SSM2305

Data Sheet

Rev. B | Page 12 of 16

GAIN

The SSM2305 has a default gain of 18 dB that can be reduced by

using a pair of external resistors with a value calculated as follows:

POP-AND-CLICK SUPPRESSION

Voltage transients at the output of audio amplifiers can occur when

shutdown activates or deactivates. Voltage transients as low as

10 mV can be heard as audio pops in the speaker. Clicks and

pops can also be classified as undesirable audible transients gener-

ated by the amplifier system and, therefore, as not coming from

the system input signal. Such transients can be generated when

the amplifier system changes its operating mode. For example, the

following can be sources of audible transients: system power-up/

power-down, mute/unmute, input source change, and sample rate

change. The SSM2305 has a pop-and-click suppression

architecture that reduces these output transients, resulting in

noiseless activation and deactivation.

OUTPUT MODULATION DESCRIPTION

The SSM2305 uses three-level, Σ-Δ output modulation. Each

output is able to swing from GND to VDD, and vice versa. Ideally,

when no input signal is present, the output differential voltage is

0 V because there is no need to generate a pulse. In a real-world

situation, there are always noise sources present. Due to this

constant presence of noise, a differential pulse generates when it

is required in response to this stimulus. A small amount of current

flows into the inductive load when the differential pulse is

generated. However, most of the time output differential voltage is

0 V due to the Analog Devices patented three-level, Σ-Δ output

modulation. This feature ensures that the current flowing through

the inductive load is small.

When the user wants to send an input signal, an output pulse is

generated to follow the input voltage. The differential pulse density

is increased by raising the input signal level. Figure 34 depicts

three-level, Σ-Δ output modulation with and without input stimuli.

OUTPUT > 0V

+5V

0V

OUT+

+5V

0V

OUT–

+5V

0V

VOUT

OUTPUT < 0V

+5V

0V

OUT+

+5V

0V

OUT–

0V

–5V

VOUT

OUTPUT = 0V

OUT+

+5V

0V

+5V

0V

OUT–

+5V

–5V

0V

VOUT

Figure 34. 3-Level, Σ-Δ Output Modulation with and Without Input Stimuli

LAYOUT

As output power continues to increase, care needs to be taken

to lay out PCB traces and wires properly between the amplifier,

load, and power supply. A good practice is to use short, wide

PCB tracks to decrease voltage drops and minimize inductance.

Ensure that track widths are at least 200 mil for every inch of

track length for lowest dc resistance (DCR), and use 1 oz or 2 oz

of copper PCB traces to further reduce IR drops and inductance.

A poor layout increases voltage drops, consequently affecting

efficiency. Use large traces for the power supply inputs and

amplifier outputs to minimize losses due to parasitic trace

resistance.

Proper grounding guidelines help improve audio performance,

minimize crosstalk between channels, and prevent switching noise

from coupling into the audio signal. To maintain high output swing

and high peak output power, the PCB traces that connect the

output pins to the load and supply pins should be as wide as

possible to maintain the minimum trace resistances. It is also

recommended that a large ground plane be used for minimum

impedances.

In addition, good PCB layouts isolate critical analog paths from

sources of high interference. Separate high frequency circuits

(analog and digital) from low frequency circuits.

Properly designed multilayer PCBs can reduce EMI emission

and increase immunity to the RF field by a factor of 10 or more

compared with double-sided boards. A multilayer board allows

a complete layer to be used for the ground plane, whereas the

ground plane side of a double-sided board is often disrupted with

signal crossover.

If the system has separate analog and digital ground and power

planes, place the analog ground plane underneath the analog

power plane, and, similarly, place the digital ground plane

underneath the digital power plane. There should be no overlap

between analog and digital ground planes or analog and digital

power planes.

INPUT CAPACITOR SELECTION

The SSM2305 does not require input coupling capacitors if the

are required if the input signal is not biased within this recom-

mended input dc common-mode voltage range, if high-pass

filtering is needed, or if using a single-ended source. If high-pass

filtering is needed at the input, the input capacitor, together with

the input resistor of the SSM2305, forms a high-pass filter

whose corner frequency is determined by the following

equation:

The input capacitor can significantly affect the performance of

the circuit. Not using input capacitors degrades both the output

offset of the amplifier and the dc PSRR performance.