AN-1368
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Ferrite Bead Demystified
By Jefferson Eco and Aldrick Limjoco
INTRODUCTION
High resolution, high performance converters and radio frequency
(RF) systems require a low noise design of the power supply to
achieve optimum performance. Good filtering of power supply
noise and high frequency crosstalk reduction between analog and
digital domains is required, especially on mixed-signal converters
and transceivers.
Figure 1 shows an example of a filtering scheme often used in
mixed-signal ICs to separate analog and digital supplies. The
scheme is suitable for any power distribution network; however,
understanding its effectiveness and limitations helps users to
avoid characteristics that may, for example, be detrimental in high
performance converter applications.
Power supply noise is more evident with switching regulators,
which generate undesired output artifacts that can be harmful
to any noise sensitive system. However, due to their high power
conversion efficiency, switching regulators are often found in
portable devices where prolonged battery life is needed and in
systems where thermal limitations exist.
This application note discusses the important considerations that
system designers need to be aware of when using ferrite beads
in power supply systems, including response characteristics of
the ferrite bead, a simplified ferrite bead model and simulation, dc
current considerations, LC resonance effects, and damping
methods. An understanding of these elements can make the
system design approach fast and effective.
The Analog Devices, Inc., products used to demonstrate the
effects of ferrite beads as output filters are the 1.25 A, 1.2 MHz
synchronous step-down switching regulator (ADP2120) and the
2 A/1.2 A dc-to-dc switching regulator with independent positive
and negative outputs (ADP5071).
An effective method for filtering high frequency power supply
noise and cleanly sharing similar supply rails is the use of ferrite
beads. A ferrite bead is a passive device that filters high frequency
noise energy over a broad frequency range. It becomes resistive
over its intended frequency range and dissipates the noise energy in
the form of heat. The ferrite bead is connected in series with the
power supply rail and is often combined with capacitors to ground
on either side of the bead. This forms a low-pass filter network,
further reducing the high frequency power supply noise.
Rev. 0 | Page 1 of 11
DIGITAL
SUPPLY
ANALOG
SUPPLY
POWER
SUPPLY
FERRITE
BEAD
13359-001
Similar digital and analog voltage rails of mixed-signal ICs are
often powered from different power domains. This approach
helps to prevent fast digital switching noise coupling onto the
sensitive analog supply rail and degrading the converter
performance, but it increases system level complexity and cost.
With proper high frequency isolation of supply domains, power
supplies can be shared between analog and digital domains,
simplifying the design and reducing the cost.
Figure 1. Sample Filter Scheme for a Mixed-Signal IC
AN-1368
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
LC Resonance Effect .........................................................................6
Revision History ............................................................................... 2
Damping Methods.............................................................................9
Ferrite Bead Response Characteristics .......................................... 3
Conclusion....................................................................................... 11
Ferrite Bead Simplified Model and Simulation ............................ 3
References ........................................................................................ 11
DC Bias Current Considerations .................................................... 4
REVISION HISTORY
8/15—Revision 0: Initial Version
Rev. 0 | Page 2 of 11
Application Note
AN-1368
Ferrite beads are categorized by three response regions: inductive,
resistive, and capacitive. These regions can be determined by
looking at a ZRX plot, where Z is the impedance, R is the
resistance, and X is the reactance of the bead. To reduce high
frequency noise, the bead must be in the resistive region; this is
especially desirable for electromagnetic interference (EMI) filtering
applications. The component acts like a resistor, which impedes
the high frequency noise and dissipates it as heat. The resistive
region occurs after the bead crossover frequency (X = R) and up
to the point (shown in Figure 2) where the bead becomes
capacitive. This capacitive point occurs at the frequency where
the absolute value of capacitive reactance is equivalent to R.
