WAN_0176 A.C. Coupling Capacitor Selection

WAN_0176
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A.C. Coupling Capacitor Selection
INTRODUCTION
This application note discusses the various capacitor types available which are suitable for a.c.
coupling of audio signals in and out of Wolfson CODECs. The choice is becoming more difficult as
parts are getting smaller. A good choice will avoid signal attenuation or distortion.
REQUIREMENTS
In many audio circuits the input or output has a d.c. bias on it. To connect this to another circuit,
which may have a different bias voltage, a capacitor is used. This capacitor blocks the d.c. path, but
passes the a.c. audio signal. This capacitor is required with Wolfson ADC inputs, DAC outputs and
usually headphone drivers.
Figure 1 High-pass Network Formed by a.c. Coupling
The value of capacitance required depends on the source/load impedance and the frequency range.
In the a.c. coupling situation the capacitor will form part of a high-pass filter, so it is the bass cut-off
frequency that is important. If the capacitance is too small, bass response is made worse. The cut-off
frequency, at which the signal is attenuated by 3dB, is defined by the following equation:
f −3dB =
1
2πRC
The table below shows the main two applications. For DAC line outputs, the load impedance may be
the traditional 47kΩ, or it may be around 10kΩ (like SCART). For ADC line inputs, the impedance
may also go below 10kΩ, depending on the ADC amplifier gain selected. These issues affect the
choice of coupling capacitor, depending on the acceptable cut-off frequency.
APPLICATION
LOAD
IMPEDANCE
COUPLING
CAPACITOR
10kΩ
10µF
1.6Hz
47kΩ
1µF
3.4Hz
16Ω
220µF
45Hz
32Ω
220µF
23Hz
Line In/Out
Headphone
-3DB CUT-OFF
FREQUENCY
Table 1 Cut-off Frequencies
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CAPACITOR DIELECTRICS
It is important to understand about the dielectric (the insulator between the metal plates of the
capacitor) and how its behaviour deviates from ideal. If the capacitance is not stable, it can lead to
signal distortion.
CERAMIC
Ceramic capacitors are by far the most popular type of capacitor as they are available in small
surface-mount packages at low cost. They are most commonly used for decoupling ICs, but are also
used in RF applications and some audio applications.
Ceramic capacitors are available in a number of dielectrics. The most common dielectrics are C0G
(same as NP0), X7R and Y5V.
C0G gives a very stable capacitance value with varying temperature and voltage. Unfortunately it is
only available in smaller values. It is good for the signal path.
X7R is not so stable, but is available in much higher values. Its capacitance varies a little with
temperature and d.c. voltage.
Y5V is a poor choice. The temperature range is smaller and it has huge variations of capacitance
with voltage.
U2J is a newer dielectric which is nearly as good as C0G, with narrower temperature range, but is
available in higher capacitance values. It is primarily applicable in audio low-pass filters.
X5R is a newer dielectric, replacing Y5V with better performance. It is essentially the same as X7R
but with reduced temperature range and more d.c.-voltage dependency.
DIELECTRIC
C0G
U2J
X7R
X5R
Y5V
Capacitance range
0.5pF to 0.1µF
3pF to 0.1µF
1nF to 22µF
0.1µF to 100µF
0.1µF to 100µF
Initial tolerance
±0.25pF to ±5%
±10% or ±20%
±10% or ±20%
±10% or ±20%
+80 to -20%
Temperature range
-55 to +125°C
-55 to +85°C
-55 to +125°C
-55 to +85°C
-30 to +85°C
Capacitance change
with temperature
0±30ppm/°C
-750±120ppm/°C
±15%
±15%
+22 to -82%
Capacitance change
with d.c. voltage
Not Significant
Not Significant
Significant – see
individual part
graph
Significant – see
individual part
graph
Very bad – do
not use for a.c.
coupling
Table 2 Ceramic Dielectric Properties
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X7R
X5R
X5R
Y5V
Figure 2 Ceramic Capacitance vs. d.c. Bias Voltage
Figure 2 shows 1µF 16V 0805 capacitors and how their capacitances vary with voltage. The first
(blue) is X7R, second (red) is X5R, third (black) is thicker X5R, fourth (magenta) is Y5V. As you can
see, the Y5V capacitance changes greatly with voltage and is only suitable for decoupling low fixed
voltages like VMID on our CODECs. X5R shows a big difference between parts and the right one
must be chosen for best performance. X7R is the most stable and best choice for performance.
X5R
X5R
X7R
Y5V
Figure 3 Ceramic Capacitance vs. Temperature
Figure 3 shows the same 1µF 16V capacitors and how their capacitances vary with temperature. As
you can see the Y5V capacitance changes greatly with temperature. In a product with elevated
temperatures, the cut-off frequency could be adversely affected.
