Xilinx Spartan-6 FPGA Series Design And Pin Planning Manual page 29

To determine the effective in-system parasitic inductance (L
The values from the example are used to determine the mounted capacitor resonant
frequency (F
F
shifts the effective-frequency band down.
A decoupling capacitor is most effective at the narrow-frequency band around its resonant
frequency, and thus, the resonant frequency must be reviewed when choosing a capacitor
collection to build up a decoupling network. This being said, capacitors can be effective at
frequencies considerably higher and lower than their resonant frequency. Recall that
capacitors of differing values in the same package share the same inductance curve. As
shown in
capacitors are equally effective.
X-Ref Target - Figure 2-8
Spartan-6 FPGA PCB Design and Pin Planning
UG393 (v1.1) April 29, 2010
F = 53 MHz (capacitor data sheet parameter)
RSELF
L = 0.8 nH (based on PCB mounting geometry)
MOUNT
L
IS
). Using Equation
RIS
------------------------------------------------------------------------------------
F =
RIS
(
2π 1.7
is 53 MHz, but F
RSELF RIS
Figure 2-8, for any given frequency along the inductive portion of the curve, the
Inductance (Z)
Figure 2-8: Effective Frequency Example
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= L + L = 0.9 nH + 0.8 nH
SELF MOUNT
L = 1.7 nH
IS
2-1:
1
F = ------------------------ -
RIS
2π L C
IS
1
=
– –
9 6
×10 H ) ⋅ ( ×10 F )
0.01
is lower at 38 MHz. The addition of mounting inductances
0805
0.47 μF
0805
4.7 μF
Frequency
Basic PDS Principles
), add the via parasitics:
IS
Equation 2-3
Equation 2-4
6
×10 Hz
38 Equation 2-5
Z Value at F 2 is Equal
Inductive
Portion
F
2
ug393_c2_08_091809
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