1200
RESISTIVE
INDUCTIVE
CAPACITIVE
1000
The Tyco Electronics BMB2A1000LN2 multilayer ferrite bead is
used as an example. Figure 4 shows the measured ZRX response
of the BMB2A1000LN2 for a zero dc bias current using an
impedance analyzer. The circuit simulator suite used was the
ADIsimPE, a simulation tool optimized for the design and
development of analog and mixed-signal circuits. ADIsimPE is
powered by SIMetrix/SIMPLIS.
1200
Z = R; RAC
1000
IMPEDANCE (Ω)
FERRITE BEAD RESPONSE CHARACTERISTICS
800
Z
600
Z ≈ XL; LBEAD
R
Z ≈ |XC|;
CPAR
400
CROSSOVER
FREQUENCY
Z
200
600
0
R
1
400
10
X
100
FREQUENCY (MHz)
13359-004
IMPEDANCE (Ω)
X
800
1000
Figure 4. BMB2A1000LN2 ZRX Plot
200
1
10
100
1000
FREQUENCY (MHz)
For the region on the measured ZRX plot where the bead appears
most inductive (Z ≈ XL; LBEAD), the bead inductance is calculated
by the following equation:
13359-002
0
Figure 2. Tyco Electronics BMB2A1000LN2 ZRX Plot
At relatively low frequencies below crossover, the bead response
is inductive. At high frequencies, the bead response is capacitive. In
this case, the falling slope of the impedance vs. frequency plot is
defined by the parasitic capacitance associated with the component.
FERRITE BEAD SIMPLIFIED MODEL AND
SIMULATION
LBEAD =
(1)
where:
f is the frequency point anywhere in the region the bead appears
inductive. In this example, f = 30.7 MHz.
XL is the reactance at 30.7 MHz, which is 233 Ω.
Equation 1 yields an inductance value (LBEAD) of 1.208 μH.
A ferrite bead can be modeled as a simplified circuit consisting
of resistors, an inductor, and a capacitor, as shown in Figure 3.
RDC corresponds to the dc resistance of the bead. CPAR, LBEAD,
and RAC are the parasitic capacitance, the bead inductance, and
the ac resistance (ac core losses) associated with the bead.
For the region where the bead appears most capacitive (Z ≈ |XC|;
CPAR), the parasitic capacitance is calculated by the following
equation:
CPAR =
CPAR
1
2×π × f × | XC |
where:
f is the frequency point anywhere in the region the bead appears
capacitive. In this example, f = 803 MHz.
|XC| is the reactance at 803 MHz, which is 118.1 Ω.
LBEAD
13359-003
RDC
RAC
XL
2×π × f
Figure 3. Simplified Circuit Model
Equation 2 yields a parasitic capacitance value (CPAR) of 1.678 pF.
In some cases, the simplified circuit model shown in Figure 3 can
be used to approximate the ferrite bead impedance characteristic
up to the sub-GHz range.
Rev. 0 | Page 3 of 11
(2)
AN-1368
Application Note
The dc resistance (RDC), which is 300 mΩ, is acquired from the
manufacturer data sheet. The ac resistance (RAC) is the peak
impedance where the bead appears to be purely resistive.
Calculate RAC by subtracting RDC from Z. Because RDC is very
small compared to the peak impedance, it can be neglected.
Therefore, in this case RAC is 1.082 kΩ. The ADIsimPE circuit
simulator tool was used to generate the impedance vs. the
frequency response. Figure 5 shows both the actual measurement
and simulated measurement for the impedance vs. the frequency
response at zero dc bias current and Figure 6 shows the circuit
simulation model with the calculated values. In this example, the
impedance curve from the circuit simulation model closely
matches the measured one.
1200
IMPEDANCE (Ω)
1000
800
DC BIAS CURRENT CONSIDERATIONS
Selecting the right ferrite bead for power applications requires
careful consideration not only of the filter bandwidth, but also
of the impedance characteristics of the bead with respect to dc
bias current. In most cases, manufacturers only specify the
impedance of the bead at 100 MHz and publish data sheets with
frequency response curves at zero dc bias current, similar to
those shown in Figure 2. However, when using ferrite beads for
power supply filtering, the load current going through the bead
is never zero, and as dc bias current increases from zero, all of
these parameters change significantly.
