ad73322arz-reel7 Analog Devices, Inc., ad73322arz-reel7 Datasheet - Page 32

• Manufacturer Part Number: ad73322arz-reel7
• Description: Low Cost, Low Power Cmos General-purpose Dual Analog Front End
• Manufacturer: Analog Devices, Inc.

AD73322
Interfacing to an Electret Microphone
Figure 34 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, there-
fore this output signal must be capacitively coupled to remove
the power supply (dc) component. In this circuit the AD73322
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.
Analog Output
The AD73322’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 mini-
mum resistive load on the output section is 150 .) 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 35 shows a simple circuit providing a
differential output with ac coupling. The capacitors in this cir-
cuit (C
follows:
where f
10 F
+5V
R
Figure 34. Electret Microphone Interface Circuit
R
A
OUT
B
C
= desired cutoff frequency.
C2
) are optional; if used, their value can be chosen as
ELECTRET
PROBE
R1
VOUTN1
REFOUT
VOUTP1
REFCAP
C
REFCAP
C1
R2
C
VFBN1
VINN1
VINP1
VFBP1
V
OUT
REF
2
+6/–15dB
GAIN
PGA
f R
C
1
1
LOAD
REFERENCE
CONTINUOUS
LOW-PASS
FILTER
TIME
AD73322
V
REF
0/38dB
PGA
–32–
Figure 36 shows an example circuit for providing a single-ended
output with ac coupling. The capacitor of this circuit (C
not optional if dc current drain is to be avoided.
Differential to Single-Ended Output
In some applications it may be desireable to convert the full
differential output of the decoder channel to a single-ended
signal. The circuit of Figure 37 shows a scheme for doing this.
Figure 37. Example Circuit for Differential to Single-
Ended Output Conversion
R
LOAD
Figure 36. Example Circuit for Single-Ended Output
Figure 35. Example Circuit for Differential Output
R
LOAD
R
R
LOAD
F
R
C
C
C
C
REFCAP
F
OUT
OUT
OUT
REFCAP
VOUTP1
VOUTN1
REFOUT
R
R
REFCAP
VOUTP1
VOUTN1
REFOUT
0.1 F
I
I
VFBP1
VFBN1
VINP1
VINN1
VFBP1
VFBN1
VINP1
VINN1
REFOUT
VOUTP1
VOUTN1
REFCAP
VFBN1
VFBP1
0.1 F
VINP1
VINN1
V
V
REF
REF
V
REF
+6/–15dB
+6/–15dB
PGA
PGA
+6/–15dB
REFERENCE
PGA
REFERENCE
GAIN
GAIN
REFERENCE
1
1
CONTINUOUS
CONTINUOUS
GAIN
LOW-PASS
LOW-PASS
FILTER
FILTER
1
TIME
TIME
CONTINUOUS
LOW-PASS
FILTER
TIME
AD73322
AD73322
AD73322
V
REF
OUT
REV. B
0/38dB
PGA
) is