wm8400 Wolfson Microelectronics plc, wm8400 Datasheet - Page 198

• Manufacturer Part Number: wm8400
• Description: Wolfson Audioplustm Hi-fi Audio Codec And Power Management Unit For Mobile Multimedia
• Manufacturer: Wolfson Microelectronics plc

WM8400
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APPLICATIONS INFORMATION
RECOMMENDED EXTERNAL COMPONENTS
AUDIO INPUT PATHS
The WM8400 provides 4 stereo pairs of analogue audio inputs. Each of these inputs is referenced to
the internal DC reference, VMID. A DC blocking capacitor is required for each input pin used in the
target application. The choice of capacitor is determined by the filter that is formed between that
capacitor and the input impedance of the input pin. The circuit is illustrated in Figure 101.
Figure 101 Audio Input Path DC Blocking
Using a large capacitance will result in a lower filter cut-off frequency, with no attenuation of the audio
frequencies of interest. However, a large capacitance is undesirable in a portable application due to
its large physical size. A small capacitance will raise the cut-off frequency, which may result in some
of the audio bandwidth being compromised. A judgement needs to be made regarding what
frequencies require to be passed by the filter. For hi-fi audio applications, typical circuits aim to
provide a cut-off frequency at 20Hz.
If the input impedance is known, and the cut-off frequency is known, then the minimum capacitor
value may be derived easily. However, it can be seen from the representation in Figure 101 that the
input impedance is not fixed in all applications but can vary with gain and boost amplifier settings.
The choice of capacitor for a 20Hz cut-off frequency is shown in Table 105 for different input
impedance conditions. The applicable input impedance can be found in the “Electrical
Characteristics” section of this datasheet.
Table 104 Audio Input DC Blocking Capacitors
Using the figures in Table 105, it follows that a 1µF capacitance for all input connections will give
good results in most cases. Tantalum electrolytic capacitors are particularly suitable as they offer
high stability in a small package size.
Ceramic equivalents are a cost effective alternative to the superior tantalum packages, but care must
be taken to ensure the desired capacitance is maintained at the AVDD operating voltage. Also,
ceramic capacitors may show microphonic effects, where vibrations and mechanical conditions give
rise to electrical signals. This is particularly problematic for microphone input paths where a large
signal gain is required.
A single capacitor is required for a line input or single-ended microphone connection. In the case of a
differential microphone connection, a DC blocking capacitor is required on both input pins.
IMPEDANCE
15kΩ
30kΩ
60kΩ
2kΩ
MINIMUM CAPACITANCE
FOR 20HZ PASS BAND
0.27 µF
0.13 µF
0.5 µF
4 µF
PP, April 2009, Rev 3.0
Pre-Production
198