Headphone Output - Analog Devices ADAU1961 Manual

ADAU1961

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
Register R30 (playback headphone right volume control register,
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 44 depicts a typical playback path in
a capless headphone configuration. Table 18 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.
MX3LM
LEFT
DAC
MX7[1:0]
MX4RM
RIGHT
DAC
Figure 44. Capless Headphone Configuration Diagram
Table 18. Capless Headphone Register Settings
Register Bit Name
R36 DACEN[1:0]
R22 MX3EN
MX3LM
R24 MX4EN
MX4RM
R28 MX7EN
MX7[1:0]
R33 MONOM
MOMODE
R29 LHPVOL[5:0]
LHPM
R30 HPMODE
RHPVOL[5:0]
RHPM
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LHPVOL[5:0]
MIXER 3
LHP
MX3EN
MONOM
MIXER 7
MONOOUT
MX7EN
MOMODE
RHPVOL[5:0]
MIXER 4
RHP
MX4EN
Setting
11 = both DACs on
1 = enable Mixer 3
1 = unmute left DAC input
1 = enable Mixer 4
1 = unmute right DAC input
1 = enable Mixer 7
00 = common-mode output
1 = unmute mono output
1 = headphone output
Desired volume for LHP output
1 = unmute left headphone output
1 = headphone output
Desired volume for RHP output
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 45 and Figure 46 illustrate the
headphone power-up/power-down sequencing.
For capless headphones, configure the MONOOUT pin before
unmuting the headphone outputs.
HPMODE
1 = HEADPHONE
RHPM AND LHPM
1 = UNMUTE
INTERNAL
PRECHARGE
Figure 45. Headphone Output Power-Up Timing
RHPM AND LHPM
0 = MUTE
HPMODE
0 = LINE OUTPUT
Figure 46. 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:
f = 1/(2π × R × C)
3dB
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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