How Do I Determine the DC Accuracy of a NI Digitizer/Oscilloscope for a Given Input Value?Hardware: Modular Instruments>>HighSpeed Digitizers (Scopes)
Problem: I need to determine the DC accuracy of a NI Digitizer/Oscilloscope. How do I accomplish this? Solution: NI specification documents specify the DC accuracy in terms of a percent of the input voltage plus a small voltage offset. If you need the actual voltage range of the DC accuracy for a given input value, then you will need to perform the calculations as shown below. Definition:
Accuracy is defined as the ability of a device to indicate the true value of a measured signal. In order to calculate the DC accuracy of a NI Digitizer/Oscilloscope, you need to have several quantities: the input range, the input voltage level, the gain error, and the offset error.
Expected Voltage Calculation: DC Accuracy = ± (Gain Error * Applied Voltage + Offset Error) The expected measured voltage for a given applied voltage at the input can be calculated by applying the following formula: Expected Voltage = Applied Voltage ± DC Accuracy = Applied Voltage ± (Gain Error * Applied Voltage + Offset Error) As an example, take a look at Figure 1 for the DC accuracy of a NI PXI5124. If we have configured the digitizer to use the 10 V_{pkpk }range, then 0.65% corresponds to our Gain Error and 10.0 mV corresponds to our Offset Error. If we then apply 7.95 V to the input, our expected voltage will be: Expected Voltage = 7.95 ± (0.0065 * 7.95 + .01) V = 7.95 ± 0.061675 V
You can see how the right graph on Figure 2 below shows that if you applied 7.95 volts your expected voltage can be anywhere between the green and red light. Figure 2. Expected Voltage when 7.95 V is applied on the input of NI PXI5124 configured for a 10 V_{pkpk }range. DC Drift Calculation: DC Drift = ± (0.00057 * 7.95 + 0.00006 * 10 + 0 .0009) * 2 V = ± 0.0060315 V
To get our overall uncertainty we need to add the DC drift to our previous Expected Voltage calculation as shown below. Expected Voltage = 7.95 ± 0.061675 ± 0.0060315 V = 7.95 ± 0.0677065 V We can see in Figure 4 below how the DC drift extends out the range of the Expected Voltage. The yellow and blue lines represent the uncertainty attributed to both the accuracy and the DC drift.
Special Cases: Related Links: Product Specifications: NI 5153/5154 Specifications KnowledgeBase 42AJ3IJQ: Troubleshooting Steps to Improve Scope Accuracy KnowledgeBase 3ZID3RSE: Calculating the Error of the Two Channel Measurements in NISCOPE Specifications Explained: NI Oscilloscopes and Digitizers Attachments:
Report Date: 07/31/2008 Last Updated: 10/04/2017 Document ID: 4NUGHDD2
