Chapter 3 Configuring for Measurements 59
the multimeter has a power line frequency of 60 Hz and the device being
measured has a power line frequency of 50 Hz. For this application you can
achieve NMR by setting the reference frequency to 50 Hz as follows:
OUTPUT 722;"LFREQ 50"
Remember that whenever power is cycled or the front panel Reset key is
pressed, the reference frequency returns to the rounded value of 50 or 60 Hz.
Setting the Integration
Time
Integration time is the period of time that the A/D converter measures the
input signal. For DC or ohms measurements, the integration time determines
the measurement speed, accuracy, maximum digits of resolution, and the
amount of NMR for noise at the A/D converter's reference frequency, You
can specify integration time in terms of power line cycles (PLCs) using the
NPLC command or directly (in seconds) using the APER command. Since
the NPLC and APER commands both set the integration time, executing
either will cancel the integration time previously established by the other.
Specifying Power Line
Cycles
The multimeter achieves NMR for noise at the A/D converter's reference
frequency when the integration time is ³1 power line cycles. You can specify
integration time in terms of power line cycles (PLCs) using the NPLC
command. The multimeter multiplies the specified number of PLCs by the
period of the A/D converter's reference frequency (LFREQ command) to
determine the integration time. For example, the period of a 50 Hz power
line is l/50 = 20 msec. If you specify 10 PLCs, the integration time is 200
msec. In the power-on state, integration time is set to 10 PLCs. In the
PRESET NORM state, integration time is set to 1 PLC. To set the integration
time for the fastest measurements (with the lowest accuracy, lowest
resolution. and no NMR), send:
OUTPUT 722; "NPLC 0"
To specify the most accuracy, highest resolution, and 80dB of NMR for DC
or ohms measurements (with the slowest measurement speed), send:
OUTPUT 722;"NPLC 1000"
You can specify power line cycles in the following ranges:
• 0 - 1 PLC (in .000006 PLC steps for 60Hz ref. frequency or. 000005 PLC
steps for 50Hz ref. frequency)
• 1- 10 PLC in 1 PLC steps
• 10 - 1000 PLCs in 10 PLC steps
Note For integration times greater than 10 PLCs, the multimeter averages a
number of readings made using 10 PLCs of integration time. For example,
if you specify 60 PLCs, the multimeter averages six 10 PLC readings.
The wide range of PLC settings provides flexibility in the selection of
measurement speed. accuracy, resolution, and NMR. Typically, you should
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