ADAU1977WBCPZ-R7 datasheet(21/68 Pages) AD

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Data Sheet

ADAU1977

Rev. C | Page 21 of 68

Microphone Inputs

This section describes some ways to connect the ADAU1977 for

microphone input applications. The MICBIAS voltage and the

bias resistor value depend on the ECM selected. The ADAU1977

can provide the MICBIAS from 5 V up to 9 V in 0.5 V steps. In

an application requiring multiple microphones, care must be

taken not to exceed the MICBIAS output current rating.

ECM Balanced or Differential Input DC-Coupled Case

For example, in a typical ECM, the output signal swing depends

on the MICBIAS voltage. With a typical 8.5 V supply, the ECM can

output a 2 V rms differential signal. The signal at each input pin

has a 1 V rms or 2.8 V p-p signal swing. With a common-mode dc

level of 2/3 × MICBIAS on the AINxP and 1/3 × MICBIAS on

the AINxN pins, this results in around −14 dBFS (ac only with

dc high-pass filter) at the ADC output because the input is 14 dB

below the full-scale input of 10 V rms differential. See Figure 21.

ECM Pseudo Differential Input AC-Coupled Case

For a typical MEMS ECM module, the output signal swing is

low. With a typical 3.3 V supply, the ECM module can output a

2 V rms differential signal. The signal at the input pin has a 1 V rms

or 2.8 V p-p signal swing. For this application, it is recommended

to bias the input pins using resistors to 7 V dc, similar to the

case described in the Line Input Unbalanced or Single-Ended

Pseudo Differential AC-Coupled Case section. See Figure 22.

Figure 21. Connecting the Microphone Inputs—Differential Input DC-Coupled Case

Figure 22. Connecting the Microphone Inputs—Pseudo Differential Input AC-Coupled Case

ATTENUATOR

14dB

AINx+

ADAU1977

AINx–

MICBIAS

VIN = 2V rms AC DIFFERENTIAL

VCM+ ≈ 2/3 × MICBIAS

VCM– ≈ 1/3 × MICBIAS

R = TYPICAL

300Ω TO 500Ω

MICROPHONE

TYPICAL

ECM MODULE

NOTES

1. THE DIAGNOSTICS FEATURE IS AVAILABLE.

ATTENUATOR

14dB

AINx+

ADAU1977

AINx–

MICBIAS

VMAX = 5V rms AC

VDD

TYPICAL ECM

WITH PREAMP

MODULE

NOTES

1. THE DIAGNOSTICS FEATURE IS NOT AVAILABLE.

ADAU1977WBCPZ-R7 Datasheet(PDF) 21 Page - Analog Devices

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