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AS3421 AS3422

Datasheet - D e t a i l e d D e s c r i p t i o n

9.2 Microphone Input

The AFE offers two microphone inputs and one low noise microphone voltage supply (microphone bias). The inputs can be switched to single

ended or differential mode.

Figure 18. Microphone Input

9.2.1

Gain Stage & Limiter

The Mic Gain Stage has programmable Gain within -11.25dB…+36dB in 128 steps of 0.375dB.

As soft-start function is implemented for an automatic gain ramping implemented with steps of 4ms to fade in the audio at the end of the start-up

sequence.

A limiter automatically attenuates high input signals. The AGC has 127 steps with 0.375dB with a dynamic range of the full gain stage. In some

design it is necessary to switch of the AGC functionality. This can be done in register 0x17 by setting bit ‘1’.

In monitor mode, the gain stage can be set to an fixed (normally higher) gain level or be controlled by the VOL pin.

9.2.2

Microphone Supply

The MICS charge pump is providing a proper microphone supply voltage for low supply voltage supply. The integrated microphone supply

supports basically 3 different modes.

The first mode, also called SWITCH-MODE, for 1.8V supply is to have a direct connection with an integrated switch from VBAT to MICS. The

enough to operate a microphone properly. This mode is more commonly used in devices with a fixed power supply like in Bluetooth headsets.

The internal microphone charge pump is switched off in this mode. This can help also reducing the external filter capacitors for the microphone

supply. Please mind that the sensitivity of your microphone can be reduced in this mode.

Gain Ramp Rate

Button Mode, Tinit=400ms

80

ms/step

Monitor Mode

8

Limiter Activation Level

HPL/R start of neg. clipping

Limiter Release Level

HPL/R

VNEG

+0.3

Limiter Attack Time

4

s

Limiter Decay Time

8

ms

Table 4. Line Input Parameter (Continued)

Symbol

Parameter

Condition

Min

Typ

Max

Unit

AGND

MICL

stereo mode

MICR

QMICR

QMICL

mono differential mode

AGND

MICL

MICR

QMICR

QMICL

AGND