As the dc bias current increases, the core material begins to
saturate, which significantly reduces the inductance of the ferrite
bead. The degree of inductance saturation differs depending on
the material used for the core of the component. Figure 7 shows
the typical dc bias dependency of the inductance for two ferrite
beads. With 50% of the rated currents, the inductance decreases
by up to 90%.
450
600
UP TO 90% DECREASE
IN BEAD INDUCTANCE
AT 50% RATED CURRENT
400
400
350
INDUCTANCE (nH)
SIMULATION
1
10
100
1000
FREQUENCY (MHz)
13359-005
ACTUAL MEASUREMENT
0
300
250
200
MPZ1608S101A
(100Ω, 3A, 0603)
150
Figure 5. Actual Measurement vs. Simulation
IMPEDANCE
50
C1
1.208µF
R2
L1
0
0
1.082kΩ
R1
1
2
3
4
DC BIAS CURRENT (A)
5
Figure 7. Effect of DC Bias on Bead Inductance
13359-205
AC 1A
I1
300mΩ
742 792 510
(70Ω, 6A, 1812)
100
1.678pF
13359-006
200
Figure 6. Circuit Simulation Model
The ferrite bead model can be useful in noise filtering circuit
design and analysis. For example, approximating the inductance
of the bead can be helpful in determining the resonant frequency
cut-off when combined with a decoupling capacitor in a low-pass
filter network. However, the circuit model specified in this
application note is an approximation with a zero dc bias current.
This model may change with respect to dc bias current, and in
other cases, a more complex model is required.
For effective power supply noise filtering, use ferrite beads at
about 20% of their rated dc current. As shown in these two
examples, the inductance at 20% of the rated current drops to
about 30% for the 6 A bead and to about 15% for the 3 A bead.
The current rating of ferrite beads is an indication of the maximum
current the device can take for a specified temperature rise and
not a real operating point for filtering purposes.
Rev. 0 | Page 4 of 11
Application Note
AN-1368
In addition, the effect of dc bias current can be observed in the
reduction of impedance values over frequency, which in turn
reduces the effectiveness of the ferrite bead and its ability to
remove EMI. Figure 8 and Figure 9 show how the impedance of
the ferrite bead varies with dc bias current.
160
System designers must be fully aware of the effect of dc bias
current on bead inductance and effective impedance, as this can
be critical in applications that demand high supply current.
Figure 10 shows the measured attenuation for a 100 Ω, 3 A
rated ferrite bead plus a 1 μF capacitor in various dc bias
currents, and Figure 11 shows the test circuit model.
10
140
0mA
100mA
250mA
500mA
750mA
1000mA
1500mA
2000mA
3000mA
100
80
INSERTION LOSS (dB)
IMPEDANCE (Ω)
120
IBIAS = 0mA
IBIAS = 100mA
IBIAS = 250mA
0
60
40
–10
–20
–30
–40
–50
20
100
1000
FREQUENCY (MHz)
–70
0.01
Figure 8. TDK MPZ1608S101A Impedance Curves with Respect to DC Bias
Current
120
IMPEDANCE (Ω)
80
60
1
10
100
FREQUENCY (MHz)
Figure 10. Measured Response of the Bead and Capacitor Low-Pass Filter vs.
DC Bias Current
FERRITE BEAD:
TDK MPZ1608S101A
(100Ω, 3A, 0603)
0mA
100mA
250mA
500mA
1000mA
1500mA
2000mA
3000mA
4000mA
5000mA
100
0.1
13359-009
10
CAPACITOR:
MURATA
GRM188R61A105KA61
(1µF, X5R, 0603)
IBIAS
13359-209
1
13359-007
–60
0
Figure 11. Test Circuit Model
40
For a 3 A bead with 250 mA of bias current, the resonance
cutoff moves to the right mainly due to the inductance drop
described in Figure 7.
