For this example, we will assume the following:
- Our NI 4071 was externally calibrated between 90 days and 2 years ago
- Our NI 4071 is within 1°C from the temperature at last self-calibration or external calibration
- We are looking for DC voltage accuracy in the 100 V range
- We want to know the accuracy at 5½-digit resolution
- We are using the default aperture time
- First, you will need to determine the aperture time for your DMM. Default aperture times are documented in NI Digital Multimeters Help » Devices » NI 4071 » DMM Measurements » DMM Measurement Cycle » Aperture Time.
Table 1: NI 4071 5½-Digit Resolution Default Aperture Times from DMM Help
Since we are using an NI 4071 and 5½-digit resolution, we can use Table 1 to determine that our aperture time is 500 µs.
- Next, we need to add in additional noise in error by looking at the graph from the specifications. We can find it on page three of the NI 4071 DMM Specifications page:
Graph 1: Additional Noise Error from 4071 Specifications
Looking at the graph, the RMS Noise is related to the number of power line cycles (NPLC), not seconds. We will convert our aperture time into PLC. In countries that have a mains power frequency of 60 Hz, 1 PLC equals 16.67 ms. In countries that have a mains power frequency of 50 Hz, 1 PLC equals 20 ms.
We will assume that we're operating in the USA, which uses 60 Hz powerline signals. From this we can calculate that 500 µs equals 0.03 PLC. (500us / 16.67 ms = 0.03 PLC)
Now, we look at Graph 1 for the RMS Noise that correlates with 0.03 PLC. Please note that both axes are logarithmic. We see that a 0.03 PLC integration time corresponds to just under 1 ppm of RMS noise, but for simplicity's sake in this example we will use 1 ppm of the range.
Now we will find the correct multiplier. We will use the table provided on page three of the specifications:
Table 2: RMS Noise from 4071 Specifications
As mentioned above, we are interested in the 100 V range so our multiplier is 6. However, we are also interested in the peak-to-peak range, so we need to multiply it by an additional 6. Since accuracy is specified in terms of ±, we actually want half so we will divide by 2. Our multiplier has become 6 x 6 ÷ 2 = 18.
The RMS Noise was 1 ppm of the range. Multiplying this by our multiplier: 1 x 18 = 18 ppm of the range.
Lastly, we need to determine our accuracy equation. First, use the accuracy table provided in the specifications in order to get a starting point:
Table 3: NI 4071 DC Voltage Accuracy Specifications
We are interested in the 100 V range, our device was last calibrated between 90 days and 2 years ago, and it is within 1°C from the temperature at last calibration. At 7½-digit resolution, our coefficients would be 20 + 2, making our equation be DC Voltage ± (20 ppm of reading + 2 ppm of the range).
We will add the 18 ppm of the range to our range coefficient: 18 ppm + 2 ppm = 20 ppm of the range. Our final equation is DC Voltage ± (20 ppm of reading + 20 ppm of the range).