Specifications

5.12 Test signal voltage enhancement
When measuring the impedance of test signal level dependent devices, such as liquid crystals, induc-
tors, and high value ceramic capacitors, it is necessary to vary the test signal voltage. Many of the
auto-balancing bridge instruments employ a test signal source whose output is variable, typically
from 5 mV to 1V rms. Particularly, the E4980A precision LCR meter with Option E4980A-001 can
output a test signal voltage of up to 20 V rms and is the most suitable for this application.
In some cases, measurement needs exist for evaluating impedance characteristics at large test signal
voltages beyond the maximum oscillator output level of the instrument. For auto-balancing bridge
instruments, output voltage enhancement is possible if the test signal is amplified as shown in
Figure 5-43. A voltage divider is also required so that the input voltage of the Hp terminal is the
same as the output voltage of the Hc terminal. The DUT’s impedance is a concern. Because the
current flowing through the DUT is also amplified and flows directly into the Rr circuit, it should
not exceed the maximum allowable input current of the Lc terminal. Typically, this is 10 mA. For
example, when a 10 V rms signal is applied to the DUT, the minimum measurable impedance is
10 V/10 mA = 1 kΩ. Also, it should be noted that measured impedance is 1/A (gain of amplifier) of
an actual DUT’s impedance. For example, when a 10 pF capacitor is measured using ×10 amplifier,
displayed value will be 100 pF.
Note: For RF I-V instrument, it is impossible to amplify the test signal because at the test port the
signal source output is not separate from the voltmeter and current meter inputs.
Figure 5-43. Schematic diagram of test signal voltage enhancement circuit
Figure 5-44 shows a measurement setup example to boost the test signal voltage by factor of
10 (A = 10). The amplifier used in this application should have constant gain in the measurement
frequency range and output impedance less than 100 Ω. R
3
in Figure 5-44 needs to be adjusted to
compensate for the magnitude error in measured impedance and C
2
needs to be adjusted for flat fre-
quency response. This can be accomplished by comparing the measured values with known values
of a reference device. For better accuracy, perform the open/short/load compensation at a test sig-
nal level below 1 V rms (not to cause an excessive current to flow in short condition.) The required
circuit constants of the divider are different depending on the input impedance of the Hp terminal
of the instrument.
5-32