SSM2275/SSM2475

REV. A

–13–

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 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 k

Ω 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

L CHANNEL

MIC IN

10 F

4

6

5

8

7

10 F

1/2

SSM2275

10 F

+5V

1/2

SSM2275

0.1 F

1

2

3

R2

10k

R CHANNEL

MIC IN

R1

100

0.1 F

29

35/36

34/37

+5V

LMIC

GND

RMIC

AD1848

32

28

10k

10k

+5V

2N4124

10 F

2k

2k

R1

100

R2

10k

Figure 38. Low Noise Microphone Preamplifier for

Multimedia Soundcard Codec

FREQUENCY – Hz

1

0.1

0.001

20

20k

100

1k

0.01

10k

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.

The AD1862 has a built in 3 k

Ω 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

VmA

k

R

OUT MAX

FB

,

=×

+

()

13

Ω

(10)

100pF

TO LPF

SSM2275-A

+12V

16

12

11

10

(OPTIONAL)

ACOM

AD1862

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.

A1

1.15k

10 F

A2

10 F

1.15k

1.05k

237

4.7nF

562

1.05k

0.1 F

OUT

13 OR 18

12 OR 17

14

1

15

28

18

+5V

+5V

OUT–

OUT+

FILT

GND

GND

AD1855

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