Ver 1.5

17/03/29

11

ES166

LCR/DMM analog front

1.5 Basic impedance theory

The general DMM could measure DC resistance only, but the LCR meter could

measure AC impedance. The impedance consists of resistance (real part) and reactance

(imaginary part). For example, Zs represents the impedance in series mode. Zs can be

defined a combination of resistance Rs and reactance Xs. It also could be defined as a |Z|

of magnitude with a phase angle

θ.

Zs = Rs + jXs

| Zs |

Real axis

Imaginary axis

(series mode)

Rs

Xs

θ

Zs

Xs

1

Rs

1

θ

1

θ > 0

Zs = Rs + jXs

| Zs |

Real axis

Imaginary axis

(series mode)

Rs

Xs

θ

Zs

Xs

1

Rs

1

θ

1

θ > 0

Zs = Rs + jXs or |Zs|∠

θ

|Z| =

2

2

Xs

Rs

+

Rs = |Zs| cos

θ

Xs = |Zs| sin

θ

Xs/Rs = tan

θ

If

θ > 0, the reactance is inductive. In other words, if θ < 0, the reactance is capacitive.

C

2

1

f

π

(C = Capacitance)