0
1
10
100
FREQUENCY (MHz)
1000
13359-008
20
Figure 9. Wurth Elektronik 742 791 510 Impedance Curves with Respect to DC
Bias Current
As shown in Figure 8 and Figure 9, the effective impedance
at 100 MHz dramatically drops from 100 Ω to 10 Ω for the
TDK MPZ1608S101A (100 Ω, 3 A, 0603) and from 70 Ω to
15 Ω for the Wurth Elektronik 742 792 510 (70 Ω, 6 A, 1812) by
applying just 50% of the rated current.
In addition, the effective attenuation of the network reduces by
as much as 15 dB at 1 MHz, where most modern switching
regulators operate. This reduction can cause problems if the
system engineer relies solely on the data sheet, which shows the
impedance at zero dc bias current only. Therefore, to validate the
effectiveness of the ferrite bead filter and for accurate results, model
the inductance over a range of dc bias current and identify its
impedance characteristics under the actual operating conditions.
Rev. 0 | Page 5 of 11
AN-1368
Application Note
C1
0.1µF
CIN
22µF
X5R
6.3V
R1
10Ω
ADP2120ACPZ-2.5-R7
EN 10
1 VIN
2 PVIN
SYNC/MODE 9
MPZ1608S101A
VOUT
2.5V
COUT1
22µF
X5R
6.3V
C
1µF
COUT2
10µF
X5R
6.3V
3 SW
L
1.5µH
PGOOD 8
4 PGND
RL
10Ω
5 GND
R2
10kΩ
TRK 7
FB 6
13359-010
VIN
5V
Figure 12. ADP2120 Application Circuit with Bead and Capacitor Low -Pass Filter Implementation for DC Bias Effect
70
60
ADP2120 NO LOAD OUTPUT SPECTRUM
ADP2120 + UNDAMPED BEAD-C, ILOAD = 250mA
ADP2120 + UNDAMPED BEAD-C, ILOAD = NO LOAD
AMPLITUDE (dBµV)
50
40
30
20
10
–10
0.01
0.1
1
10
FREQUENCY (MHz)
100
Resonance peaking is possible when implementing a ferrite
bead together with a decoupling capacitor. This commonly
overlooked effect can be detrimental because it may amplify
ripple and noise in a given system instead of attenuating it. In
many cases, this peaking occurs around popular switching
frequencies of dc-to-dc converters.
Peaking occurs when the resonant frequency of a low-pass
filter network, formed by the ferrite bead inductance and the
high Q decoupling capacitance, is below the crossover frequency
of the bead. The resulting filter is underdamped. Figure 14
shows the measured impedance vs. frequency plot of the
TDK MPZ1608S101A. The resistive component, which is
depended upon to dissipate unwanted energy, does not become
significant until about 20 MHz to 30 MHz. Below this frequency,
the ferrite bead still has a very high Q and acts like an ideal
inductor. LC resonant frequencies for typical bead filters are
generally in the 0.1 MHz to 10 MHz range. For typical switching
frequencies in the 300 kHz to 5 MHz range, additional damping is
required to reduce the filter Q.
160
13359-011
0
LC RESONANCE EFFECT
140
Z
Figure 13. Measured Response of the Bead and Capacitor Low-Pass Filter vs.
DC Bias Current
IMPEDANCE (Ω)
As the load current increases in Figure 13, the filter cutoff
frequency moves to the right from approximately 180 kHz with
no load to approximately 370 kHz with 250 mA (red trace vs.
green trace). The fundamental ripple at around 1.2 MHz
attenuates by 30 dB with no load but reduces to about 18 dB
with a load of 250 mA.
