Pre-Production
WM8985
Headphone Output using DC Blocking Capacitors
DC Coupled Headphone Output
Figure 26 Recommended Headphone Output Configurations
When DC blocking capacitors are used, their capacitance and the load resistance together
determine the lower cut-off frequency of the output signal, fc. Increasing the capacitance lowers fc,
improving the bass response. Smaller capacitance values will diminish the bass response.
Assuming a 16Ω load and C1, C2 = 220µF:
fc = 1 / 2π RLC1 = 1 / (2π x 16Ω x 220µF) = 45 Hz
In the DC coupled configuration, the headphone pseudo-ground is connected to the buffered midrail
reference pin (LOUT2 or ROUT2). The L/ROUT2 pins can be configured as a DC output driver by
setting the LOUT2MUTE and ROUT2MUTE register bits. The DC voltage on VMID in this
configuration is equal to the DC offset on the LOUT1 and ROUT1 pins therefore no DC blocking
capacitors are required. This saves space and material cost in portable applications.
It is not recommended to use DC-coupling to line inputs of another device. Although the built-in short
circuit protection on the headphone outputs would be tolerant of shorts to ground, such a connection
could be noisy, and may not function properly if the other device is grounded. DC-coupled
configurations should only be used with headphones.
PP, Rev 3.4, October 2006
65
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
