WM8731 datasheet(20/56 Pages) WOLFSON

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WM8731

Advanced Information

WOLFSON MICROELECTRONICS LTD

AI Rev 2.0 February 2001

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 and the driving audio equipment. C2 together with the input

impedance of the WM8731 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 11. 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 11 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)

Or alternatively to calculate the value of Rmic to achieve a given gain, G1.

Rmic = (50k/G1) – 10k

The internal 50k and 10k resistors have a tolerance of 15%. For Rmicext = 90k G = 0.5 (-6dB) and

for Rmicext = 0 G = x10 (14dB).

The 2

nd stage comprises a 0dB gain stage that can be software configured to provide a fixed 20dB of

gain for low sensitivity microphones.

The microphone input can therefore be configured with a variable gain of between -6dB and 14dB on

the 1

st stage, and an additional fixed 0dB or 20dB on the 2nd stage. This allows for all gains to the

input signal in the range –6dB to 34dB to be catered for.

The ADC Full Scale input is 1.0V rms at AVDD = 3.3 volts. Any voltage greater than full scale will

possibly overload the ADC and cause distortion. Note that the full scale input tracks directly with

AVDD. Stage 1 and Stage 2 gains should be configured so that the ADC receives a maximum signal

equal to its full scale for maximising the signal to noise.

The software control for the MICIN is shown in Table 3. Note that the Microphone Mute only mutes

the input to the ADC, this will still allow the Microphone Input signal to pass to the line output in

Sidetone Mode.

WM8731 Datasheet(PDF) 20 Page - Wolfson Microelectronics plc

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