120
100
CROSSOVER FREQUENCY
80
60
R
X
INDUCTIVE
REGION
40
20
0
1
10
100
FREQUENCY (MHz)
1000
Figure 14. Measured Impedance vs. Frequency Plot for the
TDK MPZ1608S101A
Rev. 0 | Page 6 of 11
13359-012
Figure 12 shows an application circuit that uses the ADP2120
buck regulator running in forced pulse-width modulation (FPWM)
mode with the ferrite bead filter of Figure 11. With the ADP2120
in FPWM mode, the switching noise spectral output does not
change significantly with respect to load current. For more details
about FPWM mode, see the ADP2120 data sheet. The spectral
output is plotted in Figure 13, showing the effects of dc bias on
ferrite bead response.
Application Note
AN-1368
An undamped ferrite bead filter can exhibit peaks from
approximately 10 dB to approximately 15 dB depending on the Q
of the filter circuit. In Figure 15, peaking occurs at around
2.5 MHz with as much as 10 dB gain.
As an example of this effect, Figure 15 shows the S21 frequency
response of the bead and capacitor low-pass filter, which displays a
peaking effect. The ferrite bead used is a TDK MPZ1608S101A
(100 Ω. 3 A, 0603) and the decoupling capacitor used is a Murata
GRM188R71H103KA01 low ESR ceramic capacitor (10 nF , X7R,
0603). Load current is in the microampere range.
In addition, signal gain can be seen from 1 MHz to 3.5 MHz.
This peaking is problematic if it occurs in the frequency band in
which the switching regulator operates. This amplifies the
unwanted switching artifacts, which can wreak havoc on the
performance of sensitive loads such as the phased-lock loop
(PLL), voltage-controlled oscillator (VCO), and high resolution
analog-to-digital converter (ADC). The result shown in Figure 15
has been taken with a very light load (in the microampere
range), but this is a realistic application in sections of circuits
that need just a few microamperes to 1 mA of load current or
sections that are turned off to save power in some operating
modes. This potential peaking creates additional noise in the
system that can create unwanted crosstalk.
20
10
0
–20
–30
–40
–50
–60
1k
10k
100k
1M
As an example, Figure 17 shows an ADP5071 application circuit
with an implemented bead filter and Figure 18 shows the spectral
plot at the positive output. The switching frequency is set at
2.4 MHz, the input voltage is set at 9 V, the output voltage is set
at 16 V, and the load current is set at 5 mA.
13359-013
–70
100
10M
FREQUENCY (Hz)
Figure 15. S21 Response for Bead and Capacitor Low-Pass Filter
DC BIAS
CURRENT
(µA)
CAPACITOR:
MURATA
GRM188R7H103KA01
(10nF, X7R, 0603)
13359-213
FERRITE BEAD:
TDK MPZ1608S101A
(100Ω, 3A, 0603)
Figure 16. Bead and Capacitor Low-Pass Filter Test Model
ADP5071
RC1
118kΩ
CC1
820pF
CIN1
10µF
RC2
57.6kΩ
CC2
2.7nF VREG
INBK
L1
2.2µH
COMP1
EN1
CVREG
1µF
VIN
+9V
SS
VREG
FB1
PVIN1
PVIN2
PVINSYS
PGND
EN2
VREF
COMP2
SYNC/FREQ
SLEW
SEQ
AGND
D1
MBR130T1G
MPZ1608S101A
+16V
SW1
RFT1
68.1kΩ
RFB1
3.57kΩ
COUT1
10µF
C
10nF
RL
3.2kΩ
CVREF
1µF
RFB2
3.09kΩ
FB2
COUT2
10µF
RFT2
64.9kΩ
–16V
SW2
D2
MBR130T1G
L2
6.8µH
13359-014
GAIN (dB)
–10
Figure 17. ADP5071 Application Circuit with Bead and Capacitor Low-Pass Filter Implementation on Positive Output
Rev. 0 | Page 7 of 11
AN-1368
Application Note
60
Figure 21 shows a circuit simulation changing from a 1 MΩ
load to a 10 Ω load using the same source resistance. Peaking is
significantly reduced and damped for lower load resistance.
