SSM2135
An 18-Bit Stereo CD-DAC Output Amplifier
A Single Supply Differential Line Receiver
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.
+5V SUPPLY
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 kΩ 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 kΩ precision resistor network.
10kΩ
+5V
10µF+0.1µF
1
2
3
4
5
6
7
8
V
L
LL
DL
CK
DR
LR
18-BIT
DAC
VBL
1/2
SSM2135
16
15
3
7.68kΩ
9.76kΩ
2
330pF
7.68kΩ
100pF
4
8
1
47kΩ
220µF
LEFT
CHANNEL
OUTPUT
20kΩ
2
3
8
1
1/2
4
SSM2135
18-BIT
SERIAL
REG.
V
REF
VOL
14
13
DIFFERENTIAL
AUDIO IN
20kΩ
10kΩ
20kΩ
10µF
AUDIO
OUT
6
7
5
2.0V
10Ω
AD1868
18-BIT
SERIAL
REG.
V
REF
AGND
12
11
1/2
SSM2135
1
+5V
8
4
3
2
VOR
DGND
VBR
18-BIT
DAC
V
S
7.68kΩ
10
7.68kΩ
9
330pF
5
9.76kΩ
6
7
100pF
220µF
RIGHT
CHANNEL
OUTPUT
1µF
7.5kΩ
+5V
5kΩ
100Ω
1/2
SSM2135
47kΩ
0.1µF
2.5kΩ
1/2
SSM2135
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.
1kΩ
+5V
10µF+0.1µF
2
100µF
AUDIO
IN
1kΩ
6
10kΩ
2.0V
7
5
3
4
8
1
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.
V
S
+ = +5V
→
+12V
R3
2.5kΩ
C1
0.1µF
R1
5kΩ
2
8
1/2
SSM2135
1kΩ
DIFFERENTIAL
AUDIO OUT
1/2
SSM2135
2.5kΩ
0.1µF
100Ω
1
3
4
5kΩ
R2
5kΩ
+5V
8
2
+5V
7.5kΩ
1/2
SSM2135
3
4
R4
100Ω
1
V
S
+
C2
1µF
2
OUTPUT
1µF
1/2
SSM2135
Figure 34. Single Supply Differential Line Driver
Figure 36. Pseudo-Reference Generator
REV. D
–9–
AD [ ANALOG DEVICES ]
