27 II/28 II
Users Manual
16
When measuring voltage, the Meter acts approximately
like a 10-MΩ (10,000,000 Ω) impedance in parallel with
the circuit. This loading effect can cause measurement
errors in high-impedance circuits. In most cases, the error
is negligible (0.1 % or less) if the circuit impedance is
10 kΩ (10,000 Ω) or less.
For better accuracy when measuring the dc offset of an
ac voltage, measure the ac voltage first. Note the ac
voltage range, then manually select a dc voltage range
equal to or higher than the ac range. This procedure
improves the accuracy of the dc measurement by
ensuring that the input protection circuits are not
activated.
Zero Input Behavior of True-rms Meters (28 II)
True-rms meters accurately measure distorted
waveforms, but when the input leads are shorted together
in the ac functions, the Meter displays a residual reading
between 1 and 30 counts. When the test leads are open,
the display readings may fluctuate due to interference.
These offset readings are normal. They do not affect the
Meter’s ac measurement accuracy over the specified
measurement ranges.
Unspecified input levels are:
• AC voltage: below 3 % of 600 mV ac, or 18 mV ac
• AC current: below 3 % of 60 mA ac, or 1.8 mA ac
• AC current: below 3 % of 600 μA ac, or 18 μA ac
Low-Pass Filter (28 II)
The 28 II is equipped with an ac low-pass filter. When
measuring ac voltage or ac frequency, press to
activate the low-pass filter mode (). The Meter
continues measuring in the chosen mode, but now the
signal diverts through a filter that blocks unwanted
voltages above 1 kHz, refer to Figure 3. The lower
frequency voltages pass with reduced accuracy to the
measurement below 1 kHz. The low-pass filter can
improve measurement performance on composite sine
waves that are typically generated by inverters and
variable frequency motor drives.
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