SSM2135SZ Datasheet, PDF (11/16 Pages) Analog Devices – Dual Single-Supply Audio Operational Amplifier

Low Noise Microphone Preamplifier
The 5.2 nV/√Hz input noise in conjunction with low distortion
make the SSM2315 an ideal device for amplifying low level signals
such as those produced by microphones. Figure 34 illustrates a
stereo microphone input circuit feeding a multimedia sound
codec. The gain is set at 100 (40 dB), although it can be set to
other gains depending on the microphone output levels. Figure 33
shows the harmonic distortion performance of the preamplifier
with 1 V rms output, while operating from a single 5 V supply.
The SSM2135 is biased to 2.25 V by the VREF pin of the AD1845
codec. The same voltage is buffered by the 2N4124 transistor to
provide phantom power to the microphone. A typical electrets
condenser microphone with an impedance range of 100 Ω to
1 kΩ works well with the circuit. This power booster circuit can
be omitted for dynamic microphone elements.
1
V_S = 5V
A_V = 40dB
V_OUT = 1V rms
80kHz LOW-PASS FILTER
0.1
SSM2135
0.01
10
100
1k
FREQUENCY (Hz)
10k 20k
Figure 33. MIC Preamp THD + N Performance
LEFT CHANNEL
MIC IN
10µF
2kΩ 10kΩ
5V
2N4124
10kΩ
100Ω
5V 10µF
28
1
3
1/2
4 ^{SSM2135 5V}
0.1µF
RIGHT CHANNEL
10kΩ
MIC IN
2kΩ _10µF
10µF
0.1µF
5
7
6
1/2
SSM2135
29
L_MIC
35/36
V_CC
34/37
GNDA
32
V_REF
AD1845
28
R_MIC
100Ω
10kΩ
Figure 34. Low Noise Microphone Preamp for Multimedia Sound Codec
Rev. G | Page 11 of 16