How Do I Calculate Absolute Accuracy or System Accuracy?Hardware: Multifunction DAQ (MIO)
Problem: I am aware of the online accuracy calculator, but how does it calculate my system accuracy based on the inputs I give it? Also, what if for some reason I cannot use the online accuracy calculator such as for cascaded SCXI modules? How do I calculate the absolute accuracy of my components and the system accuracy of my entire measurement? Solution: There are three steps when calculating the system accuracy of a measurement:
Step 1: Identify variables affecting the calculated accuracy First, determine how each component is connected to the system and identify all pertinent variables that will affect the calculated accuracy. For this example, we'll assume an SCXI1125 isolation module is cascaded using the SCXI1352 cable to an SCXI1141 filter module. This filter module is then connected to an NI 6052E DAQ device. SCXI 1125 > SCXI 1141 > NI 6052E Assume the following:
Step 2: Calculate the Absolute Accuracy for each Component Next, calculate the Absolute Accuracy for each component. For any individual device with gain (either an amplifier or attenuator), for a specified nominal range, National Instruments provides an absolute accuracy specification in millivolts. Depending upon the presentation of different errors, there are two different equations to use to calculate the accuracy. Both set of equations are listed below:
Temperature Drift = ±(Input Voltage * % of Reading/° C + Offset/° C) * Temperature Difference Temperature effects are already accounted for in the specification values unless your ambient temperature is outside of the 15 to 35° C range. For instance, if the ambient temperature of your measurement system is at 45° C, you must account for 10° C of temperature difference. In this case, since the temperature is assumed to be 25° C, we don't have to add in anything for Temperature Drift. Please note that terminal blocks or connector blocks are not considered gain stages unless they have attenuation circuitry. Modules or DAQ devices that do not have amplifiers are also not considered gain stages. Step 3: Calculate the system accuracy and system accuracy RTI Finally, we will use the Absolute Accuracy from each component to calculate the System Accuracy and System Accuracy RTI. Like the Pythagorean Theorem, the System Accuracy is equal to the square root of the sum of the squares of each component's Absolute Accuracy. The System Accuracy Relative To Input (RTI) is calculated as follows: System Accuracy RTI = System Accuracy / Input Voltage System Accuracy Example Calculations for the above setup: Here are the absolute accuracy calculations for each component of our system: Absolute Accuracy  SCXI1125 System Accuracy System Accuracy = ( (Absolute Accuracy 1)^2 + (Absolute Accuracy 2)^2 + (Absolute Accuracy 3)^2 + ...) )^(1/2) System Accuracy RTI System Accuracy RTI = System Accuracy / Input Voltage Related Links: Support Utilities: Accuracy Calculator Developer Zone Example: Absolute Accuracy Calculator NI Developer's Zone: Using Calibration to Improve Measurement Accuracy NI Developer's Zone: Accuracy and Uncertainty KnowledgeBase 2W3ISON1. Finding the Accuracy of Measurements in the Units of a Sensor KnowledgeBase 3IHCT5LE: Absolute Accuracy of Dynamic Signal Acquisition Devices Attachments:
Report Date: 05/05/2003 Last Updated: 03/11/2010 Document ID: 2X4HGEBG
