SSM2135
Rev. G | Page 10 of 16
APPLICATIONS INFORMATION
The SSM2135 is a low voltage audio amplifier that has exception-
ally low noise and excellent sonic quality even when driving loads
as small as 25 Ω. Designed for single supply use, the inputs and
output can both swing very close to 0 V. Thus with a supply
voltage at 5 V, both the input and output swing from 0 V to 4 V.
Because of this, signal dynamic range can be optimized if the
amplifier is biased to a 2 V reference rather than at half the
supply voltage.
The SSM2135 is unity-gain stable, even when driving into a fair
amount of capacitive load. Driving up to 500 pF does not cause
any instability in the amplifier. However, overshoot in the
frequency response increases slightly.
The SSM2135 makes an excellent output amplifier for 5 V only
audio systems such as a multimedia workstation, a CD output
amplifier, or an audio mixing system. The amplifier has large
output swing even at this supply voltage because it is designed
to swing to the negative rail. In addition, it easily drives load
impedances as low as 25 Ω with low distortion.
The SSM2135 is fully protected from phase reversal for inputs
going to the negative supply rail. However, internal ESD protec-
tion diodes turn on when either input is forced more than 0.5 V
below the negative rail. Under this condition, input current in
excess of 2 mA may cause erratic output behavior, in which case,
a current limiting resistor should be included in the offending
input if phase integrity is required with excessive input voltages.
A 500 Ω or higher series input resistor prevents phase inversion
even with the input pulled 1 V below the negative supply.
Hot plugging the input to a signal generally does not present a
problem for the SSM2135, assuming that the signal does not
have any voltage exceeding the supply voltage of the device.
If so, it is advisable to add a series input resistor to limit the
current, as well as a Zener diode to clamp the input to a voltage
no higher than the supply.
APPLICATION CIRCUITS
Low Noise Stereo Headphone Driver Amplifier
Figure 31 shows the SSM2135 used in a stereo headphone driver
for multimedia applications with the AD1845, a 16-bit stereo
codec. The SSM2135 is equally well suited for the serial-bused
AD1849 stereo codec. The impedance of the headphone can be
as low as 25 Ω, which covers most commercially available high
fidelity headphones. Although the amplifier can operate at up to
±18 V supply, it is just as efficient powered by a single 5 V. At
this voltage, the amplifier has sufficient output drive to deliver
distortion-free sound to a low impedance headphone.
L_OUT
V
CC
GNDA
V
REF
AD1845
R_OUT
40
35/36
32
41
10kΩ 8.66kΩ
10kΩ 8.66kΩ
470µF
5V
0.1µF
10µF
LEFT
CHANNEL
RIGHT
CHANNEL
0.1µF
34/37
1
2
3
5
6
7
470µF
0.1µF
10µF
1/2
SSM2135
1/2
SSM2135
4
8
AGND
00349-031
V+
Figure 31. A Stereo Headphone Driver for Multimedia Sound Codec
Figure 32 shows the total harmonic distortion characteristics vs.
frequency driving into a 32 Ω load, which is a very typical
impedance for a high quality stereo headphone. The SSM2135
has excellent power supply rejection, and, as a result, is tolerant
of poorly regulated supplies. However, for best sonic quality, the
power supply should be well regulated and heavily bypassed to
minimize supply modulation under heavy loads. A minimum of
10 μF bypass is recommended.
1
0.005
0.001
0.01
0.1
10 20k10k1k100
THD + N (%)
FREQUENCY (Hz)
V
S
= 5V
80kHz LOW-PASS FILTER
00349-032
Figure 32. Headphone Driver THD + N vs. Frequency into a 32 Ω Load
