34410A/11A User’s Guide 119
Measurement Tutorial 4
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 will exhibit
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 GW). The multimeter's measuring circuitry exhibits approximately
30pA of input bias current for ambient temperatures from 0 °C to 30 °C.
Bias current will double (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 kW, or when
the multimeter's operating temperature is significantly greater than 30 °C.
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