Preliminary Technical Data

WM8737L

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AI Rev 3.0 May 2004

35

APPLICATIONS INFORMATION

LINE INPUT CONFIGURATION

In order to avoid clipping, the user must ensure that the input signal does not exceed AVDD. This

may require a potential divider circuit in some applications. It is also recommended to remove RF

interference picked up on any cables using a simple first-order RC filter, as high-frequency

components in the input signal may otherwise cause aliasing distortion in the audio band. This filter

must not have high output impedance at audio frequencies (e.g. use a LC filter) if PGA gain errors

are to be minimised when bypassing the microphone preamplifier.

When using ac signals with no dc bias they should be coupled to the WM8737L signal inputs through

a DC blocking capacitor, e.g. 470nF or 1

µF when using the microphone preamplifier, and at least

10

µF if directly driving the PGA (bigger capacitance may be required at higher gains due to the low

PGA input impedance at high gain).

MICROPHONE INPUT CONFIGURATION

R2

47KOhm

C1

220pF

C2

1uF

AGND

AGND

AGND

LINPUT1/2/3

RINPUT1/2/3

FROM

MICROPHONE

R1

680 Ohm

MICBIAS

Figure 25 Recommended Circuit for Microphone Input

For interfacing to a microphone, the ALC function should be enabled and the microphone boost

switched on. Microphones held close to a speaker’s mouth would normally use a lower boost setting

such as 13dB, while tabletop or room microphones would need a higher boost, for example 28dB.

The recommended application circuit is shown above. R1 and R2 form part of the biasing network

(refer to Microphone Bias section). R1 connected to MICBIAS is necessary only for electret type

microphones that require a voltage bias. R2 should always be present to prevent the microphone

input from charging to a high voltage which may damage the microphone on connection. R1 and R2

should be large so as not to attenuate the signal from the microphone, which can have source

impedance greater than 2k

Ω. C1 together with the source impedance of the microphone and the

WM8737L input impedance forms an RF filter. C2 is a dc blocking capacitor to allow the microphone

to be biased at a different dc voltage to the MICIN signal.