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2.5.3 Reactance Measurement 29
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2.5.3 Reactance
Measurement
In practice, no measurement of a resistive load is purely resistive. There are likely
capacitive and inductive components that must be taken into consideration to fully
characterize the load being measured. The complex number formed by adding the
resistive component (real) and the capacitive/inductive reactive component
(imaginary) is called impedance.
The Model 372 uses an on-board digital signal processor to digitally synthesize the
excitation current used for measurements. In addition, the Model 372 has the
capability to measure both the resistive, in-phase voltage and the reactive, out of
phase voltage measured across the load. The out-of-phase voltage measured across
the load is comprised of both capacitive and inductive elements, causing the
measured AC voltage to be 90 degrees out-of-phase, or “in quadrature” with the
excitation current. A reactance reading that is dominated by capacitance will be
negative while a reactance reading dominated by inductance will be positive.
Impedance (Z) is generally represented as Z = R + jX, where R is resistance in ohms, j is
the imaginary component and X is reactance, also in ohms. This is represented
slightly differently in the Model 372 and is shown below in FIGURE 2-4.
Keep in mind, the Model 372 impedance measurement assumes the load is modeled
as a series RLC circuit. There are many applications, such as determining parasitic
cable capacitance, where it is desired to measure the reactance parallel to the
resistive load. In those cases, the resistive and reactive readings can be input into the
following simple formulas to determine the equivalent parallel loads.
2.5.4 Excitation Modes The Model 372 uses current source excitation for resistance measurements. Current
excitation mode is the most fundamental measurement mode because operation is
tied directly to hardware function. For the measurement input only, the instrument
firmware is also able to simulate voltage excitation of the measured resistor to create
a second operating mode. Some users prefer voltage excitation mode because it
matches the operation of other bridges and permits convenient limiting of excitation
power. Resistance range selection is different in these two modes as described below.
Autorange is available for both modes.
FIGURE 2-4 Display showing impedance measurement