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 three different equations to use to calculate the accuracy. All equations are listed below:
Absolute Accuracy =± [(VoltageReading x GainError) + (VoltageRange x OffsetError) + NoiseUncertainty]
- GainError = ResidualGainError + (GainTempco x TempChangeFromLastInternalCal) + (ReferenceTempco x TempChangeFromLastInternalCal)
- OffsetError = Residual OffsetError + (OffsetTempco x TempChangeFromLastInternalCal) + INLError
- NoiseUncertainty* = (RandomNoise x 3) ÷ (√100)
*for a coverage factor of 3 σ and averaging 100 points
You can obtain the parameter values in the above equation by looking at the specifications found in each component's manual, specifications or datasheet.
Absolute Accuracy =± [(InputVoltage x %ofReading) + (VoltageRange x Offset) + SystemNoise +TemperatureDrift]
- Input Voltage is the voltage range the device is configured for. For example, for +- 10V, Input Voltage = 1
- %ofReading is a raw % accuracy based on the input gain. This accounts for gain error
- Offset is the maximum offset error. Many times, this offset can be in ppm instead of % so in order to change this into %, use this conversion: 1% = 10,000 ppm
- SystemNoise is the error introduced to the measurement by the device itself. This will often depend on filter settings or whether a single sample is taken as opposed to multiple samples being averaged
- TemperatureDrift** =± [(InputVoltage x %ofReading/ °C) + (%Offset/ ° C)]*
**This accounts for errors introduced by ambient temperature variation. See notes in additional information section.
You can obtain the parameter values in the above equation by looking at the information found in each component's specifications, manual or datasheet.
C Series Modules provide concise specifications for users to determine their accuracy throughout the entire operating temperature range (Calibrated Maximum -40 to 70). This accuracy entry accounts for temperature variations, worst case component tolerances, thermal hysteresis, etc.
Absolute Accuracy = ± [(InputReading x GainError) + (Range x OffsetError) + InputNoise]
- Input Reading- is the value that the user is trying to measure
- Gain Error- a raw % accuracy based on the input gain
- Range- the reading span that the device is configured for. For example for 0 to 10V, Range= 10
- OffsetError- the maximum offset error. In the datasheet it might be specified in ppm instead of %, for the conversion to percentage use 1%= 10,000ppm.
- InputNoise- is the error introduced to a measurement by the device itself. See notes in additional information section. See the image below for an example of what values to use.
All of the above specifications can be found in a datasheet or specifications of a component.