SSM2135

Rev. G | Page 13 of 16

Digital Volume Control Circuit

Logarithmic Volume Control Circuit

Working in conjunction with the AD7528 dual 8-bit DAC,

the SSM2135 makes an efficient audio attenuator, as shown in

Figure 38. 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 internal R-2R ladder switches of the DACs operating prop-

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

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

shown in Figure 38, 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.

Figure 39 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 attenuation levels

for each input code.

DATA IN

AND

CONTROL

LEFT AUDIO

IN

RIGHT AUDIO

IN

DGND

5V

0.1µF

AGND

DGND

AD7111

3

5V

0.1µF

3

14

14

16

16

10

10

10

47µF

47µF

AGND

D1

D1

AD7111

5V

1/2

SSM2135

1/2

SSM2135

47µF

2kΩ

5V

2V

7.5kΩ

5kΩ

100Ω

0.1µF

1µF

2V

1/2

SSM2135

5V

47µF

1

2

1

2

RIGHT AUDIO

OUT

LEFT AUDIO

OUT

10µF + 0.1µF

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 10µF + 0.1µF

6

DAC A/

DAC B

1/2

SSM2135

DAC A

OUT A

DACB

OUT B

DATA IN

15

CS

16

WR

18

CONTROL

SIGNAL

LEFT

AUDIO IN

RIGHT

AUDIO IN

DGND

17

5

5V

0.1µF

AD7528

3

2

19

20

1

RIGHT AUDIO

OUT

LEFT AUDIO

OUT

47µF

47µF

2kΩ

5V

2V

7.5kΩ

5kΩ

100Ω

0.1µF

1µF

2V

Figure 39. Single-Supply Logarithmic Volume Control

Figure 38. Digital Volume Control