AD73322L_17 datasheet(35/49 Pages) AD

Part #	AD73322L

Description	Low Cost, Low Power CMOS General-Purpose Dual Analog Front

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AD73322L

Rev. A | Page 34 of 48

INTERFACING TO AN ELECTRET MICROPHONE

Figure 39 details an interface for an electret microphone which

may be used in some voice applications. Electret microphones

typically feature a FET amplifier whose output is accessed on

the same lead which supplies power to the microphone;

therefore, this output signal must be capacitively coupled to

remove the power supply (dc) component. In this circuit, the

AD73322L input channel is being used in single-ended mode

where the internal inverting amplifier provides suitable gain to

scale the input signal relative to the ADC’s full-scale input

range. The buffered internal reference level at REFOUT is used

via an external buffer to provide power to the electret

microphone. This provides a quiet, stable supply for the

microphone. If this is not a concern, then the microphone can

be powered from the system power supply.

CONTINUOUS

TIME

LOW-PASS

FILTER

VREF

VFBN1

VINN1

VINP1

VFBP1

VOUTP1

VOUTN1

AD73322L

+6/–15dB

PGA

0/38dB

PGA

GAIN

±1

REFOUT

REFCAP

REFERENCE

VREF

R1

RB

RA

50k

Ω

CREFCAP

100pF

C2

ELECTRICITY

PROBE

10

µF

5V

Figure 39. Electret Microphone Interface Circuit

ANALOG OUTPUT

The AD73322L’s differential analog output (VOUT) is produced

by an on-chip differential amplifier. The differential output can

be ac-coupled or dc-coupled directly to a load which can be a

headset or the input of an external amplifier (the specified

minimum resistive load on the output section is 150 Ω.) It is

possible to connect the outputs in either a differential or a

single-ended configuration, but please note that the effective

maximum output voltage swing (peak to peak) is halved in the

case of single-ended connection. Figure 40 shows a simple

circuit providing a differential output with ac coupling. The

capacitors in this circuit (COUT) are optional; if used, their value

can be chosen as follows:

LOAD

OUT

R

fc

C

π

=

2

1

where fC = desired cutoff frequency.

CONTINUOUS

TIME

LOW-PASS

FILTER

VREF

VFBN1

VINN1

VINP1

VFBP1

VOUTP1

VOUTN1

AD73322L

+6/–15dB

PGA

GAIN

±1

REFOUT

REFCAP

REFERENCE

CREFCAP

RLOAD

COUT

Figure 40. Example Circuit for Differential Output

Figure 41 shows an example circuit for providing a single-ended

output with ac coupling. The capacitor of this circuit (COUT) is

not optional if dc current drain is to be avoided.

CONTINUOUS

TIME

LOW-PASS

FILTER

VREF

VFBN1

VINN1

VINP1

VFBP1

VOUTP1

VOUTN1

AD73322L

+6/–15dB

PGA

GAIN

±1

REFOUT

REFCAP

REFERENCE

0.1

µF

RLOAD

COUT

Figure 41. Example Circuit for Single-Ended Output

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