Relationship Between NPLC and Reading Rate in DMM Specification

Updated Mar 12, 2019

Reported In

Hardware

  • PXIe-4081
  • PXIe-4080
  • PXIe-4082
  • PXI-407X
  • PXI-4071
  • PXI-4070
  • PXI-4072
  • PXI-4065
  • PXI-4060

Issue Details

I am using the PXI-4065. What are mean NPLC and reading rate in the specification? And I would like to know the relationship between NPLC and reading rate.

Solution

NPLC (Number of Power Line Cycles)
NPLC stands for Number of Power Line Cycles. NPLC is a measurement term that indicates the accuracy with which a voltage or current is displayed on a device such as a digital multimeter. When determining the value of a DC signal, AC noise is often present by power lines. Since this noise is periodic, integrating the DC signal over one cycle can help eliminate this noise. For Power Line Cycle is 1, the Power Line Frequency is 60 Hz (16.67 ms) for the United States (US) and 50 Hz (20 ms) for other countries except for the US.

Aperture time
Aperture time is the period during which the ADC is reading the input signal. Aperture time is basis of time, and it is same concept with NPLC. NPLC is defined by the power line cycles, the aperture time is defined as the time.

Relationship between NPLC (or aperture time) and reading rate
If you want to measure signal, you should wait for aperture time and setting time until the signal is finally measured. As you can see as below specification sheet, the shorter the aperture time, the larger the reading rate. In conclusion, the shorter the aperture time, the shorter the set-up time, which leads to an increase in the number of samples that can be read.

Note: The below table is the specification of PXI-4065. Other DMM modules may have different values, so please refer to each module's specification.
 

Additional Information

It should be noted here that setting the aperture time to a shorter value is not always good. If you set the aperture time to less than 16.67ms (or NPLC = 1), remember that you can measure the shaky (unstable) signal.