FEDL2302DIGEST-05
OKI Semiconductor
ML2302
ANALOG INPUT AMPLIFIER CIRCUIT
The ML2302 contains two OP amplifiers to amplify a voice signal from a microphone. Each OP amplifier is
provided with the inverting input pin and output pin.
The analog circuit reference voltage SG (signal ground) is input internally to the non-inverting input of each
amplifier.
For amplification, form an inverting amplifier circuit and adjust the amplification ratio by using external resistors.
C1
VDD
VLO
0.01 µF
0.01 µF
MOUT
R3
R4
LIN
VLOUT (Max)
1/2VDD
VLO
MIN
LOUT
VLOUT (Min)
GND
OP amplifier 1
OP amplifier 2
SG
Figure 6.1
The constants of R4 and C1 are determined by f = 1/2πR·C. C1 is 39.89 pF ≅ 39 pF when R4 is 200 kΩ, where the
cut-off frequency of LPF is f = 20 kHz.
During recording, the output VLO of the OP amplifier is input to LPF.
Adjust the amplification ratio by an external resistor so that the output voltage VLOUT may be in the
LOUT-permissible input voltage range.
If VLOUT is not in this range, the waveform of the LPF output may be deformed.
Table 6.1 shows an examples of LOUT-permissible input voltage ranges of the ML2302.
Table 6.1
LOUT-permissible voltage range VLOUT
Model name
ML2302
Supply voltage VDD
3 V
LOUT-permissible range
1.5 Vpp
MIN
MAX
0.75 V
2.25 V
The load resistance ROUTA of the OP amplifier is 50 kΩ. Therefore, the feedback resistors R4 and R3 of the
amplifying circuit must be 50 kΩ or higher.
16/24
OKI [ OKI ELECTRONIC COMPONETS ]
