WM8991 datasheet(78/174 Pages) WOLFSON

78 / 174 page

WM8991

Pre-Production

PP, May 2008, Rev 3.1

When driving a handset ear speaker using OUT3 and OUT4 other than from RXP/RXN, the required

phase difference can be provided by inverting one of the DAC outputs or alternatively by mixing Left

and Right channels together using either LOMIX or ROMIX and muting the opposite channel.

Note that a differential output will provide an additional 6dB gain at the output pins. Register bits

OUT3ATTN and OUT4ATTN can be used to compensate for this gain if required.

Fully differential headphone drive is possible between LOUT and OUT3 and between ROUT and

OUT4. Routing LOPGA to OUT3 and ROPGA to OUT4 results in a phase inversion at LOUT with

respect to OUT3 and at ROUT with respect to OUT4. This allows fully differential headset drive, with

greatly improved crosstalk performance, improved bass response, increased noise immunity and

removing the need for large and expensive DC-blocking capacitors.

To ensure fully balanced differential operation, LOUT and OUT3 must be set to the same gain as

each other, and ROUT and OUT4 must be set to the same gain as each other. This is best achieved

by setting OUT3ATTN and OUT4ATTN to 0dB, whilst setting volume controls LOPGAVOL and

LOUTVOL at matching levels and setting volume controls ROPGAVOL and ROUTVOL at matching

levels.

Some example headphone output configurations are shown below.

Figure 32 AC-Coupled Headphone Drive

Figure 33 Capless Mode Headphone Drive

Figure 34 Headphone and Ear Speaker Drive

Figure 35 Fully Differential Headphone Drive

WM8991 Datasheet(PDF) 78 Page - Wolfson Microelectronics plc