This article shows you how to calculate the accuracy of a signal for a digital multimeter (DMM). Accuracy calculations for the NI DMMs are provided below, along with examples. Sample calculations focus on DC Voltage accuracy, but the general calculation can be applied for DC current, resistance, AC voltage, and AC current.
You will need to know the following:
PXIe-4080 Specifications
PXIe-4081 Specifications
PXIe-4082 Specifications
The accuracy specification comes in a (a + b) format, where:
a is the ppm of reading component
b is the ppm of range component
Accuracy = ±(ppm of reading + ppm of range)
= ±((a/1,000,000) * reading + (b/1,000,000) * range))
Assumptions:
20 + 6
or
20 ppm of reading + 6 ppm of range
Accuracy = ±(ppm of reading + ppm of range)
= ±(20 ppm of reading + 6ppm of range)
= ±(20 ppm of 7 V + 6 ppm of 10 V)
= ±((7 V * (20/1,000,000) + (10 V *(6/1,000,000))
= ±200 µV
The reading should be within ±200 µV of the actual input voltage.
PXIe-4070 Specifications
PXIe-4072 Specifications
The accuracy specification comes in a (a + b) format, where:
a is the ppm of reading component
b is the ppm of range component
Accuracy = ±(ppm of reading + ppm of range + additional noise error ppm of range)
= ±((a/1,000,000) * reading + (b/1,000,000) * range + (additional noise error/1,000,000) * range))
Assumptions:
20 + 6
or
20 ppm of reading + 6 ppm of range
10 ppm of range
Accuracy = ±(ppm of reading + ppm of range + additional noise ppm of range)
= ±(20 ppm of 0.5 V + 6 ppm of 1 V + 10 ppm of 1 V)
= ±((0.5 V * (20/1,000,000) + (1 V * (6/1,000,000) + 1 V * (10/1,000,000))
= ±26 µV
The reading should be within ±26 µV of the actual input voltage.
PXI-4065 Specifications
PXI-4071 Specifications
The accuracy specification comes in a (a + b) format, where:
a is the ppm of reading component
b is the ppm of range component
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.
Assumptions:
18 + 2
or
18 ppm of reading + 2 ppm of range
1.6 ms
NPLC = (aperture time/16.67 ms) (for mains power frequency of 60Hz)
NPLC = (1.6 ms)/(16.67 ms)
= 0.096
Estimate RMS Noise (ppm of range) from the Additional Noise Error graph.
RMS Noise = ~0.3 ppm of range
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.