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

GNDA

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

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

20k

10k

1k

100

FREQUENCY (Hz)

80kHz LOW-PASS FILTER

Figure 32. Headphone Driver THD + N vs. Frequency into a 32 Ω Load