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TANTALUM ELECTROLYTIC
Tantalum capacitors are available in a number of variants. They are generally more expensive than
ceramic capacitors but offer greater stability at higher capacitance values, albeit with higher ESR
(Equivalent Series Resistance). These features make them preferable over ceramic types in most
cases.
TYPE
STANDARD
MICROCHIP
Capacitance range
0.1µF to 1500µF
0.47µF to 220µF
Initial tolerance
±10% or ±20%
±10% or ±20%
Temperature range
-55 to +125°C
-55 to +125°C
Capacitance change
with temperature
-5 to +8%
-5 to +8%
Capacitance change
with d.c. voltage
Not Significant
Not Significant
Table 3 Tantalum Electrolytic Properties
Figure 4 Tantalum Capacitance vs. Temperature
ALUMINIUM ELECTROLYTIC
Standard aluminium parts are much lower cost than tantalum, but are usually physically bigger. They
offer good stability of capacitance with voltage and temperature and have similar ESR, so are
recommended where there is enough space.
ALUMINIUM ORGANIC/POLYMER ELECTROLYTIC
These parts offer very good performance at a higher price. If the application permits, these are the
preferred choice.
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WHY IS THE CAPACITANCE CHANGE WITH D.C. VOLTAGE IMPORTANT?
If a large low-frequency signal is applied to the capacitor, it looks like a slowly-changing d.c. offset.
Consider a 2Vrms DAC output at 50Hz. The typical d.c. offset will be 4.5V if the supply is 9V. When
the DAC is producing a full-scale waveform, the output is going between 0 and 9V. This means the
capacitor sees an effective change in d.c. bias from 0 to 9V. For a ceramic Y5V capacitor, the
capacitance changes between 1.25µF and 0.22µF, which means that charge is stored and released
in a non-linear fashion, causing signal distortion.
RECOMMENDATIONS
LINE INPUTS AND OUTPUTS
The load impedance is usually >1kΩ, so the higher ESR of electrolytic capacitors is not significant.
To achieve good frequency response with 47kΩ load, 1µF is a minimum requirement. For best
performance choose tantalum or aluminium electrolytic. Where space is limited and high
performance is not required, use X7R ceramic.
To achieve good frequency response with 10kΩ load, 1µF is a minimum requirement, but 10µF
would be better for higher-quality applications. For best performance choose tantalum or aluminium
electrolytic. Where space is limited and high performance is not required, use X7R ceramic. For
lowest cost, use X5R ceramic, but THD at low frequencies will be worse.
HEADPHONE OUTPUTS
The load impedance is usually 16-32Ω, so the higher ESR of tantalum capacitors is significant for
power loss. To achieve good frequency response, 100µF is a minimum requirement, 220µF better.
For best performance choose low-ESR tantalum or aluminium electrolytic. ESR under 1Ω would be
best for power efficiency. Where space is limited, use microchip tantalum. For lowest cost, use X5R
ceramic, but THD at low frequencies will be worse.
TYPE
LINE IN/OUT
LINE IN/OUT
HEADPHONE
Capacitance
1µF
10µF
220µF
High
performance
Sanyo OSCON
30SC1M
Sanyo OSCON
16SC10M
Sanyo OSCON
10SA220M
Medium
performance
AVX tantalum
TAJA105M020
AVX tantalum
TAJA106M010
AVX tantalum
TAJD227M006
Medium
performance
Small Size
X7R
Murata
GRM21BR71C105KA01
Tantalum Microchip
AVX
TACH106M010
Tantalum Microchip
AVX
TLCT227M004
Low cost
X5R
Murata
GRM219R61A105KA01
X7R
Murata
GRM32DR71C106KA01
Standard aluminium
Low cost
Small Size
X5R
Murata
GRM188R61C105KA93
X5R
Murata
GRM21BR60J106KE15
X5R (100µF)
Murata
GRM32ER60J107ME20
Table 4 Recommended Components
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CONCLUSION
There are a variety of capacitors available for audio applications. With careful choice, the cost can be
optimised for the appropriate size and performance level.
APPLICATION SUPPORT
If you require more information or require technical support, please contact the Wolfson
Microelectronics Applications group through the following channels:
Email:
Telephone Apps:
Fax:
Mail:
[email protected]
(+44) 131 272 7070
(+44) 131 272 7001
Applications Engineering at the address on the last page
or contact your local Wolfson representative.
Additional information may be made available on our web site at:
http://www.wolfsonmicro.com
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