Note that in actual applications (see Figure 13), higher dc load
current significantly affects ferrite bead performance. The
inductance of the bead drops as dc current increases and affects
the resonant frequency cutoff.
40
30
20
ADP5071 OUTPUT SPECTRUM
WITH BEAD AND C LOW-PASS
FILTER (UNDAMPED)
0
ADP5071 OUTPUT SPECTRUM
WITHOUT BEAD AND C
LOW-PASS FILTER
40
10Ω LOAD
1MΩ LOAD
1
10
100
FREQUENCY (MHz)
Figure 18. ADP5071 Spectral Output at 5 mA Load
Resonant peaking occurs at around 2.4 MHz due to the inductance
of the bead and the 10 nF ceramic capacitor. Instead of attenuating
the fundamental ripple frequency, a gain of 10 dB occurs.
Other factors that have an effect on the resonant peaks are the
series and load impedances of the ferrite bead filter. Figure 19
shows the circuit simulation result comparing a 10 Ω resistor
and a 0.1 Ω series source resistor. Peaking significantly reduces
and damps for higher source resistance. However, the load
regulation degrades with this approach, making it unrealistic in
practice. The dc voltage at the load droops with load current due
to the drop from the series resistance.
INSERTION LOSS (dB)
0.1
13359-015
20
–10
0.01
–20
–40
–60
–80
–100
1
10
1k
10k
100k
1M
10M
100M
Figure 21. Circuit Simulation Comparison of 10 Ω Load vs. 1 MΩ Load
200fF
100mΩ 30mΩ
IN
10Ω SOURCE
0.1Ω SOURCE
20
100
FREQUENCY (Hz)
40
INSERTION LOSS (dB)
0
+ R3
R5
C2
800nH
L1
150Ω
AC1
V1
0
R4
–20
OUT
C1
10nF
L2
2nH
R1
10mΩ
LOAD
R2
Figure 22. Circuit Simulation Model of 10 Ω Load vs. 1 MΩ Load
–40
–60
1
10
100
1k
10k
100k
FREQUENCY (Hz)
1M
10M
100M
13359-016
–80
–100
Figure 19. Circuit Simulation Comparison of 0.1 Ω Source vs. 10 Ω Source
200fF
+ R3
30mΩ
C2
800nH
R5
L1
150Ω
AC1
V1
R4
C1
10nF
L2
2nH
R1
10mΩ
OUT
R2
1MΩ
13359-216
SOURCE
IN
13359-116
10
13359-316
50
AMPLITUDE (dBµV)
Load impedance also affects the peaking response. Peaking is
worst for light load conditions.
PEAKING OCCURS
AT ~2.5MHz
DUE TO RESONANCE
Figure 20. Circuit Simulation Model of 0.1 Ω Source vs. 10 Ω Source
Rev. 0 | Page 8 of 11
Application Note
AN-1368
FERRITE BEAD:
TDK MPZ1608S101A
(100Ω, 3A, 0603)
DAMPING METHODS
This section describes three damping methods that a system
engineer can use to reduce the level of resonant peaking
significantly (see Figure 23).
