wm9090 Wolfson Microelectronics plc, wm9090 Datasheet - Page 72

• Manufacturer Part Number: wm9090
• Description: Ultra Low Power Audio Subsystem
• Manufacturer: Wolfson Microelectronics plc

WM9090
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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 18.
Figure 18 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 18 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.
Table 35 Audio Input DC Blocking Capacitors
Using the figures in Table 35, it follows that a 1μF 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.
INPUT IMPEDANCE
15kΩ
30kΩ
60kΩ
2kΩ
MINIMUM CAPACITANCE
FOR 20HZ PASS BAND
0.27 μF
0.13 μF
0.5 μF
4 μF
PP, January 2010, Rev 3.0
Pre-Production
72