WM8731SEDSSLASHRV datasheet(24/65 Pages) WOLFSON

24 / 65 page

WM8731 / WM8731L

Production Data

PD, Rev 4.9, October 2012

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 4. 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.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

0000100

Analogue Audio

Path Control

MICBOOST

Microphone Input Level Boost

1 = Enable Boost

0 = Disable Boost

MUTEMIC

Line Input Mute to ADC

1 = Enable Mute

0 = Disable Mute

Table 4 Microphone Input Software Control

The microphone input is biased internally through the operational amplifier to VMID. Whenever the

line inputs are muted the MICIN input is kept biased to VMID using special anti-thump circuitry. This

reduces any audible clicks that may otherwise be heard when re-activating the input.

The application drawing for the microphone is shown in Figure 13.

AGND

MICIN

MICBIAS

AGND

FROM

MICROPHONE

Rmic

Figure 13 Microphone Input and Bias Application Drawing

Recommended component values are C1 = 220pF (npo ceramic), C2 = 1

F, R1 = 680 Ω, R2 = 47k.

Rmic values depends on gain setting (see above).

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