34410A/11A/L4411A User’s Guide 119
Measurement Tutorial 4
Other Sources of Measurement Error
Settling Time Effects
The multimeter has the ability to insert automatic measurement settling
delays. These delays are adequate for resistance measurements with less than
200 pF of combined cable and device capacitance. This is particularly
important if you are measuring resistances above 100 kΩ. Settling due to RC
time constant effects can be quite long. Some precision resistors and
multi–function calibrators use large parallel capacitors (1000 pF to 0.1
μF)
with high resistor values to filter out noise currents injected by their internal
circuitry. Non–ideal capacitances in cables and other devices may have much
longer settling times than expected just by RC time constants due to dielectric
absorption (soak) effects. Errors are measured when settling after the initial
connection and after a range change.
Loading Errors (ac volts)
In the ac voltage function, the input of the multimeter appears as a 1 MΩ
resistance in parallel with 100 pF of capacitance. The cabling that you use to
connect signals to the multimeter also adds capacitance and loading. The table
below shows the multimeter's approximate input resistance at various
frequencies.
For low frequencies, the loading error is:
At high frequencies, the additional loading error is:
Input Frequency Input Resistance
100 Hz 1 MΩ
1 kHz 850 kΩ
10 kHz 160 kΩ
100 kHz 16 kΩ
UG_ED5.book Page 119 Thursday, March 1, 2012 11:28 AM
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