WM9090ECSSLASHR datasheet(75/81 Pages) WOLFSON

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Production Data

WM9090

PD, November 2010, Rev 4.1

AUDIO INPUT PATHS

The WM9090 provides 4 analogue audio inputs. 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 17.

Figure 17 Audio Input Path DC Blocking Capacitor

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

value may be derived easily. However, it can be seen from the representation in Figure 17 that the

input impedance is not fixed in all applications but can vary with gain and boost amplifier settings.

The choice of capacitor for a 20Hz cut-off frequency is shown in Table 35 for different input

impedance conditions. The applicable input impedance can be found in the “Electrical

Characteristics” section of this datasheet.

INPUT IMPEDANCE

MINIMUM CAPACITANCE

FOR 20HZ PASS BAND

μF

15k

0.5

μF

30k

0.27

μF

60k

0.13

μF

Table 35 Audio Input DC Blocking Capacitors

Using the figures in Table 35, it follows that a 1

μF capacitance for all input connections will give good

results in 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 AVDD 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 connection, a DC blocking capacitor is required on both input pins.

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