Specifications
4-2-1. Offset compensation
When a measurement is affected by only a single component of the residuals, the effective value can
be obtained by simply subtracting the error value from the measured value. For example, in the
case of the low value capacitance measurement shown in Figure 4-2, the stray capacitance Co paral-
leled with the DUT’s capacitance Cx is significant to the measurement and can be compensated by
subtracting the stray capacitance value from the measured capacitance value Cm. The stray capaci-
tance value is obtained with the measurement terminals left open.
Figure 4-2. Offset compensation
4-2-2. Open and short compensations
Open and short compensations are the most popular compensation technique used in recent LCR
measurement instruments. This method assumes that the residuals of the test fixture can be repre-
sented by the simple L/R/C/G circuit. The method is represented in Figure 4-3 (a). When the
UNKNOWN terminals are open, as shown in Figure 4-3 (b), stray admittance Go+jωCo is measured
as Yo because residual impedance Zs is negligible. When the UNKNOWN terminals are shorted, as
shown in Figure 4-3 (c), the measured impedance represents residual impedance Zs= Rs+jωLs
because Yo is bypassed. As a result, each residual parameter is known and, the DUT’s impedance,
Zdut, can be calculated from the equation given in Figure 4-3 (d).
Note: Agilent’s impedance measurement instruments actually use a slightly different equation.
Refer to APPENDIX B for more detailed information.
This compensation method can minimize the errors when the actual residual circuit matches the
assumed model in the specific situations listed below:
• Measurement by connecting an Agilent test fixture to the UNKNOWN terminals
• Measurement with an Agilent test fixture connected by an Agilent test cable that is compensat-
ed for electrical length
In other situations, the open/short compensation will not thoroughly correct the measured values.
In addition, this method cannot correlate measurement results from different instruments. To
resolve these compensation limitations, the open, short and load compensations are required. Refer
to Paragraph 4-2-4 for details.
4-2