SSM2135
APPLICATION CIRCUITS
A Low Noise Microphone Preamplifier
A Low Noise Stereo Headphone Driver Amplifier
Figure 29 shows the SSM2135 used in a stereo headphone
driver for multimedia applications with the AD1848, a 16-bit
stereo codec. The SSM2135 is equally well suited for the serial-
bused AD1849 stereo codec. The headphone’s impedance can
be as low as 25
Ω,
which covers most commercially available high
fidelity headphones. Although the amplifier can operate at up to
±
18 V supply, it is just as efficient powered by a single +5 V. At
this voltage, the amplifier has sufficient output drive to deliver
distortion-free sound to a low impedance headphone.
L
OUT
V
CC
GND
V
REF
40
35/36
34/37
10kΩ
2
1
3
8.66kΩ
470µF
The SSM2135’s 4.7 nV/√Hz input noise in conjunction with
low distortion makes it an ideal device for amplifying low level
signals such as those produced by microphones. Figure 31 illus-
trates a stereo microphone input circuit feeding a multimedia
sound codec. As shown, the gain is set at 100 (40 dB), although
it can be set to other gains depending on the microphone output
levels. Figure 32 shows the preamplifier’s harmonic distortion
performance with 1 V rms output while operating from a single
+5 V supply.
The SSM2135 is biased to 2.25 V by the V
REF
pin of the
AD1848 codec. The same voltage is buffered by the 2N4124
transistor to provide “phantom power” to the microphone. A
typical electret condenser microphone with an impedance range
of 100
Ω
to 1 kΩ works well with the circuit. This power booster
circuit may be omitted for dynamic microphone elements.
10kΩ
+5V
0.1µF
1/2
SSM2135
10µF
L CH
32
0.1µF
10µF
5
8
R CH
0.1µF
7
AGND
470µF
+5V
L CHANNEL
MIC IN
100Ω
10µF
3
2kΩ
+5V
2N4124
10kΩ
2
8
AD1848
10µF
1
29
+5V
0.1µF
35/36
34/37
32
6
1/2
4
SSM2135
R
OUT
41
10kΩ
8.66kΩ
1/2
4
SSM2135
LMIC
V
CC
GND
V
REF
Figure 29. A Stereo Headphone Driver for Multimedia
Sound Codec
R CHANNEL
MIC IN
10µF
2kΩ
10kΩ
5
7
10µF
100Ω
6
0.1µF
AD1848
28
RMIC
Figure 30 shows the total harmonic distortion characteristics
versus frequency driving into a 32
Ω
load, which is a very typical
impedance for a high quality stereo headphone. The SSM2135
has excellent power supply rejection, and as a result, is tolerant
of poorly regulated supplies. However, for best sonic quality, the
power supply should be well regulated and heavily bypassed to
minimize supply modulation under heavy loads. A minimum of
10
µF
bypass is recommended.
1/2
SSM2135
10kΩ
Figure 31. Low Noise Microphone Preamp for Multimedia
Sound Codec
Figure 30. Headphone Driver THD+N vs. Frequency into a
32
Ω
Load (V
S
= +5 V, with 80 kHz Low-Pass Filter)
Figure 32. MIC Preamp THD+N Performance (V
S
= +5 V,
A
V
= 40 dB, V
OUT
= 1 V rms, with 80 kHz Low-Pass Filter)
–8–
REV. D
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