WM8731SEDSSLASHR datasheet(23/63 Pages) WOLFSON

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WM8731 / WM8731L

Production Data

PD Rev 4.3 August 2006

AGND

LINEIN

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 = 1

µF.

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

10k

VMID

MICIN

ADC

VMID

20dB GAIN BOOST

Figure 12 Microphone Input Schematic

There are 2 stages of gain made up of two low noise inverting operational amplifiers.

The 1

st 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.

G1 = 50k/ (Rmic + 10k)

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