WM8944BECSSLASHR datasheet(158/165 Pages) CIRRUS

158 / 165 page

WM8944B

158

Rev 4.3

APPLICATIONS INFORMATION

RECOMMENDED EXTERNAL COMPONENTS

AUDIO INPUT PATHS

The WM8944B provides up to 2 analogue audio inputs (including the auxiliary input AUX). Each of

these inputs is referenced to the internal DC reference, VMID. A DC blocking capacitor is required for

each input pin used in the target application. The choice of capacitor is determined by the filter that is

formed between that capacitor and the input impedance of the input pin. The circuit is illustrated in

Figure 53. (Note that capacitors are not required on any unused audio input.)

IN1/

DMICDAT

PGA

AUX

Fc = high pass 3dB cut-off frequency

Fc =

2 RC

Figure 53 Audio Input Path DC Blocking Capacitor

When the input impedance is known, and the cut-off frequency is known, then the minimum capacitor

value may be derived easily. For practical use, a 1

F capacitance for all audio inputs can be

recommended for most cases. Tantalum electrolytic capacitors are particularly suitable as they offer

high stability in a small package size.

Ceramic equivalents are a cost effective alternative to the superior tantalum packages, but care must

be taken to ensure the desired capacitance is maintained at the LDOVOUT operating voltage. Also,

ceramic capacitors may show microphonic effects, where vibrations and mechanical conditions give

rise to electrical signals. This is particularly problematic for microphone input paths where a large

signal gain is required.

A single capacitor is required for a line input or single-ended microphone connection. In the case of a

differential microphone connection, a DC blocking capacitor is required on both input pins.

LINE OUTPUT PATHS

The WM8944B provides three outputs (LINEOUT, SPKOUTP and SPKOUTN). Each of these outputs

is referenced to the internal DC reference, VMID. In any case where a line output is used in a single-

ended configuration (i.e. referenced to GND), a DC blocking capacitor is required in order to remove

the DC bias. In the case where a pair of line outputs is configured as a BTL differential pair, then the

DC blocking capacitor should be omitted.

The choice of capacitor is determined from the filter that is formed between the capacitor and the load

impedance. A 1

F capacitance would be a suitable choice for a line load. Tantalum electrolytic

capacitors are again particularly suitable but ceramic equivalents are a cost effective alternative. Care

must be taken to ensure the desired capacitance is maintained at the appropriate operating voltage.

WM8944B

LINEOUT

GND

1 uF

GND = 0V

1 uF

SPKOUTP

SPKOUTN

Figure 54 DC-blocking Components for Line Output

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