Absolute Accuracy of Dynamic Signal Acquisition Devices
Hardware: Modular Instruments>>Dynamic Signal Acquisition and Analysis (DSA)
Problem: How can I calculate the absolute accuracy of a NI Dynamic Signal Acquisition (DSA) board?
Solution: Absolute accuracy is used to define the overall uncertainties of a measurement. For a DSA board, it can be computed with the following formula:
Absolute Accuracy =
Gain Error +
Offset Error +
Flatness
This formula uses the three major sources of error in the system:
- Gain Error
A measure of the deviation of the gain of an amplifier from the ideal gain. Expressed in dB or % of a reading, it can be viewed as the deviation m from the voltage transfer function of mx+b.
- Offset Error
A constant error added to a measurement along the whole transfer curve. In mx+b, the offset error is b.
- Flatness
A term expressed in dB to specify the limits within which the amplitude of a signal varies across a given frequency range. It measures the "frequency response" of the board. It is referenced to a measurement on a 1 kHz signal. It does not apply to DC signals. It only applies to waveform tones.
As an example, the Absolute Accuracy of the NI 4461 is calculated for both the Analog Input and Analog Output is shown below. For Analog Input, we assume we are acquiring a 1 kHz sine wave with 10 V amplitude. For Analog Output, assume we are generating the same frequency signal.
IMPORTANT NOTE: The values used in this KnowledgeBase are the typical specifications, not the maximum error specifications. These values should be reasonable if the device has been setup properly. Specifically, it is important that the device has been allowed to warm up for the correct time and self calibrated within 24 hours of taking the measurements. The reasons for each of these factors are explained below:
- Warm-up Time:
The highly linear Analog to Digital Converter (ADC) that the NI DSA devices use is a Sigma-Delta type converter. One inherent property of this type of ADC is that it is very sensitive to temperature changes. By allowing the card to reach a steady state temperature, this sensitivity can be reduced. See the specifications manual for your NI DSA device for the needed warm-up time.
- Self Calibration:
Once the NI DSA device has come to a steady temperature state, a self calibration should be performed to eliminate any differences since the last calibration. This is important because the constant sources of error may be slightly different since the last calibration. The specifications used below assume a self calibration was performed within 24 hours and ±5°C of the measurement. Refer to the NI 446x Calibration Procedure for more information.
Analog Input To find the AI Gain Amplitude Accuracy, AI Offset and AI Flatness refer to the
NI 446x Specifications.
For a 1 kHz sine wave with an amplitude of 10V, the following values apply:
AI Gain Amplitude Accuracy = ±0.03 dB max
AI Offset (with no Gain applied) = ±0.7 mV max
AI Flatness (with no Gain applied) = ±0.003 dB typical
Gain Error is calculated based on this Gain Amplitude Accuracy of a 1kHz input tone.
Gain Amplitude Accuracy = ±0.03 dB max
Gain Amplitude Accuracy(dB) = 20 log (1 + (Gain Error(%)/100))
±0.03 dB = 20 log (1 + (Gain Error(%)/100))
Gain Error(%) = 0.3460
Gain Error(V) = Input Voltage x Gain Error(%) = 10 x 0.003460 = 34.60 mV
Where the Input Voltage refers the the voltage range the device is configured for.
Flatness is converted to voltage in an identical manner.
Flatness = ±0.003 dB max
Flatness(dB) = 20 log (1 + (Flatness(%)/100))
±0.003 dB = 20 log (1 + (Flatness(%)/100))
Flatness(%) = 0.0345
Flatness(V) = Input Voltage x Flatness(%) = 10 x 0.000345 = 3.45 mV
From these two calculation the total AI Accuracy is calculated as follows:
AI Accuracy = AI Gain + AI Offset + AI Flatness
AI Accuracy = 34.60 mV + 0.7 mV + 3.45 mV
AI Accuracy = ±38.75 mV
Analog Output The analog output accuracy can be calculated in the same way. See the following sample calculation:
AO Gain Amplitude Accuracy = ±0.04 dB max
AO Offset = ±1 mV max
AO Flatness = ±0.008 dB max
Gain Error (calculated using formula above) = 46.16 mV
Flatness (calculated using formula above) = 9.21 mV
The final calculation for the analog output accuracy is as follows:
AO Accuracy = AO Gain + AO Offset + AO Flatness
AO Accuracy = 46.16 mV + 1 mV + 9.21 mV
AO Accuracy = ±56.37 mV
Related Links: Dynamic Signal Acquisition Support PageProduct Manuals: NI 446x Specifications (December 2008)Product Manuals: NI 446X Calibration Procedure (March 2006)Product Manuals: NI Dynamic Signal Acquisition User Manual (August 2008)
Attachments: 
Report Date: 02/18/2005
Last Updated: 07/22/2009
Document ID: 3IHCT5LE
