WM8994ECSSLASHR datasheet(144/363 Pages) CIRRUS

144 / 363 page

WM8994

144

Rev 4.6

band under certain conditions. Possible sources of these instabilities include the inductive load of a

headphone coil or an active load in the form of an external line amplifier. The capacitance of lengthy

cables or PCB tracks can also lead to amplifier instability. The zobel network should comprise of a

 resistor and 100nF capacitor in series with each other, as illustrated in Figure 36.

If any ground-referenced headphone output is not used, then the zobel network components can be

omitted from the corresponding output pin, and the pin can be left floating. The respective headphone

driver(s) should not be enabled in this case.

AGND = 0V

WM8994

Ω

100 nF

AGND = 0V

HPOUT1L

HPOUT1R

HPOUT1FB

Ω

100 nF

Figure 36 Zobel Network Components for HPOUT1L and HPOUT1R

The headphone output incorporates a common mode, or ground loop, feedback path which provides

rejection of system-related ground noise. The return path is via HPOUT1FB. This pin must be

connected to ground for normal operation of the headphone output. No register configuration is

required.

Note that the HPOUT1FB pin should be connected to GND close to the headphone jack, as illustrated

in Figure 36.

EARPIECE DRIVER OUTPUT CONFIGURATIONS

The earpiece driver outputs HPOUT2P and HPOUT2N are driven by the HPOUT2MIX output mixer,

which can take inputs from the mixer output PGAs MIXOUTLVOL and MIXOUTRVOL, or from the low

power, differential Direct Voice path IN2LP/VRXN and IN2RP/VRXP. Fine volume control is available

on the output mixer paths using MIXOUTLVOL and MIXOUTRVOL. A selectable -6dB attenuation is

available on the HPOUT2MIX output, as described in Table 67 (

refer to the “Analogue Output Signal

Path

” section).

The earpiece outputs are designed to operate in a BTL configuration, driving 50mW into a typical 16



ear speaker.

For suppression of pop noise there are two separate enables for the earpiece driver; HPOUT2_ENA

enables

the

output

stage

and

HPOUT2_IN_ENA

enables

the

mixer

and

input

stage.

HPOUT2_IN_ENA should be enabled a minimum of 50

s before HPOUT2_ENA – see “Control Write

Sequencer

” section for an example power sequence.

LINE OUTPUT CONFIGURATIONS

The four line outputs LINEOUT1P, LINEOUT1N, LINEOUT2P and LINEOUT2N provide a highly

flexible combination of differential and single-ended configurations, each driven by a dedicated output

mixer. There is a selectable -6dB gain option in each mixer to avoid clipping when mixing more than

one signal into a line output. Additional volume control is available at other locations within each of the

supported signal paths. For more information about the line output mixing options, refer to the

“Analogue Output Signal Path” section.

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