Other Sources of Measurement Error
Settling Time Effects
The multimeter can 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 when
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.
Input Frequency Input Resistance (kΩ)
100 Hz 941
1 kHz 614
10 kHz 137
100 kHz 15.7
For low frequencies, the loading error is:
At high frequencies, the additional loading error is:
Measurements Below Full Scale
You can make the most accurate AC measurements when the multimeter is at or near the full scale of the selected
range. Autoranging occurs at 10% (down–range) and 120% (up–range) of full scale. This enables you to measure
some inputs at full scale on one range and 10% of full scale on the next higher range. In general, the accuracy is better
on the lower range; for the highest accuracy, select the lowest manual range possible for the measurement.
Agilent Truevolt Series DMM Operating and Service Guide 97
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