ad73411 Analog Devices, Inc., ad73411 Datasheet - Page 33

• Manufacturer Part Number: ad73411
• Description: Low-power Analog Front End With Dsp Microcomputer
• Manufacturer: Analog Devices, Inc.

Interfacing to an Electret Microphone
Figure 30 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 that supplies power to the microphone; therefore,
this output signal must be capacitively coupled to remove the
power supply (dc) component. In this circuit the AD73411
input channel is being used in single-ended mode where the
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 pro-
vides a quiet, stable supply for the microphone. If this is not a
concern, the microphone can be powered from the system
power supply.
MICROPHONE
ELECTRET
10k
10k
0.1 F
0.1 F
10 F
R
B
100
100
0.047
0.047
F
F
5V
R
C2
REFOUT
0.1 F
REFOUT
0.1 F
REFCAP
REFCAP
A
VOUTP
VOUTN
VOUTP
VOUTN
R1
VINN
VINN
VINP
VINP
R2
C1
REFOUT
REFCAP
VOUTP
VOUTN
C
REFCAP
VINP
VINN
+6/–15dB
+6/–15dB
PGA
PGA
REFERENCE
REFERENCE
+6/–15dB
CONTINUOUS
CONTINUOUS
PGA
LOW-PASS
LOW-PASS
FILTER
FILTER
TIME
TIME
REFERENCE
CONTINUOUS
LOW-PASS
FILTER
TIME
AD73411
AD73411
V
V
REF
REF
0/38dB
0/38dB
PGA
PGA
V
REF
0/38dB
PGA
Analog Output
The AD73411’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 that can be a
headset or the input of an external amplifier. 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 31 shows a simple circuit providing a differ-
ential output with ac coupling. The capacitors in this circuit
(C
where f
Figure 32 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.
OUT
R
) are optional; if used, their value can be chosen as follows:
LOAD
C
R
LOAD
= desired cutoff frequency.
C
C
REFCAP
REFCAP
C
C
C
OUT
OUT
OUT
REFOUT
REFOUT
REFCAP
REFCAP
VOUTP
VOUTN
VOUTP
VOUTN
VINN
VINP
VINN
VINP
C
OUT
=
2 π
+6/–15dB
+6/–15dB
PGA
PGA
f R
C
REFERENCE
REFERENCE
1
CONTINUOUS
CONTINUOUS
LOAD
LOW-PASS
LOW-PASS
FILTER
FILTER
TIME
TIME
AD73411
AD73411
AD73411
OUT
) is