SSM2356 datasheet(14/16 Pages) AD

14 / 16 page

SSM2356

Data Sheet

Rev. A | Page 14 of 16

The measurements for Figure 36 and Figure 37 were taken in

an FCC-certified EMI laboratory with a 1 kHz input signal,

producing 0.5 W output power into an 8 Ω load from a 5 V

supply. Cable length was 12 cm, unshielded twisted pair

speaker cable. Note that reducing the supply voltage greatly

reduces radiated emissions.

OUTPUT MODULATION DESCRIPTION

The SSM2356 uses three-level, Σ-Δ output modulation. Each

output can 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 is

generated, when required, in response to this stimulus. A small

amount of current flows into the inductive load when the differ-

ential pulse is generated. However, most of the time, output

differential voltage is 0 V, due to the Analog Devices 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 input voltage. The differential pulse density

is increased by raising the input signal level. Figure 38 depicts

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

stimulus.

OUTPUT > 0V

+5V

OUT+

+5V

OUT–

+5V

VOUT

OUTPUT < 0V

+5V

OUT+

+5V

OUT–

–5V

VOUT

OUTPUT = 0V

OUT+

+5V

OUT–

+5V

–5V

VOUT

Figure 38. Three-Level, Σ-Δ Output Modulation With and

Without Input Stimulus

LAYOUT

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

lay out PCB traces and wires properly among 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 the lowest dc resistance (DCR), and use 1 oz. or

2 oz. 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 to 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

layout isolates critical analog paths from sources of high inter-

ference. High frequency circuits (analog and digital) should be

separated 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 by

signal crossover.

If the system has separate analog and digital ground and power

planes, the analog ground plane should be directly beneath the

analog power plane, and, similarly, the digital ground plane should

be directly beneath the digital power plane. There should be no

overlap between analog and digital ground planes or between

analog and digital power planes.

INPUT CAPACITOR SELECTION

The SSM2356 does not require input coupling capacitors if the

input signal is biased from 1.0 V to VDD − 1.0 V. Input capacitors

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 a single-ended source is used. If high-

pass filtering is needed at the input, the input capacitor and the

input resistor of the SSM2356 form a high-pass filter whose

corner frequency is determined by the following equation:

fC = 1/(2π × RIN × CIN)

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.

PROPER POWER SUPPLY DECOUPLING

To ensure high efficiency, low total harmonic distortion (THD),

and high PSRR, proper power supply decoupling is necessary.

Noise transients on the power supply lines are short-duration

voltage spikes. These spikes can contain frequency components

that extend into the hundreds of megahertz. The power supply

input must be decoupled with a good quality, low ESL, low ESR

capacitor, greater than 4.7 μF. This capacitor bypasses low freq-

uency noises to the ground plane. For high frequency transient

noises, use a 0.1 μF capacitor as close as possible to the VDD

pin of the device. Placing the decoupling capacitor as close as

possible to the SSM2356 helps to maintain efficient

performance.

SSM2356 Fiches technique(PDF) 14 Page - Analog Devices

SSM2356 Fiches technique(HTML) 14 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Fiches technique Télécharger

Lien URL