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How Do I Determine the DC Accuracy of a NI Digitizer/Oscilloscope for a Given Input Value?



Hardware: Modular Instruments>>High-Speed 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:

The DC accuracy for a given applied voltage at the input of a NI Digitizer/Oscilloscope is given by the equation:

 

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 PXI-5124.  If we have configured the digitizer to use the 10 Vpk-pk 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

 


Figure 1.  DC Accuracy Table as Found in the NI PXI/PCI-5124 Specifications document.

 

 

 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 PXI-5124 configured for a 10 Vpk-pk range.

 

DC Drift Calculation:

The previous calculation of the scope DC accuracy was done assuming the room temperature is within ± 5 °C of the room temperature during the last self-calibration of the device.   Outside of this temper
ature range we need to account for DC drift.  DC drift is a slow change in our accuracy due to temperature change.

For example, assume that a PXI-5124 was last self-calibrated at a room temperature of 28 °C.  However, the room temperature is now 35 °C.  This would mean that we are 2 °C outside of the ± 5 °C range where we don’t need to account for DC drift.  If we are still applying a voltage of 7.95 V to the device and we are still configured to use the 10 Vpk-pk range then we can see from Figure 3 below that the DC drift can be calculated using the equation below.

DC Drift = ± (0.00057 * 7.95 + 0.00006 * 10 + 0 .0009) * 2 V = ± 0.0060315 V



Figure 3.  DC Drift Table as Found in the NI PXI/PCI-5124 Specifications document.

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.



Figure 4.  Expected Voltage when 7.95 V is applied on the input of NI PXI-5124 configured for a 10 Vpk-pk range 7°C above the room temperature during the last self-calibration .

 

 

Special Cases: 

The NI 5153 and the NI 5154 have a little different approach for calculating the DC accuracy.  DC drift is taken into account when the measurement is taken in an environment with a temperature different from the last external calibration’s temperature.   Refer to the NI 5153/5154 specifications document for more information.
 

Offset Error is equal to a gain times the full scale of the range being used, sometimes our digizer/oscilloscope specifications call this out as a constant for the chosen voltage range and sometimes as a gain times the full scale.



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 NI-SCOPE
Specifications Explained: NI Oscilloscopes and Digitizers

Attachments:





Report Date: 07/31/2008
Last Updated: 10/04/2017
Document ID: 4NUGHDD2

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