AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 7

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

Inductors
MOTOROLA
Another factor that affect the effectiveness of the decoupling capacitor is the
dielectric material of the capacitor. Two common materials are used in the
manufacture of decoupling capacitors: barium titanate ceramic (Z5U) and
strontium titanate (NPO). Z5U has a larger dielectric constant, with a self-
resonant frequency from 1MHz to 20MHz. NPO has a lower dielectric constant,
but a higher self-resonant frequency (greater than 10MHz). Therefore, Z5U is
more suitable for low frequency decoupling, while NPO is good for decoupling
at over 50MHz.
A common practice is to use two decoupling capacitors in parallel. This
configuration can provide a wider spectral distribution to reduce the switching
noise induced by the power supply networks. Multiple decoupling capacitors
connected in parallel can provide 6dB improvement to suppress RF currents
generated by active device switching.
The multiple decoupling capacitors not only provide wider spectral distribution,
but also provide greater trace width such that to reduce lead inductance.
Therefore, it will significantly improves the effectiveness of decoupling. The
value of the two capacitors should differ by two orders of magnitude to provide
effective decoupling (e.g. 0.1 µ F + 0.001 µ F connected in parallel).
A point should be noted for digital circuit decoupling. A low ESR value is more
important than the self-resonant frequency because a low ESR value can
provide a lower impedance path to ground such that to provide adequate
decoupling by the capacitor when the capacitor becomes inductive when over
the self-resonant frequency.
The inductor is the component which forms a link between magnetic and
electric fields, hence are potentially more susceptible than other components
as they have an inherent ability to interact with magnetic fields. Similar to
capacitors, the inductor, when used intelligently, can provide a cure to many
EMC problems.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
Capacitor Value
For More Information On This Product,
1000 pF
0.01 µ F
100 pF
1.0 µ F
0.1 µ F
10 pF
Go to: www.freescale.com
Table 1. Capacitor Self-Resonant Frequencies
PART 2: COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES
Through-hole (0.25 leads)
250 MHz
800 MHz
2.5 MHz
25 MHz
80 MHz
8 MHz
Surface mount (0805)
160 MHz
500 MHz
1.6 GHz
16 MHz
50 MHz
5 MHz
AN2321/D
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