output level around a fixed DC offset. For example, modeling 5 V power supply noise requires analog outputs that can simulate a small amount of noise (sub-mV) on a 5 VDC signal. With a traditional multifunction data acquisition device that has a single output range without offsets, you would have to set the range at 0 to 10 V. Even with 16-bit resolution, the digital-to-analog converter (DAC) could only represent 153 μV changes.

The programmable analog output offset feature helps you achieve much finer resolution around a specified offset value. When you use it with the programmable reference feature, the programmable offset enables an M Series device to model 5 V power supply noise by using all 16 bits of resolution on a 4 to 6 V range (5 V offset with a ±1 V reference). This improves the minimum code width to 31 μV – almost a 5X increase over nonprogrammable offset and range devices. Figure 7 shows the low-cost and high-accuracy M Series devices generating a ±1 mV, 50 Hz sine wave on top of a 5 VDC signal. The low-cost M Series device without programmable offsets and ranges uses a range of ±10 V to best represent the signal. The high-accuracy device with the programmable offsets and ranges uses an offset of 5 V and a range of ±1 V. The offset and range features of high-accuracy M Series devices deliver a more accurate representation of the actual signal.The previous five technologies combine to deliver the absolute accuracy specifications of the high-accuracy M Series device. The term “absolute accuracy” defines the worst-case error of a measurement for a given data acquisition device at a specified range, and it provides useful information about the reliability of a measurement in voltage units. Absolute accuracy is a function of temperature, voltage reading, offset, gain, and noise uncertainty. As shown in Figure 8, the high-accuracy M Series devices have a remarkable worst-case absolute accuracy on the analog inputs of 980 μV at the ±10 V range and 28 μV at ±100 mV. View the high-accuracy M Series device data sheets and specifications for more information.

In the high-tech world that we live in today, making mistakes in data acquisition can be expensive. In some applications, you might only have one chance to acquire all of the required data because of the high cost for each test run. The high-accuracy M Series devices make sure that the data from your data acquisition application will not be compromised. View the detailed specifications of the full line of high-accuracy M Series devices and start taking uncompromisingly accurate measurements.
Related Links:
Learn more about high-accuracy M Series DAQ.
See how the NI-MCal calibration technique increases measurement accuracy.
Increase system accuracy with signal conditioning.
Watch the interactive M Series tutorial and learn from the engineers.

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