REV. E

–10–

SSM2135

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 5 shows the implementation of an 18-bit

stereo 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 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

W with

no additional buffering required.

Single Supply Differential Line Driver

Signal distribution and routing is often required in audio systems,

particularly portable digital audio equipment for professional

applications. Figure 6 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

W line termination at a reduced output amplitude.

+5V

1/2

SSM2135

1/2

SSM2135

AUDIO IN

100 F

1k

DIFFERENTIAL

AUDIO OUT

+5V

1/2

SSM2135

1k

2.5k

100

+5V

7.5k

5k

1 F

0.1 F

2.0V

10k

1k

10 F+0.1 F

Figure 6. Single-Supply Differential Line Driver

47k

1/2

SSM2135

AD1868

18-BIT

DAC

18-BIT

SERIAL

REG.

DL

LL

DR

LR

18-BIT

SERIAL

REG.

18-BIT

DAC

CK

DGND

VBR

VBL

VOL

AGND

VOR

1/2

SSM2135

5V SUPPLY

7.68k

9.76k

7.68k

9.76k

7.68k

7.68k

330pF

330pF

100pF

100pF

220 F

220 F

LEFT

CHANNEL

OUTPUT

RIGHT

CHANNEL

OUTPUT

47k

Figure 5. 5 V Stereo 18-Bit DAC

Single-Supply Differential Line Receiver

Receiving a differential signal with minimum distortion is achieved

using the circuit in Figure 7. Unlike a difference amplifier (a

subtractor), the circuit has a true balanced input impedance

regardless of input drive levels. That is, each input always pre-

sents a 20 k

W 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

W precision resistor network.

1/2

SSM2135

AUDIO

OUT

1/2

SSM2135

10k

+5V 10 F+0.1 F

+5V

1/2

SSM2135

DIFFERENTIAL

AUDIO IN

100

5k

1 F

0.1 F

2.5k

+5V

7.5k

10

10 F

20k

2.0V

20k

20k

Figure 7. Single-Supply Balanced Differential

Line Receiver

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 8 provides a supply splitter function with low

output impedance. The 1

mF 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

mF 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 a

5 V supply, limited only by the 100

W output resistor. Reducing

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.