Agilent Technologies 34401A manual Common Mode Errors, AC Current Measurement Errors

Chapter 7 Measurement Tutorial

Common Mode Errors

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 for the Agilent 34401A), 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 (see page 201).

AC Current Measurement Errors

Burden voltage errors, which apply to dc current, also apply to ac current measurements. However, the burden voltage for ac current is larger due to the multimeter’s series inductance and your measurement connections. The burden voltage increases as the input frequency increases. Some circuits may oscillate when performing current measurements due to the multimeter’s series inductance and your measurement connections.

212