WM8731 / WM8731L
Production Data
C2
R1
LINEIN
C1
R2
AGND
AGND AGND
Figure 11 Line Input Application Drawing
For interfacing to a typical CD system, it is recommended that the input is scaled to ensure that there
is no clipping of the signal. R1 = 5.6k, R2 = 5.6k, C1 = 220pF, C2 = 1F.
R1 and R2 form a resistive divider to attenuate the 2 Vrms output from a CD player to a 1 Vrms level,
so avoiding overloading the inputs. R2 also provides a discharge path for C2, thus preventing the
input to C2 charging to an excessive voltage which may otherwise damage any equipment connected
that is not suitably protected against high voltages. C1 forms an RF low pass filter for increasing the
rejection of RF interference picked up on any cables. C2 forms a DC blocking capacitor to remove the
DC path between the WM8731/L and the driving audio equipment. C2 together with the input
impedance of the WM8731/L form a high pass filter.
MICROPHONE INPUT
MICIN is a high impedance, low capacitance input suitable for connection to a wide range of
monophonic microphones of different dynamics and sensitivities.
The MICIN includes programmable volume adjustments and a mute function. The scheme is shown in
Figure 12. Passive RF and active Anti-Alias filters are also incorporated within the microphone
inputs. These allow a matched interface to the multi-bit oversampling ADC and preventing high
frequencies aliasing into the audio band or otherwise degrading performance.
50k
20dB GAIN BOOST
MICIN
10k
VMID
To
ADC
VMID
Figure 12 Microphone Input Schematic
There are 2 stages of gain made up of two low noise inverting operational amplifiers.
The 1st stage comprises a nominal gain of G1 = 50k/10k = 5. By adding an external resistor (Rmic) in
series with MICIN the gain of stage can be adjusted. For example adding Rmic = 40K sets the gain of
stage 1 to x1 (0dB). The equation below can be used to calculate the gain versus Rmic.
PD, Rev 4.9, October 2012
23
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