IN
OUT
PARALLEL RESISTOR
ACROSS FERRITE BEAD
20
CAPACITOR:
MURATA
GRM188R71H103KA01
(10nF, X7R, 0603)
13359-019
10
Figure 25. Parallel Resistor Across Ferrite Bead
C
160
A
–20
BEAD ONLY,
DC BIAS 0mA
R
X
Z
140
B
120
IMPEDANCE (Ω)
–30
–40
–60
UNDAMPED
METHOD A: SERIES 10Ω RESISTOR TO DECOUPLING CAPACITOR
METHOD B: PARALLEL 10Ω RESISTOR ACROSS FERRITE BEAD
METHOD C: DAMPED: C DAMP (1µF) + RDAMP (2Ω)
BEAD: TDK MPZ1608S101A (100Ω, 3A, 0603)
CAPACITOR: MURATA GRM188R71H103KA01 (10nF, X7R, 0603)
–70
0.0001
0.001
0.01
0.1
1
60
40
10
FREQUENCY (MHz)
20
Figure 23. Actual Frequency Response for Various Damping Methods
0
1M
Method A consists of adding a series resistor to the decoupling
capacitor path (see Figure 24), which dampens the resonance of
the system but degrades the bypass effectiveness at high
frequencies.
10M
1G
100M
FREQUENCY (Hz)
Figure 26. MPZ1608S101A Impedance Curve
12
FERRITE BEAD:
TDK MPZ1608S101A
(100Ω, 3A, 0603)
10
SERIES
RESISTOR TO
DECOUPLING
CAPACITOR
IMPEDANCE (Ω)
OUT
CAPACITOR:
MURATA
GRM188R71H103KA01
(10nF, X7R, 0603)
13359-018
IN
80
Figure 24. Series Resistor to Decoupling Capacitor
8
BEAD ONLY,
DC BIAS 0mA
R
X
Z
BEAD WITH
10Ω RESISTOR,
DC BIAS 0mA
R
X
Z
6
4
2
Method B consists of adding a small parallel resistor across the
ferrite bead (see Figure 25), which also dampens the resonance
of the system. However, the attenuation characteristic of the filter is
reduced at high frequencies. Figure 26 and Figure 27 show the
impedance vs. frequency curve of the MPZ1608S101A with and
without a 10 Ω parallel resistor. The green dashed curve is the
overall impedance of the bead with a 10 Ω resistor in parallel. The
impedance of the bead and resistor combination is significantly
reduced and is dominated by the 10 Ω resistor. However, the
3.8 MHz crossover frequency for the bead with the 10 Ω parallel
resistor is much lower than the crossover frequency of the bead
on its own at 40.3 MHz. The bead appears resistive at a much
lower frequency range, lowering the Q for improved damped
performance.
Rev. 0 | Page 9 of 11
0
1M
10M
100M
1G
FREQUENCY (Hz)
Figure 27. MPZ1608S101A Impedance Curve, Zoom View
13359-022
–50
BEAD WITH
10Ω RESISTOR,
DC BIAS 0mA
R
X
Z
100
13359-021
–10
13359-017
INSERTION LOSS (dB)
0
AN-1368
Application Note
Method C consists of adding a large capacitor (CDAMP) with a
series damping resistor (RDAMP), which is often optimum (see
Figure 28).
Figure 29 shows the ADP5071 positive output spectral plot with
Method C damping implemented on the application circuit shown
in Figure 17. The CDAMP and RDAMP used are a 1 μF ceramic
capacitor and a 2 Ω SMD resistor, respectively. The fundamental
ripple at 2.4 MHz is reduced by 5 dB as opposed to the 10 dB
gain shown in Figure 18.
FERRITE BEAD:
TDK MPZ1608S101A
(100Ω, 3A, 0603)
OUT
CDAMP
CAPACITOR:
MURATA
GRM188R71H103KA01
(10nF, X7R, 0603)
60
50
Figure 28. Additional RC Decoupling Filter
Adding the capacitor and resistor dampens the resonance of the
system and does not degrade the bypass effectiveness at high
frequencies, as shown in Figure 23. Implementing this method
avoids excessive power dissipation on the resistor due to a large
dc blocking capacitor. The capacitor must be much larger than
the sum of all decoupling capacitors, which reduces the required
damping resistor value. The capacitor impedance must be
sufficiently smaller than the damping resistance at the resonant
frequency to reduce the peaking. Determine the RDAMP range by
using Equation 3 and Equation 4. Using a ratio of 16 or higher
between damping capacitance (CDAMP) and decoupling capacitance
(CDECOUP) provides a range of damping resistance that satisfies
both equations.
