122 34410A/11A/L4411A User’s Guide
4 Measurement Tutorial
Common Mode Errors
Errors are generated when the multimeter's input LO terminal is driven with
an ac voltage relative to earth. The most common situation where unnecessary
common mode voltages are created is when the output of an ac calibrator is
connected to the multimeter "backwards." Ideally, a multimeter reads the same
regardless of how the source is connected. Both source and multimeter effects
can degrade this ideal situation. Because of the capacitance between the input
LO terminal and earth (approximately 200 pF), the source will experience
different loading depending on how the input is applied. The magnitude of the
error is dependent upon the source's response to this loading.
The multimeter's measurement circuitry, while extensively shielded, responds
differently in the backward input case due to slight differences in stray
capacitance to earth. The multimeter's errors are greatest for high–voltage,
high–frequency inputs. Typically, the multimeter exhibits about 0.06%
additional error for a 100 V, 100 kHz reverse input. You can use the grounding
techniques described for dc common mode problems to minimize ac common
mode voltages.
Leakage Current Errors
The multimeter's input capacitance will "charge up" due to input bias currents
when the terminals are open–circuited (if the input resistance is >10 GΩ). The
multimeter's measuring circuitry exhibits approximately 30pA of input bias
current for ambient temperatures from 0 °C to 30 °C. Bias current doubles
(x2) for every 8 °C change in ambient temperature above 30 °C. This current
generates small voltage offsets dependent upon the source resistance of the
device–under–test. This effect becomes evident for a source resistance of
greater than 100 kΩ, or when the multimeter's operating temperature is
significantly greater than 30 °C.
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