SSM2275 데이터 시트보기 (PDF) - Analog Devices
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SSM2275 Datasheet PDF : 16 Pages
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SSM2275/SSM2475
Multimedia Soundcard Microphone Preamplifier
The low distortion and low noise figures of the SSM2275 make
it an excellent device for amplifying low level audio signals. Fig-
ure 38 shows how the SSM2275 can be configured as a stereo
microphone preamplifier driving the input to a multimedia
sound codec, the AD1848. The SSM2275 can be powered from
the same +5 V single supply as the AD1848. The VREF pin on
the AD1848 provides a bias voltage of 2.25 V for the SSM2275.
This voltage can also be used to provide phantom power to a
condenser microphone through a 2N4124 transistor buffer and
2 kΩ resistors. The phantom power circuitry can be omitted for
dynamic microphones. The gain of SSM2275 amplifiers is set
by R2/R1 which is 100 (40 dB) as shown. Figure 39 shows the
device’s THD+N performance with a 1 VRMS output.
R2
10k⍀
L CHANNEL
MIC IN
10F
2k⍀ 10k⍀
+5V
2N4124
R CHANNEL
MIC IN
2k⍀ 10k⍀
10F
+5V 10F
R1
100⍀ 2
8
1
3
1/2
4 SSM2275 +5V
0.1F
29
LMIC
35/36
VCC
34/37
GND
5
6
R1
100⍀
10F
0.1F
7
1/2
SSM2275
32
VREF
AD1848
28
RMIC
R2
10k⍀
Figure 38. Low Noise Microphone Preamplifier for
Multimedia Soundcard Codec
1
AV = +40dB
VSY = ±2.5V
VIN = –40dBV
RL > 10k⍀
BW = 22kHz
0.1
The AD1862 has a built in 3 kΩ resistor that is connected from
the inverting input to the output of the amplifier. The full-scale
output current of the AD1862 is ± 1 mA, resulting in a maximum
output voltage of ± 3 V. Additional feedback resistance can be
added in the feedback loop to increase the output voltage. With
RFB connected the maximum output voltage will be:
( ) VOUT ,MAX = 1mA × 3 kΩ + RFB
(10)
16
VCC
AD1862
12
ACOM
11
IOUT
10
RF
+12V
TO LPF
SSM2275-A
100pF
RFB
(OPTIONAL)
NOTE: ADDITIONAL PIN CONNECTIONS OMITTED FOR CLARITY
Figure 40. A High Performance I-V Converter for a 20-Bit DAC
In Figure 41, the SSM2275 is used as a low-pass filter for one
channel of the AD1855, a 24-bit 96 kHz stereo sigma-delta
DAC, which uses a complementary voltage output. The filter is
configured as a second order low-pass Bessel filter with a cutoff
frequency of 50 kHz. This provides a phase linear response from
dc to 24 kHz, which is ideal for high quality audio applications.
The SSM2275 can be connected to the same +5 V power sup-
ply source, that the AD1855 is connected to, eliminating the
need for extra power circuitry. The FILT output (Pin 14) from
the AD1855 provides a common reference voltage equal to half
of the supply voltage for the SSM2275.
Amplifier A1 is used as a unity-gain inverter for the positive out-
put of the AD1855. The output of A1 is combined with a nega-
tive output of the AD1855 into the active low pass filter around
A2. The output impedance of each output of the AD1855 is
100 Ω which must be taken into account to achieve proper dc
gain, which in Figure 41 is unity gain. In this configuration the
SSM2275 can drive reasonable capacitive loads, making the de-
vice suitable for the RCA jack line outputs found in most con-
sumer audio equipment.
0.01
0.001
20
100
1k
10k 20k
FREQUENCY – Hz
Figure 39. THD+N vs. Frequency (VSY = +5 V, AV = 40 dB,
VOUT = 1 V rms)
High Performance I-V Converters and Filters for 20-Bit DACs
Because of the increasing resolution and lower harmonic distor-
tions required by more audio applications, the need for high
quality amplifiers at the output of D/A converters becomes criti-
cal. The SSM2275 and SSM2475 can be used as current-to-
voltage converters and smoothing filters for 18- and 20-bit
DACs, achieving 0.0006% THD+N figures while running from
the same +5 V or +12 V source used to power the D/A con-
verter. Figure 40 shows how the SSM2275 can be used with the
AD1862, a current output 20-bit DAC.
+5V
28
VDD
18
VDD
13 OR 18
OUT–
AD1855
12 OR 17
OUT+
1.05k⍀
1.05k⍀
1.15k⍀
A1
562⍀
4.7nF
237⍀
A2
1.15k⍀
10F
OUT
14
FILT
1
GND
15
GND
+5V
0.1F 10F
A1 AND A2 ARE SSM2275
OR 1/2 SSM2475
NOTE: ADDITIONAL PIN CONNECTIONS
OMITTED FOR CLARITY
Figure 41. Low-Pass Filter for a 24-Bit Stereo Sigma-
Delta DAC
REV. A
–13–
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