ADAU1761BCPZ-R7 datasheet(37/93 Pages) AD

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ADAU1761

Rev. C | Page 36 of 92

HEADPHONE OUTPUT

The LHP and RHP pins can be driven by either a line output

driver or a headphone driver by setting the HPMODE bit in

Address 0x4024). The headphone outputs can drive a load of at

least 16 Ω.

Separate volume controls for the left and right channels range

from −57 dB to +6 dB. Slew can be applied to all the playback

volume controls using the ASLEW[1:0] bits in Register R34

(playback pop/click suppression register, Address 0x4028).

Capless Headphone Configuration

The headphone outputs can be configured in a capless output

configuration with the MONOOUT pin used as a dc virtual

ground reference. Figure 45 depicts a typical playback path in

a capless headphone configuration. Table 19 lists the register

settings for this configuration. As shown in this table, the

MONOOUT pin outputs common mode (AVDD/2), which

is used as the virtual headphone reference.

LHP

RHP

MONOOUT

MIXER 3

LEFT

DAC

MX3LM

LHPVOL[5:0]

MX3EN

MIXER 4

RIGHT

DAC

MX4RM

RHPVOL[5:0]

MX4EN

MIXER 7

MX7[1:0]

MONOM

MOMODE

MX7EN

Figure 45. Capless Headphone Configuration Diagram

Table 19. Capless Headphone Register Settings

Bit Name

Setting

R36

DACEN[1:0]

11 = both DACs on

MX3EN

1 = enable Mixer 3

R22

MX3LM

1 = unmute left DAC input

MX4EN

1 = enable Mixer 4

R24

MX4RM

1 = unmute right DAC input

MX7EN

1 = enable Mixer 7

R28

MX7[1:0]

00 = common-mode output

MONOM

1 = unmute mono output

R33

MOMODE

1 = headphone output

LHPVOL[5:0]

Desired volume for LHP output

R29

LHPM

1 = unmute left headphone output

HPMODE

1 = headphone output

RHPVOL[5:0]

Desired volume for RHP output

R30

RHPM

1 = unmute right headphone output

Headphone Output Power-Up/Power-Down Sequencing

To prevent pops when turning on the headphone outputs, the

user must wait at least 4 ms to unmute these outputs after

enabling the headphone output with the HPMODE bit. This is

because of an internal capacitor that must charge before these

outputs can be used. Figure 46 and Figure 47 illustrate the

headphone power-up/power-down sequencing.

For capless headphones, configure the MONOOUT pin before

unmuting the headphone outputs.

4ms

USER

DEFINED

HPMODE

1 = HEADPHONE

INTERNAL

PRECHARGE

RHPM AND LHPM

1 = UNMUTE

Figure 46. Headphone Output Power-Up Timing

HPMODE

0 = LINE OUTPUT

RHPM AND LHPM

0 = MUTE

USER DEFINED

Figure 47. Headphone Output Power-Down Timing

Ground-Centered Headphone Configuration

The headphone outputs can also be configured as ground-

centered outputs by placing coupling capacitors on the LHP

and RHP pins. Ground-centered headphones should use the

AGND pin as the ground reference.

When the headphone outputs are configured in this manner,

the capacitors create a high-pass filter on the outputs. The

corner frequency of this filter, at which point its attenuation

is 3 dB, is calculated by the following formula:

f3dB = 1/(2π × R × C)

where:

C is the capacitor value.

R is the impedance of the headphones.

For a typical headphone impedance of 16 Ω and a 47 μF

capacitor, the corner frequency is 211 Hz.

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