SSM2135

REV. D

–9–

An 18-Bit Stereo CD-DAC Output Amplifier

The SSM2135 makes an ideal single supply stereo output

amplifier for audio D/A converters because of its low noise and

distortion. Figure 33 shows the implementation of an 18-bit ste-

reo DAC channel. The output amplifier also provides low-pass

filtering for smoothing the oversampled audio signal. The filter’s

cutoff frequency is set at 22.5 kHz and it has a maximally flat

response from dc to 20 kHz.

As mentioned above, the amplifier’s outputs can drive directly

into a stereo headphone that has impedance as low as 25

Ω with

no additional buffering required.

6

7

5

100pF

330pF

16

15

14

13

12

11

10

9

18-BIT

DAC

V

REF

18-BIT

SERIAL

REG.

VOL

AGND

18-BIT

SERIAL

REG.

18-BIT

DAC

V

REF

VOR

VBL

DGND

VBR

LR

DR

LL

DL

CK

V

L

V

S

1

2

3

4

5

6

7

8

AD1868

220

µF

47k

Ω

RIGHT

CHANNEL

OUTPUT

330pF

100pF

220

µF

LEFT

CHANNEL

OUTPUT

+5V SUPPLY

1

3

2

4

8

1/2

SSM2135

1/2

SSM2135

47k

Ω

7.68k

Ω

7.68k

Ω

7.68k

Ω

7.68k

Ω

9.76k

Ω

9.76k

Ω

Figure 33. +5 V Stereo 18-Bit DAC

A Single Supply Differential Line Driver

Signal distribution and routing is often required in audio

systems, particularly portable digital audio equipment for

professional applications. Figure 34 shows a single supply line

driver circuit that has differential output. The bottom amplifier

provides a 2 V dc bias for the differential amplifier in order to

maximize the output swing range. The amplifier can output a

maximum of 0.8 V rms signal with a +5 V supply. It is capable

of driving into 600

Ω line termination at a reduced output

amplitude.

1µF

6

5

8

7

1/2

SSM2135

0.1µF

100

Ω

1/2

SSM2135

1

2

3

10µF+0.1µF

+5V

4

1k

Ω

8

1

3

2

4

DIFFERENTIAL

AUDIO OUT

2.5k

Ω

7.5k

Ω

5k

Ω

+5V

10k

Ω

2.0V

1k

Ω

+5V

1/2

SSM2135

1k

Ω

100µF

AUDIO

IN

Figure 34. Single Supply Differential Line Driver

A Single Supply Differential Line Receiver

Receiving a differential signal with minimum distortion is

achieved using the circuit in Figure 35. Unlike a difference

amplifier (a subtractor), the circuit has a true balanced input

impedance regardless of input drive levels. That is, each input

always presents a 20 k

Ω impedance to the source. For best

common-mode rejection performance, all resistors around the

differential amplifier must be very well matched. Best results

can be achieved using a 10 k

Ω precision resistor network.

1µF

6

5

8

7

1/2

SSM2135

10µF

+5V

0.1µF

20k

Ω

1/2

SSM2135

1

2

3

10µF+0.1µF

+5V

4

2.0V

1/2

SSM2135

DIFFERENTIAL

AUDIO IN

8

1

3

2

4

+5V

2.5k

Ω

AUDIO

OUT

20k

Ω

20k

Ω

10k

Ω

10k

Ω

100

Ω

5k

Ω

7.5k

Ω

10

Ω

Figure 35. Single Supply Balanced Differential Line

Receiver

A Pseudo-Reference Voltage Generator

For single supply circuits, a reference voltage source is often

required for biasing purposes or signal offsetting purposes. The

circuit in Figure 36 provides a supply splitter function with low

output impedance. The 1

µF output capacitor serves as a charge

reservoir to handle a sudden surge in demand by the load as

well as providing a low ac impedance to it. The 0.1

µF feedback

capacitor compensates the amplifier in the presence of a heavy

capacitive load, maintaining stability.

The output can source or sink up to 12 mA of current with

+5 V supply, limited only by the 100

Ω output resistor. Reduc-

ing the resistance will increase the output current capability.

Alternatively, increasing the supply voltage to 12 V also

improves the output drive to more than 25 mA.

C1

0.1µF

R2

5k

Ω

C2

1µF

R4

100

Ω

OUTPUT

+

2

1/2

SSM2135

1

8

4

2

R3

2.5k

Ω

3

R1

5k

Ω

+

= +5V

→ +12V

Figure 36. Pseudo-Reference Generator