wm8400 Wolfson Microelectronics plc, wm8400 Datasheet - Page 199

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

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Pre-Production
Figure 102 Audio Output Path Components
AUDIO OUTPUT PATHS
The WM8400 provides 4 line outputs (LOP, LON, ROP, RON) and 4 headphone outputs (LOUT,
ROUT, OUT3, OUT4). Each of these outputs is referenced to the internal DC reference, VMID.
These outputs may be connected in a number of different ways, including differential and capless
options (see “Analogue Output Paths”).
In the case of any line output or headphone output used in a single-ended configuration (i.e.
referenced to AGND), a DC blocking capacitor will be required in order to remove the VMID bias. In
the case of capless outputs (using a disabled or muted output as a ‘virtual ground’) or a pair of
outputs that are configured as a differential pair, then the DC blocking capacitor should be omitted.
The choice of capacitor is determined from the filter that is formed between the capacitor and the
load impedance. The line outputs are intended to drive a typical line load of 10kΩ or 47kΩ
impedance. The headphone outputs would typically be connected to a load of 16Ω or 32Ω per
left/right channel. Similar to the choice of audio input capacitor, a judgement needs to be made
regarding what frequencies require to be passed, and what the anticipated load impedance will be.
The choice of capacitor for a 20Hz cut-off frequency is shown in Table 105 for different impedance
conditions.
Table 105 Line Output Frequency Cut-Off
Using the figures in Table 105, it follows that a 220µF capacitance would be a suitable choice for a
32Ω headphone load and that a 1µF capacitance would be a suitable choice for a line load. 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.
For best audio performance, it is recommended to connect a zobel network to the audio output pins.
This network should comprise a 50Ω resistor and 22nF capacitor in series with each other. These
components have the effect of dampening high frequency oscillations or instabilities that can arise
outside the audio band under certain conditions. Possible sources of these instabilities include the
inductive load of a headphone coil or an active load in the form of an external line amplifier.
For best pop suppression performance, it is recommended to connect a 47kΩ resistor between the
load and AGND. This will ensure that the output capacitor does not become charged as a result of
unpredictable behaviour in the load, or as a result of memory effects in the capacitor. The inclusion
of this resistor ensures best compatibility with the WM8400 pop & click suppression circuits.
The complete set of recommended components for single-ended headphone and line outputs is
illustrated in Figure 102. Note that, as described above, not all of these components will be required
in all circumstances.
16Ω
32Ω
10kΩ
47kΩ
Output
IMPEDANCE
22nF
50
C
47k
498 µF
248 µF
0.8 µF
0.17 µF
MINIMUM CAPACITANCE
FOR 20HZ PASS BAND
R
Fc =
Note that it is assumed that the load
resistance R will dominate the
frequency response calculation.
Therefore, the 47k resistor is
ignored in this equation.
Fc = high pass 3dB cut-off frequency
2 RC
1
PP, April 2009, Rev 3.0
WM8400
199