WM8987LGECOSLASHV datasheet(56/58 Pages) WOLFSON

56 / 58 page

WM8987L

Pre-Production

PP Rev 3.0 January 2007

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

WM8987L 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.

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 WM8987L through a DC blocking capacitor, e.g. 1

µF.

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.

POWER UP

•

Switch on power supplies. By default the WM8987L is in Standby Mode, the DAC is

digitally muted and the Audio Interface and analogue 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.

POWER MANAGEMENT EXAMPLES

POWER MANAGEMENT (1)

POWER MANAGEMENT (2)

PGAs

ADCs

DACs

Output Buffers

OPERATION MODE

Low-Power Standby

BTL Headset Stereo Playback

BTL Headset Phone Call

Single-ended Headset Stereo Playback

Single-ended Headset Phone Call

Single-ended Headset Stereo Playback

(capless)

Single-ended Headset Phone Call

(capless)

Table 46 Example Register Settings for Power Management

WM8987LGECO/V Datasheet(PDF) 56 Page - Wolfson Microelectronics plc

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