WM7121PE

Rev 4.1

7

APPLICATIONS INFORMATION

RECOMMENDED EXTERNAL COMPONENTS

OUTPUT

VDD

GND

1

F

-

+

C

R

WM7121P

Figure 1 WM7121P Recommended External Components

A DC-blocking output capacitor is required on the OUTPUT pin, as illustrated in Figure 1. A single

capacitor is required for a single-ended connection. The capacitor must be correctly selected as it

affects the cut-off frequency of the output path. A low cut-off frequency is desirable as it means there

is no significant filtering of the audio bandwidth.

The 3dB cut-off frequency of the output path is given by the equation below, where C is the output

capacitance and R is the input resistance of the other circuit.

3dB filter roll-off frequency =

1

2 RC

p

A typical recommended configuration, with 1uF DC-

blocking capacitor and 20kΩ minimum input circuit

impedance, gives a 3dB cut-off frequency of 10Hz or less. Tantalum electrolytic capacitors are

particularly suitable for the DC-blocking components as they offer high stability in a small package

size.

OPTIMISED SYSTEM RF DESIGN

For optimised RF design please refer to document WAN0278 (Recommended PCB Layout for

Microphone RF Immunity in Mobile Cell Phone Applications) for further information.