Agilent Technologies 4294A Specifications download pdf (Page 17)

Figure 2-1. Impedance measurement method (3 of 3)

2-3

The current, flowing through the DUT, also flows through resistor R. The

potential at the “L” point is maintained at zero volts (thus called a “virtual

ground”), because the current through R balances with the DUT current by

operation of the I-V converter amplifier. The DUT impedance is calculated

using voltage measurement at High terminal and that across R.

Note: In practice, the configuration of the auto balancing bridge differs for

each type of instrument.

Generally LCR meters, in a low frequency range typically below 100

kHz, employ a simple operational amplifier for its I-V converter. This

type of instrument has a disadvantage in accuracy, at high frequen-

cies, because of performance limits of the amplifier. Wideband LCR

meters and impedance analyzers employ the I-V converter consist-

ing of sophisticated null detector, phase detector, integrator (loop

filter) and vector modulator to ensure a high accuracy for a broad

frequency range over 1 MHz. This type of instrument can attain to a

maximum frequency of 110 MHz.

Auto balancing bridge method

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Agilent Technologies 4294A Specifications Page 17