58 Chapter 3 Configuring for Measurements
Configuring the A/D
Converter
The A/D converter's configuration determines the measurement speed,
resolution, accuracy, and normal mode rejection
1
for DC or ohms
measurements. The factors that affect the A/D converter's configuration are
the reference frequency, the specified integration time, and the specified
resolution.
The Reference Frequency When power is applied, the multimeter measures the power line frequency,
rounds the value to 50 Hz or 60 Hz, and sets the A/D converter's reference
frequency to the rounded value. (For a 400Hz power line frequency, the
multimeter uses 50Hz as the reference frequency, which is a subharmonic of
400Hz.) For DC or ohms measurements, the multimeter achieves normal
mode rejection (NMR) for noise at the reference frequency when the
integration time is ³1 power line cycle. See Setting the Integration Time
(following) for more information.
Changing the Reference
Frequency
For most operating conditions, the power-on reference frequency allows for
excellent NMR. However, for maximum NMR you should set the reference
frequency to the exact power line frequency. (If your power line frequency
is subject to drift, you may have to periodically correct the reference
frequency.) The following command measures the power line frequency and
sets the reference frequency to the exact measured value (for a 400Hz line
frequency. the multimeter divides the measured value by 8 and uses that as
the reference frequency).
OUTPUT 722;"LFREQ LINE”
You can also use the LFREQ command to directly specify the reference
frequency. This is particularly useful when the multimeter has a different
power line frequency than the device being measured. Suppose, for example,
Figure 14. 4-Wire ohms measurement connections
1. Normal mode rejection (NMR) is the multimeter’s ability to reject noise at the power line frequency from DC
or ohms measurements.
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