SSM2135

REV. D

–10–

A Digital Volume Control Circuit

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

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

attenuator, as shown in Figure 37. The circuit works off a single

+5 V supply. The DAC’s are biased to a 2 V reference level

which is sufficient to keep the DAC’s internal R-2R ladder

switches operating properly. 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 fre-

quency 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.

1µF

47µF

8

47µF

2k

Ω

100

Ω

0.1µF

1/2

SSM2135

1

2

3

10µF+0.1µF

+5V

4

1/2

SSM2135

8

1

3

2

4

+5V

5k

Ω

L AUDIO

OUT

47µF

1/2

SSM2135

R AUDIO

OUT

6

5

7

2.0V

7.5k

Ω

+5V

2.0V

3

2

19

20

1

17

5

18

4

6

15

16

DACA/

DACB

CS

WR

REF B

DAC B

FB

OUTB

DGND

0.1µF

+5V

47µF

L AUDIO

IN

DATA IN

CONTROL

SIGNAL

R AUDIO

IN

AD/PM-7528

REF A

DAC A

FB

OUTA

Figure 37. Digital Volume Control

A Logarithmic Volume Control Circuit

Figure 38 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 AD7111 data sheet for

attenuation levels for each input code.

1µF

8

47µF

2k

Ω

100

Ω

0.1µF

1/2

SSM2135

1

2

3

4

1/2

SSM2135

8

1

3

2

4

+5V

5k

Ω

L AUDIO

OUT

47µF

1/2

SSM2135

R AUDIO

OUT

6

5

7

7.5k

Ω

+5V

2.0V

0.1µF

+5V

L AUDIO

IN

DATA IN &

CONTROL

R AUDIO

IN

10µF+0.1µF

+5V

1

2

15

314

16

47µF

DGND

AD7111

FB

OUTA

AGND

1

2

0.1µF

+5V

314

16

DGND

AD7111

FB

OUTA

AGND

15

47µF

10

10

10

Figure 38. Single Supply Logarithmic Volume Control