WM8994ECSSLASHR datasheet(350/363 Pages) CIRRUS

350 / 363 page

WM8994

350

Rev 4.6

IN1L_VOL[4:0],

IN2L_VOL[4:0],

IN1R_VOL[4:0],

IN2R_VOL[4:0]

VOLUME

(dB)

INPUT RESISTANCE

(

kΩ)

SINGLE-ENDED

MODE

DIFFERENTIAL

MODE

10101

+15.0

11.9

6.5

10110

+16.5

10.3

5.6

10111

+18.0

8.9

4.8

11000

+19.5

7.7

4.1

11001

+21.0

6.6

3.5

11010

+22.5

5.6

2.9

11011

+24.0

4.8

2.5

11100

+25.5

4.1

2.1

11101

+27.0

3.5

1.8

11110

+28.5

2.9

1.5

11111

+30.0

2.5

1.3

Table 145 PGA Input Pin Resistance

The appropriate input capacitor may be selected using the PGA input resistance data provided in

Table 145, depending on the required PGA gain setting(s).

The choice of capacitor for a 20Hz cut-off frequency is shown in Table 146 for a selection of typical

input impedance conditions.

INPUT IMPEDANCE

MINIMUM CAPACITANCE

FOR 20HZ PASS BAND



F

15k



0.5

F

30k



0.27

F

60k



0.13

F

Table 146 Audio Input DC Blocking Capacitors

Using the figures in Table 146, it follows that a 1

F 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 AVDD1 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.

HEADPHONE OUTPUT PATH

The headphone output on WM8994 is ground referenced and therefore does not require the large,

expensive capacitors necessary for VMID reference solutions. For best audio performance, it is

recommended to connect a zobel network to the audio output pins. This network should comprise of a

100nF capacitor and 20ohm resistor in series with each other

(see “Analogue Outputs” section).

These components have the effect of dampening high frequency oscillations or instabilities that can

arise outside the audio band under certain conditions. Possible sources of these instabilities include

the inductive load of a headphone coil or an active load in the form of an external line amplifier.

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