How to Calibrate a Computer-Based Data Acquisition Device

With computer-based data acquisition devices, engineers and scientists can easily take transducer and signal measurements using common desktop computers or computing platforms, such as PXI, that are designed for measurement and automation. Factors such as time in service, temperature, humidity, environmental exposure, and abuse, however, all can affect the accuracy of these measurements. Calibration quantifies measurement uncertainty by comparing your measurements to a known standard. If your device’s measurements exceed the published specifications, then you adjust your device’s measurement capability until it meets the published specifications. To ensure measurement accuracy, all measurement system devices require periodic calibration. This article reviews three types of calibration for computer-based data acquisition devices—internal calibration, external calibration, and system calibration.

click the image to enlarge Figure 1: Internal device calibration using National Instruments DAQ software.

Internal calibration

The main purpose of an internal calibration is to compensate for changes in the operating environment such as temperature variances and other factors that may affect the measurement circuitry making measurements. You should perform an internal calibration of your data acquisition device after initial installation, whenever the environmental conditions change, or when the manufacturer specifies you should perform an internal calibration.
During an internal calibration, the device digitizes a high-accuracy onboard source, compares the reading to the source’s known value, and updates and stores new
correction factors in the device’s onboard calibration EEPROM (electrically erasable programmable read-only memory). Calibrating to an onboard source, which is calibrated and usually traceable to national or international standards such as NIST, helps the device maintain a high level of measurement accuracy under different environmental conditions as well as traceability to known standards. A software function call typically initiates an internal calibration, and a software-controlled internal calibration technique can lead to reduced test times.
With the broad selection of data acquisition devices available today, any device you purchase should support internal calibration and facilitate a “hands free” process – that is, the process should not require potentiometer or jumper adjustment on the device. Without internal calibration, you cannot be sure that the measurements you are making meet device specifications.

External calibration

Metrology laboratories or other facilities that maintain traceable standards usually perform external calibrations. During this process, metrologists compare the device’s measurement capabilities to its published specifications to quantify how well the device is measuring. If the device is not measuring within specs, the metrologists may need to adjust the board’s measurement capabilities. Adjustment involves determining new calibration constants and quantifying and storing the onboard reference value in the calibration EEPROM.
To maintain traceability, metrologists should perform all verifications and adjustments with standards that are traceable to national or international standards such as NIST. Like internal calibration, the external calibration procedure should not require potentiometer adjustment or jumper movement and should be well-documented by the device manufacturer.
Once they finish an external calibration, metrologists store the new calibration constants in a protected area of the device’s memory, where they cannot be retrieved. This protects the calibration integrity from accidental tampering. A data acquisition device manufacturer should provide the appropriate procedure, documentation, and calibration software required to perform an external calibration.

click the image to enlarge Figure 2: System Calibration—a black-box approach to calibrating the entire measurement path.

System calibration

Internal and external calibrations are necessary to help ensure the accuracy of a single system componentthe measurement device. However, neither calibration accounts for the inaccuracies introduced by the entire measurement path. System calibration provides a black-box approach to calibration for an entire system rather than calibrating each system component.
System calibration software guides you through the application of a series of known reference signals to stimulate the system, and, with each reference, the software records the measured values. The result is a transfer function that compensates for all measurement errors in the system. Apply this transfer function to every measurement you make using the calibrated measurement path.

Conclusion

Computer-based data acquisition devices require periodic internal and external calibrations to ensure adherence to the stated accuracy specifications of the device. A properly designed measurement device should offer software, tools, and services that help metrologists perform hands-free external calibration and help you perform hands-free internal calibrations as well as system calibrations for an entire measurement path.