WM8973L_11 Datasheet, PDF (59/63 Pages) Wolfson Microelectronics plc – Stereo CODEC for Portable Audio Applications

Production Data
WM8973L
LINE INPUT CONFIGURATION
When LINPUT1/RINPUT1 or LINPUT2/RINPUT2 are used as line inputs, the microphone boost and
ALC functions should normally be disabled.
In order to avoid clipping, the user must ensure that the input signal does not exceed AVDD. This
may require a potential divider circuit in some applications. It is also recommended to remove RF
interference picked up on any cables using a simple first-order RC filter, as high-frequency
components in the input signal may otherwise cause aliasing distortion in the audio band. AC signals
with no DC bias should be fed to the WM8973L through a DC blocking capacitor, e.g. 1F.
MICROPHONE INPUT CONFIGURATION
FROM
MICROPHONE
AGND
R2
47kOhm
R1
680 Ohm to 2.2kOhm
check microphone's specification
MICBIAS
LINPUT1/2/3
C2
RINPUT1/2/3
1uF
C1
220pF
AGND
AGND
Figure 52 Recommended Circuit for Line Input
For interfacing to a microphone, the ALC function should be enabled and the microphone boost
switched on. Microphones held close to a speaker’s mouth would normally use the 13dB gain setting,
while tabletop or room microphones would need a 29dB boost.
The recommended application circuit is shown above. R1 and R2 form part of the biasing network
(refer to Microphone Bias section). R1 connected to MICBIAS is necessary only for electret type
microphones that require a voltage bias. R2 should always be present to prevent the microphone
input from charging to a high voltage which may damage the microphone on connection. R1 and R2
should be large so as not to attenuate the signal from the microphone, which can have source
impedance greater than 2kOhm. C1 together with the source impedance of the microphone and the
WM8973L input impedance forms an RF filter. C2 is a DC blocking capacitor to allow the microphone
to be biased at a different DC voltage to the MICIN signal.
MINIMISING POP NOISE AT THE ANALOGUE OUTPUTS
To minimise any pop or click noise when the system is powered up or down, the following procedures
are recommended.
w
POWER UP
 Switch on power supplies. By default the WM8973L is in Standby Mode, the DAC is
digitally muted and the Audio Interface, Line outputs and Headphone outputs are all OFF
(DACMU = 1 Power Management registers 1 and 2 are all zeros).
 Enable Vmid and VREF.
 Enable DACs as required
 Enable line and / or headphone output buffers as required.
 Set DACMU = 0 to soft-un-mute the audio DACs.
POWER DOWN
 Set DACMU = 1 to soft-mute the audio DACs.
 Disable all output buffers.
 Switch off the power supplies.
PD, Rev 4.5, December 2011
59