"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

Decoupling
17-22
EQUIVALENT
SERIES
INDUCTANCE
Obviously from the table, tantalum electrolytic capacitors are useless for frequencies
above 1 MHz. Effective high frequency decoupling at higher frequencies demands a ceramic
capacitor. Self-resonances of the capacitor must be known and avoided or the capacitor
may not help, or even make the problem worse. Figure 17–16 illustrates the typical
self-resonance of two capacitors commonly used for bypassing — 10 µF tantalum electrolytic
and 0.01 µF ceramic.
EQUIVALENT
SERIES
RESISTANCE
L R
Z
Z R L 1
C
C
Impedance — Ω
100,000
10,000
1000
100
10
1
0.1
0.01
0.001
Figure 17–16. Capacitor Self Resonance
100 k
1 M 10 M 100 M 1 G
Frequency — Hz
10 µF
10 nF
Consider these resonances to be typical values, the characteristics of actual capacitors
can vary from manufacturer to manufacturer and grade of part to grade of part. The important
thing is to make sure that the self-resonance of the capacitor occurs at a frequency
above the range of the noise that must be rejected. Otherwise, the capacitor will enter a
region where it is inductive.
Do not assume that a single 0.1-µF capacitor will decouple all frequencies. Smaller capacitors
may work better at higher frequencies than larger ones. When poor decoupling at
higher frequencies is suspected, try a smaller capacitor rather than a larger one.
17.5.3 Decoupling at the IC Level
The method most often used to decouple the high-frequency noise is to include a capacitor,
or multiple capacitors connected from the op amp power pin to the op amp ground pin.
It is important to keep the traces on this decoupling capacitor short. If not, the traces on
the PCB will have significant self-inductance, defeating the purpose of the capacitor.