How Do I Determine the DC Accuracy of the Scopes (Digitizers)?Hardware: Modular Instruments>>High-Speed Digitizers (Scopes)
Problem: Definition: Accuracy is defined as: device ability to indicate the true value of a measured signal. Accuracy is usually expressed as a percentage of the specified value, for example: ± (0.65% of Input + 1.3 mV). In order to calculate the DC accuracy of our digitizers, you need to know several specifications like: range, your voltage input, gain error and your offset error. Accuracy:  Figure 1 DC Drift
 Figure 2 Solution: Calculation The excepted voltage can be calculated by applying the formula: expected voltage = applied voltage ± (gain error *applied voltage + offset error). As an example let’s take a look at figure 1 for the 5122 accuracy of a 10 volts peak to peak range. If my applied voltage is 5 volts my excepted voltage is going to 7.95Vā(7.95V×0.65%+10mV)=7.95Vā(0.061675V).You can see how figure 3 on the left graph shows that if you applied 7.95 volts your expected voltage can be anywhere between the green and red light.  Figure 3
DC Drift Calculation
The previous calculation of the scope DC accuracy was done assuming our room temperature is within ±5 °C of self-calibration temperature, as you can see in figure 1 the side note that the temperature reference use to account for DC Drift is the temperature at which the self-calibration was done. DC drift is a slow change due to temperature in our accuracy. You will include DC drift into this calculation when as we stated before you temperature is outside ±5 °C of self-calibration temperature. Let’s assume as an example that your self-calibration on the 5122 was done at 28 °C; you will not have to worry about DC drift if you are acquiring between 23 and 33 °C. Now if you room temperature is 2 °C over the 33 °C, let’s say 35 °C, the calculation of you accuracy will be the one shown in figure 3. This new uncertainty has to be added to the previous calculation so: 7.95V±(0.057%×7.95V+0.006%10V+100μV)×2ā=7.95±(0.0046915). From our first calculation 0.061675V + 0.0046915 would be the overall uncertainty. You can see in figure 4 how the DC drift increases a little bit your uncertainty.  Figure 4 Special Cases:
Our digitizer 5154 has a little different approach for calculating the DC accuracy. DC drift is taken into account when the measurement is done over a temperature different form external calibration’s temperature. Refer to the specification for more information. Offset Error is equal to a gain times the full scale, is good to point out that sometimes our specification gives it as a constant for the chosen voltage range or as a gain times the full scale. Related Links: NI 5154 Specification Attachments:
Report Date: 07/31/2008 Last Updated: 08/26/2008 Document ID: 4NUGHDD2 |
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