REV. E

SSM2135

–11–

1/2

SSM2135

Æ 12V

R3

2.5k

R4

100k

R2

5k

R1

5k

C1

0.1 F

2

OUTPUT

C2

1 F

Figure 8. Pseudo-Reference Generator

Digital Volume Control Circuit

Working in conjunction with the AD7528/PM7528 dual 8-bit

D/A converter, the SSM2135 makes an efficient audio attenuator,

as shown in Figure 9. The circuit works off a single 5 V supply.

The DACs are biased to a 2 V reference level, which is sufficient

to keep the DACs’ internal R-2R ladder switches operating prop-

erly. This voltage is also the optimal midpoint of the SSM2135’s

common-mode and output swing range. With the circuit as

shown, the maximum input and output swing is 1.25 V rms.

Total harmonic distortion measures a respectable 0.01% at 1 kHz

and 0.1% at 20 kHz. The frequency response at any attenuation

level is flat to 20 kHz.

Each DAC can be controlled independently via the 8-bit parallel

data bus. The attenuation level is linearly controlled by the

binary weighting of the digital data input. Total attenuation

ranges from 0 dB to 48 dB.

+5V

1/2

SSM2135

1/2

SSM2135

+5V 10 F+0.1 F

6

DACA/

DACB

1/2

SSM2135

REFA

DAC A

OUTA

FB

REFB

DACB

FB

OUTB

DATA IN

15

CS

16

WR

18

CONTROL

SIGNAL

R AUDIO

IN

L AUDIO

IN

DGND

17

5

+5V

0.1 F

AD7528/PM7528

3

2

19

20

1

R AUDIO

OUT

L AUDIO

OUT

47 F

47 F

2k

+5V

2.0V

7.5k

5k

100

0.1 F

1 F

2.0V

Figure 9. Digital Volume Control

Logarithmic Volume Control Circuit

Figure 10 shows a logarithmic version of the volume control

function. Similar biasing is used. With an 8-bit bus, the AD7111

provides an 88.5 dB attenuation range. Each bit resolves a

0.375 dB attenuation. Refer to the AD7111 data sheet for attenua-

tion levels for each input code.

OUTA

DATA IN

AND

CONTROL

R AUDIO

IN

L AUDIO

IN

DGND

+5V

0.1 F

AGND

OUTA

FB

DGND

AD7111

314

+5V

0.1 F

314

10

10

10

47 F

47 F

FB

AGND

AD7111

+5V

1/2

SSM2135

1/2

SSM2135

R AUDIO

OUT

47 F

2k

+5V

2.0V

7.5k

5k

100

0.1 F

1 F

2.0V

1/2

SSM2135

+5V 10 F+0.1 F

L AUDIO

OUT

47 F

1

2

1

2

Figure 10. Single-Supply Logarithmic Volume Control