RDAMP ≥ 2
RDAMP ≤ 0.5
LBEAD
C DAMP
(3)
LBEAD
C DECOUP
AMPLITUDE (dBµV)
USING ADDITIONAL
RC DECOUPLING FILTER
13359-020
RDAMP
RIPPLE SPUR AT 2.4MHz
IS REDUCED BY 5dB
40
30
20
10
0
–10
0.01
ADP5071 OUTPUT SPECTRUM
WITH BEAD AND C LOW-PASS
FILTER (METHOD C DAMPING)
ADP5071 OUTPUT SPECTRUM
WITHOUT BEAD AND C
LOW-PASS FILTER
0.1
1
FREQUENCY (MHz)
10
100
13359-023
IN
Figure 29. ADP5071 Spectral Output plus Bead and Capacitor Low-Pass Filter
with Method C Damping
Generally, Method C is the most elegant and is implemented by
adding a resistor in series with a ceramic capacitor rather than
buying an expensive dedicated damping capacitor. The safest
designs always include a resistor that can be tweaked during
prototyping and that can be eliminated if not necessary. The
only drawbacks are the additional component cost and greater
required board space.
(4)
where:
RDAMP is the damping resistance.
LBEAD is the bead inductance from Equation 1, including external
inductance such as the parasitic trace inductance of the board.
CDAMP is the damping capacitance.
CDECOUP is the decoupling capacitance.
Rev. 0 | Page 10 of 11
Application Note
AN-1368
CONCLUSION
REFERENCES
This application note shows key considerations that must be
taken into account when using ferrite beads. The application
note also details a simple circuit model representing the bead.
The simulation results show good correlation with the actual
measured impedance vs. the frequency response at zero dc bias
current.
AN 583 Application Note. Designing Power Isolation Filters with
Ferrite Beads for Altera FPGAs. Altera Corporation, 2009.
The application note also discusses the effect of the dc bias
current on the ferrite bead characteristics. It shows that a dc bias
current greater than 20% of the rated current can cause a significant
drop in the bead inductance. Such a current can also reduce the
effective impedance of the bead and degrade its EMI filtering
capability. When using ferrite beads in supply rail with dc bias
current, ensure that the current does not cause saturation of the
ferrite material and produce significant change of inductance.
Because the ferrite bead is inductive, do not use it with high Q
decoupling capacitors without careful attention. Doing so can
do more harm than good by producing unwanted resonance in a
circuit. However, the damping methods proposed in this
application note offer an easy solution by using a large decoupling
capacitor in series with a damping resistor across the load, thus
avoiding unwanted resonance. Applying ferrite beads correctly
can be an effective and inexpensive way to reduce high frequency
noise and switching transients.
Applications Manual for Power Supply Noise Suppression and
Decoupling for Digital ICs, Murata Manufacturing Co., Ltd., 2010.
Burket, Chris. All Ferrite Beads Are Not Created Equal –
Understanding the Importance of Ferrite Bead Material
Behavior. TDK Corporation, 2011.
Fancher, David B. ILB, ILBB Ferrite Beads: Electro-Magnetic
Interference and Electro-Magnetic Compatibility (EMI/EMC).
Vishay Dale, 1999.
Hill, Lee and Rick Meadors. Steward EMI Suppression Technical
Presentation. Steward.
Kundert, Ken. Power Supply Noise Reduction. Designer’s Guide
Consulting, Inc., 2004.
Solving Electromagnetic Interference (EMI) with Ferrites,
Fair-Rite Products Corp.
Weir, Steve. PDN Application of Ferrite Beads. IPBLOX, LLC, 2011
©2015 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN13359-0-8/15(0)
Rev. 0 | Page 11 of 11