Specifications

Figure 1-9. Resistor frequency response
As for capacitors, parasitic inductance is the prime cause of the frequency response as shown in
Figure 1-7. At low frequencies, the phase angle (q) of impedance is around –90°, so the reactance
is capacitive. The capacitor frequency response has a minimum impedance point at a self-resonant
frequency (SRF), which is determined from the capacitance and parasitic inductance (Ls) of a series
equivalent circuit model for the capacitor. At the self-resonant frequency, the capacitive and induc-
tive reactance values are equal (1/(wC) = wLs.) As a result, the phase angle is 0° and the device is
resistive. After the resonant frequency, the phase angle changes to a positive value around +90° and,
thus, the inductive reactance due to the parasitic inductance is dominant.
Capacitors behave as inductive devices at frequencies above the SRF and, as a result, cannot be
used as a capacitor. Likewise, regarding inductors, parasitic capacitance causes a typical frequency
response as shown in Figure 1-8. Due to the parasitic capacitance (Cp), the inductor has a maximum
impedance point at the SRF (where wL = 1/(wCp).) In the low frequency region below the SRF, the
reactance is inductive. After the resonant frequency, the capacitive reactance due to the parasitic
capacitance is dominant. The SRF determines the maximum usable frequency of capacitors and
inductors.
Cp
R
Ls
R
(b) Low value resistor
w
L
|Z|
Log f
Log |Z|
|Z|
1
w
Cp
Log f
Log |Z|
(a) High value resistor
Frequency
Frequency
q
q
Cp: Stray capacitance
Ls: Lead inductance
q
0
º
–90º
90º
q
0
º
–90º
90º
1-6