For data acquisition (DAQ) measurements, accuracy indicates how faithfully a DAQ device reads the value of the signal it measures.
Inaccurate measurements can spoil your research, new design, or automated test applications and reduce yields in process applications. Therefore, ensuring that your DAQ system is accurate is a crucial part of developing your application.
Accuracy is often confused with resolution, which is the smallest amount of signal change that a DAQ device can detect. The overall accuracy and resolution of a device depends on factors such as gain, offset, and time operating environment.
The resolution of a plug-in data acquisition device is only partially related to the resolution of the analog-to-digital converter (ADC) chip. For example, an improperly designed plug-in DAQ device with a 16-bit ADC chip may perform with only 12 bits of resolution and have very poor accuracy. Overall, for the best results from your data acquisition hardware, you should understand the basic underlying technologies that deliver outstanding resolution and accuracy. National Instruments (NI) data acquisition hardware incorporates five unique, critical technologies to ensure reliable, highly accurate measurements to within 0,0127% of the actual signal is achieved.
Compensation for changes in operating temperature
An electronic component's specifications within a standalone instrument or a computer depends on the operating temperature. NI chooses electronic components that are highly resistant to operating temperature changes. For operating temperatures within 15°C to 35°C, NI designed its DAQ devices to eliminate errors due to temperature. Outside this range, NI designs its DAQ devices to reduce the amount of temperature error up to 0,0006% of the actual signal per °C. All of these features help ensure that measurements are highly reliable regardless of the operating temperature of one's desktop PC or laptop.
Several NI design advantages minimise error, such as:
Temperature drift protection circuitry - The preamplifier stage of the device uses a design that causes the components on the chip to respond to temperature in such a way that temperature drift errors cancel. For example, a high-quality resistor network accurately maintains set ratios, even at high temperatures. This gives the benefit of stable programmable gains across the operating range of the device.
An onboard temperature sensor - You can read the temperature of the onboard sensor from NI LabVIEW or other application development environments, then use a simple function call to update the calibration constants stored on the device. This onboard temperature sensor ensures that measurements are made under known operating temperatures, for more reliable measurements.
Instrument class amplifier
NI's custom instrument class amplifier, the NI-PGIA, is a special IC that ensures accuracy across a wide range of gains and sampling rates. NI designed this custom component because commercial amplifiers typically lose bandwidth at high gains, causing unacceptably slow settling times. The amplifier must settle and be stable without oscillations or ringing to make accurate measurements. Without this, a 16-bit ADC can have as much as 40 least significant bits (LSBs) of error in the signal at high gains and sampling rates. Other data acquisition vendors often overlook the importance of an instrument-class amplifier, which results in lower resolution and inaccuracies at high gains and sampling rates.
Resolution improvement technologies
Because of resolution improvement technologies, NI 16-bit multifunction DAQ devices can perform with up to 18-bit resolution and NI 12-bit devices can perform with 14-bit resolution. With a technology called NI dithering, one can see four times the improvement in resolution for 12-bit DAQ devices making low-frequency measurements (eg, temperature and strain). NI dithering circuitry actually adds a small amount of Gaussian noise to the signal. Adding noise may seem counterintuitive at first, but if the device adds the right type of noise to the incoming signal and averages the digitised values, a higher resolution is achieved than the specifications of the ADC chip. A simple way to visualise the benefits of adding noise is to envision the actual signal between two integer codes of the digitiser. (Remember that a 12-bit ADC approximates the signal with an integer value from 1 to 4096.) The Gaussian noise spreads the approximation among integer codes instead of always truncating to the ADC's lower integer value. With 16-bit products from NI, dithering circuitry is not necessary because the device design inherently adds the right amount and type of noise in the signal path to achieve the same dithering effect.
Onboard self-calibration
As time passes, the operating environment of the plug-in DAQ device can change. As previously discussed, the temperature and other parameters, such as humidity and atmospheric pressure, can vary. An electronic component's operating values also can drift slightly with use and time. To protect against these sources of inaccuracies, NI data acquisition hardware includes a precise voltage reference. Using this onboard voltage reference, you can adjust the DAQ device to ensure you are making measurements within tolerance. To make this adjustment, you can use a software function call to update the onboard calibration constants.
In addition to these onboard self-calibration features, all NI products come with a certificate of conformance. NI data acquisition products also include a NIST-traceable calibration certificate.
Cabling and signal conditioning
Some components of accuracy, including cables, terminal blocks, and signal conditioning, are often overlooked. NI cables are specifically designed to preserve signal integrity and accuracy. They include unique designs with separated analog and digital sections, individually-shielded analog outputs, individually-shielded twisted pairs for all analog inputs, large conductors for voltage lines, and a double-grounded shield. If the signal is corrupted before it is digitised, then an accurate digitiser is of little value. NI also offers a generic screw terminal block with shielding and a temperature sensor for cold-junction compensation. For high-accuracy measurements, NI SCXI is a modular signal conditioning platform that works with a wide variety of sensors ranging from microphones to strain gauges to thermocouples.
A dramatic example that illustrates the importance of signal conditioning is a thermocouple measurement taken at 25°C using a National Instruments thermocouple SCXI signal conditioning module versus a generic screw terminal connector block. For a series of single readings, the NI SCXI module achieves an accuracy of 0,3°C, compared to 5,0°C accuracy with the screw terminal connector block. Thus, the SCXI module provides a thermocouple measurement 10 times more accurate than the terminal block due to pre-amplification, low-pass filtering, and a more accurate temperature sensor. Signal averaging can further improve measurement performance for both the signal conditioning module and terminal block.
Know your specs
Measurement accuracy depends on many factors including the sensor, signal conditioning, cabling, and data acquisition hardware. As a developer, you should analyse the specifications and verify that the right technology is built into your DAQ system to ensure that you meet your accuracy and measurement tolerances. In addition, there is no standard for specifying the accuracy of plug-in data acquisition hardware. Therefore, different manufacturers often list the specifications that put their products in the best light. Overall, consider the specifications and technologies that go beyond the stated ADC resolution.
Be aware that temperature drift protection, an instrumentation-class amplifier, resolution-enhancing technologies, onboard self-calibration, signal conditioning, and cable design can all significantly improve measurement accuracy.
For more information contact National Instruments South Africa, 0800 203 199, [email protected]
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