Keithley Instruments -Frequently Asked Question

Product(s):

Digital Multimeters: 2000,2000-20,2001,2002,2010,2015,2016,2700,2701,2750

SourceMeter Instruments: 2400,2410,2420,2425,2430,2440,6430

Low-Current/High-Resistance Products: 486,487,6485,6487,6514,6517A

Question: What is NPLC?

Answer: The Number of Power Line Cycles (NPLC) indicates how long an input signal is integrated to obtain a single measurement. Noise introduced from the power line tends to be periodic. If the A/D converter integrates for an amount of time equal to one cycle of the power line noise, then the signal components from the periodic noise can be canceled. Generally speaking, the longer a signal is integrated by the A/D converter, the more accurate the reading result.

The NPLC setting of an instrument allows adjustment of the tradeoff between speed and accuracy.

For 60Hz power, an instrument operating at 1 NPLC can report a new value no faster than at 16.67 msec intervals. For 50Hz power and 1 NPLC, a new value can be reported no faster than at 20 msec intervals.

Typically, the NPLC can be set for values between 0.01 and 10. However, for complete access to the full range of settings programmatic control over the bus is required. Typically, front panel operation limits the choices to SLOW (10PLC), MEDIUM (1PLC) and FAST (0.1PLC). Additionally, some measurement functions can be performed only at a subset of the available NPLC settings, e.g., continuity. Consult your hardware manuals for information on the range of NPLC supported by your specific instrument for the selected measurement function.