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What makes NI's most accurate DMM tick?

05 May 2016  | Vivek Nanda

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National Instruments is claiming that its recently announced NI PXIe-4081 7½-digit digital multi-meter (DMM) is the first PXI Express DMM available and that it is the most accurate DMM in its class.

The PXIe-4081 provides 26bits of resolution and voltage measurements that range from 10nV to 1kV. It offers 15ppm accuracy for DC voltage measurements up to two years after calibration. A solid-state current shunt configuration offers eight DC current ranges from 1µA to 3A and 6 AC RMS current ranges from 100µA to 3A. The resistance measurements range from 10µΩ to 5GΩ.

NI has taken off-the-shelf high-speed ADC technology and combined it with a custom-designed sigma-delta converter to achieve its noise, linearity and speed performance design goals. The PXIe-4081 uses what NI says is one of the most stable on-board voltage references to provide stable performance across temperature and time. And NI backs up that claim with a two-year guaranteed accuracy of 12ppm. The DMM also uses DMM measurement techniques like offset compensated ohms, high-order DC noise rejection and self-calibration to increase accuracy.

The PXIe-4081 DMM occupies a single 3U PXI slot.

Figure 1: The PXIe-4081 DMM occupies a single 3U PXI slot and can deliver 17 DMM channels in a single PXI chassis occupying 4U of rack space.

Variable sampling

The DMM architecture of the PXIe-408x family offers a continuously variable reading rate that ranges from 7S/s to 10kS/s, which allows for the selection of the sample rate and resolution needed for the application.

NI says that the PXIe-4081 model can sample 36 times faster than some benchtop DMMs that peak at 50kS/s. For high-throughput applications, the isolated digitizer mode can acquire DC-coupled waveforms in all voltage and current ranges with a 1.8MS/s maximum sample rate. By changing the digitizer sampling rate, you can vary the resolution of the digitizer from 10bits to 23bits.

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