- Determine the accuracy specification of the measurement you are taking. Refer to the accuracy tables in the Specifications document for your DMM.
The accuracy specification comes in a (a + b) format, where:
a is the ppm of reading component
b is the ppm of range component
- Determine the aperture time for your DMM and digits of resolution from NI Digital Multimeters Help: Aperture Time.
- Determine the RMS noise (ppm of range).
Convert aperture time to number of power line cycles (NPLC) based on your country’s mains power frequency:
Mains power frequency of 60 Hz: NPLC = (aperture time/16.67 ms)
Mains power frequency of 50 Hz: NPLC = (aperture time/20 ms)
Use the Additional noise error graph in the Specifications document for your DMM to estimate the RMS noise (ppm of range) based on the NPLC.
- Determine the RMS noise multiplier based on the measurement range. Skip this step if you are using a PXI-4065. For peak-to-peak noise error, multiply the RMS noise multiplier by 6.
- Apply to calculation.
Accuracy = ±(ppm of reading + ppm of range + (RMS noise ppm of range * (RMS noise multiplier/2)))
= ±((a/1,000,000) * reading + (b/1,000,000) * range + (RMS noise * (RMS noise multiplier/2) / 1,000,000) * range))
Note: The RMS noise multiplier is divided by 2 to account for the ± of the accuracy calculation.
- Using a PXI-4071 DMM
- Operating 90 days after calibration at 23 ºC ±5 ºC
- Want to know accuracy at 5½-digit resolution
- DMM is taking a DC Voltage signal
- DMM is set to the 100 V range and expecting a 70 V signal reading
- The DC Voltage accuracy specification for 90 days after calibration and a 100 V range are:
18 + 2
18 ppm of reading + 2 ppm of range
DC Voltage Accuracy Specification from PXI-4071 Specifications document
- The aperture time for the PXI-4071 with 5.5-digit resolution and 100 V range is:
Aperture Time Specification for NI DMMs from NI Digital Multimeters Help
- Determine RMS noise (ppm of range).
NPLC = (aperture time/16.67 ms) (for mains power frequency of 60Hz)
NPLC = (1.6 ms)/(16.67 ms)
Estimate RMS Noise (ppm of range) from the Additional Noise Error graph.
RMS Noise = ~0.3 ppm of range
Additional Noise Error graph from PXI-4071 Specifications document
- The RMS noise multiplier for 100 V range is x6.
RMS Noise Multiplier from PXI-4071 Specifications document
- Apply to accuracy calculation.
Accuracy = ±(ppm of reading + ppm of range +(RMS noise ppm of range * RMS noise multiplier/2)
= ±(18 ppm of 70 V + 2 ppm of 100 V + (0.3 * (6/2) ppm of 100 V))
= ±(70 V * (18/1,000,000) + 100 V * (2/1,000,000) + 100 V * (0.3*3/1,000,000))
= ±1.55 mV
The reading should be within ±1.55 mV of the actual input voltage.