Pre-Production
WM8940
HEADPHONE OUTPUT
The speaker outputs can drive a 16Ω or 32Ω headphone load, either through DC blocking
capacitors, or DC coupled without any capacitor.
Headphone Output using DC Blocking Capacitors:
DC Coupled Headphone Output:
Figure 23 Recommended Headphone Output Configurations
When DC blocking capacitors are used, then their capacitance and the load resistance together
determine the lower cut-off frequency, fc. Increasing the capacitance lowers fc, improving the bass
response. Smaller capacitance values will diminish the bass response. Assuming a 16Ω load and
C1 = 220µF:
fc = 1 / 2π RLC1 = 1 / (2π x 16Ω x 220µF) = 45 Hz
In the DC coupled configuration, the headphone “ground” is connected to the MONOOUT pin. The
MONOOUT pin can be configured as a DC output driver by setting the MONOMUTE register bit.
The DC voltage on MONOOUT in this configuration is equal to the DC offset on the SPROUTP and
SPKOUTN pins therefore no DC blocking capacitors are required. This saves space and material
cost in portable applications.
It is recommended to connect the DC coupled outputs only to headphones, and not to the line input
of another device. Although the built-in short circuit protection will prevent any damage to the
headphone outputs, such a connection may be noisy, and may not function properly if the other
device is grounded.
MONO OUTPUT
The mono output, can be used as a line output, a headphone output or as a pseudo ground for cap-
less driving of loads by SPKOUT. Recommended external components are shown below.
Figure 24 Recommended Circuit for Line Output
The DC blocking capacitors and the load resistance together determine the lower cut-off frequency,
fc. Assuming a 10 kΩ load and C1 = 1µF:
fc = 1 / 2π (RL+R1) C1 = 1 / (2π x 10.1kΩ x 1µF) = 16 Hz
Increasing the capacitance lowers fc, improving the bass response. Smaller values of C1 will
diminish the bass response. The function of R1 is to protect the line outputs from damage when
used improperly.
Pre-Production, Rev 3.0, February 2007
49
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
