FEDL6722-05
OKI Semiconductor
Analog Input Amplifier Circuit
The MSM6722 has two built-in operational amplifiers for amplifying the microphone output.
Each output amplifier is provided with an inverting input pin and output pin. The analog circuit
reference voltage SG (signal ground) is connected internally to the non-inverting input of each
output amplifier. For amplification, form an inverting amplifier circuit and adjust the amplifi-
cation ratio by using external resistors, as shown below.
MSM6722
V
IN
–
+
V
MO
R1
MIN
–
+
V
LO
R3
R4
LOUT
–
+
R2
MOUT
V
LO
V
DD
V
DD
–1
1/2 V
DD
1
GND
LIN
OP amplifier 1
OP amplifier 2
SG
R4
R3
R2 • R4
R1 • R3
V
LO
=
V
MO
=
V
IN
(V)
The output V
LO
of output amplifier 2 is connected to the input FIN of the built-in LPF. The FIN
allowable input voltage (V
FIN
) ranges from 1 V to (V
DD
–1) V. Therefore, the amplification ratio
must be adjusted so that the V
LO
amplitude can be within the FIN allowable input voltage range.
For example, if V
DD
= 5 V, V
LO
becomes 3 V
p-p
max. If V
LO
exceeds the FIN allowable input
voltage range, the output of the LPF will be a clipped waveform. The load resistance R
OUTA
of
the OP amplifier is 200 kW or more. Therefore, the feedback resistors R2 and R4 of the inverting
amplifier circuit must be 200 kW or more.
11/21
OKI [ OKI ELECTRONIC COMPONETS ]
