HIGH FREQUENCY BEHAVIOUR OF CERAMIC
MULTILAYER CAPACITORS
Ceramic multilayer capacitors (CMC) are suitable for use
at high frequencies. At frequencies below the series
resonance frequency, the CMC can be represented by an
equivalent circuit as shown in Fig.16.
In general, the quantities C, ESR and L are frequency
dependent. For most applications, C and L can be
regarded as frequency independent below 1 GHz.
The equivalent series self-inductance L is:
•Independent of the dielectric material.
•Dependent on the size of the capacitor, it increases with
increasing length and decreases with increasing width
or thickness of the product.
•The value of L is approximately:
–0.6 nH for case size 0603
–1 nH for case sizes 0805, 1206 and 1210
–1.5 nH for case sizes 1812.
These figures are accurate to within 20%.
Because of the inductance L, associated with the CMC,
there will be a frequency at which the inductive reactance
will be equal to the reactance of the capacitor.
This is known as the series resonance frequency (SRF)
and is given by:
At the SRF, the CMC will appear as a small resistor.
The transmission loss through the CMC at this series
resonance frequency will be low.
Using the values of C, L = 1 nH and the ESR at a specific
frequency (f), two often used quantities can be derived.
The impedance (Z) is given by:
The quality factor (Q) is given by:
SRF 1
2πLC
-------------------
=
Z1 2πf()
2
–LC
2jπfC
------------------------------------ ESR
+=
Q12πf)2LC(
–
2πfESRC
--------------------------------------
=