Residual Impedance

Residual Impedance

For the NI 4072 only, Impedance is a measure of opposition to the flow of alternating currents. Residual impedances, which are found in series between the DMM and the DUT, result from the connectivity required between the DMM and the DUT. The residual impedance on the test application introduces an error by dividing the voltage across the DUT. This error can affect the accuracy for inductance and capacitance measurements, particularly when the residual impedance is comparable to the impedance of the DUT. The following figure illustrates residual impedance in a typical measurement.

where

Z_P = residual impedance
Z_X = impedance of the DUT
V_M = total input voltage measured by the DMM
V_P = voltage drop across total residual impedance
V_X = voltage across DUT
V_M = V_P + V_X
Z_M = V_M/I = V_P/I + V_X/I = Z_P + Z_X

Note  Bold values denote vector quantities or complex numbers.

For example, in an application with a 10 uH inductor in series with a 1 uH cable, the meter reads a 10% error, or 11 uH (10 uH + 1 uH).

To reduce residual impedances on test fixtures and cables, take the following steps:

• Keep the cables as short as possible.
• Avoid any variations due to mechanical vibrations, handling, and temperature changes.
• Use the compensation techniques available on the NI 4072. Refer to OPEN/SHORT Compensation for more information.

In addition to residual impedance, stray admittances can also be found in test setups. Refer to Stray Admittance for more information.