SSM2000

REV. 0

–11–

SSM2000

V+

V–

20

ACOM

6

5

0.1µF

1µF

0.1µF

1µF

Figure 28a. The Dual Power Rail Connections for the

SSM2000

Powering the SSM2000—Single Supply

To operate HUSH in the single supply mode, a false ground

potential should be generated and attached to ACOM, as

shown in Figure 28b. This false ground is usually set at the

midpoint between the power rails. The false ground that will be

created for the HUSH chip must be able to both sink and

source current up to 10 mA. A low cost OP292 will perform

this function very well. Once again, a 1

µF aluminum electro-

lytic capacitor and a 0.1

µF ceramic capacitor should be use to

decouple both the false ground potential and positive supply

from the negative supply. It may be helpful to read the preced-

ing section “Powering HUSH—Dual Supply” for information

on power supply issues that affect both dual and single supply

applications.

Mute (Pin 17) and Bypass (Pin 16) potentials should be refer-

enced to DGND. Other inputs are either capacity coupled or

referenced to ACOM.

SSM2000

V+

V–

19

ACOM

GND

20

6

5

1µF

MUTE

BYPASS

10k

Ω

10k

Ω

1

2

3

1/2

OP292

1µF

Figure 28b. Powering the SSM2000—Single Supply

DETAILED PIN DESCRIPTIONS

Left Audio IN and Right Audio IN (Pins 1, 2)

L IN and R IN are the inputs for the Left and Right channels.

These are each single ended inputs with an input impedance of

10 k

Ω. The input driver for SSM2000 should be able to handle

this load level without significant harmonic distortion. These

inputs should be ac coupled with at least the minimum capaci-

tor value shown in the example circuits, which is 6.8

µF. This

sets up a high pass filter with a corner frequency of less than 3 Hz.

Choosing a lower value capacitor than 6.8

µF may result in the

loss of low frequency audio signal. On the other hand choosing

a large value will improve low frequency channel separation.

Signal Chain Position

Figure 27 illustrates a common audio signal path. The SSM2000

is normally placed just before the volume control and power

amplifier. The reason the SSM2000 should be placed as far

down the signal chain as possible is that noise that is introduced

by the previous components will be operated on by the noise re-

duction system. On the other hand, the SSM2000 works best

at the 300 mV rms level and therefore the IC should be placed

before any significant volume control. To operate the SSM2000

in a post-volume control position, please refer to the Optional

Application Circuits–Using the SSM2000 in a post-volume con-

trol application section.

TUNER

CD

MIC...

CASSETTE

VOLUME +

POWER AMP

R

L

SSM2000

PREAMP,

MIXING,

EQ...

LP

Figure 27. The Position of the SSM2000 in a Common

Audio Signal Chain

Powering the SSM2000—Dual Supply

The SSM2000 has a supply voltage range from +7 V to +18 V.

A well regulated (< 20 mV of ripple)

±9 V supply is acceptable,

even though this is value is exactly equal to the absolute maxi-

mum rating of 18 V. The HUSH has been found to be opera-

tional down to a supply range of only 4.5 V. However, for

guaranteed performance the supplies must be within the speci-

fied supply ranges.

The power supplies of choice from a performance perspective

are quality linear supplies or batteries, because of their low

noise. If a switching supply is required, or the power supply

lines are noisy, then adequate filtering of the power supply lines

should be provided. It is common to place a power supply filter

at the power supply and another at the entry point of power to

the audio system, but only one filter is necessary if these two

locations are separated by less than a foot or so.

In normal dual supply applications ACOM should be connected

to the system ground. If ACOM is not attached to the system

ground, then please refer to the section on Powering HUSH—

single supply. V+ and V– should be connected to their respec-

tive power rails. V+ and V– will need to be properly decoupled,

because the IR drops in the power rails during large signal

swings in the HUSH chip can cause additional distortion. As

shown in Figure 28a, it is recommended that a aluminum elec-

trolytic bulk storage cap of 1

µF be placed in the vicinity of the

HUSH chip and a 0.1

µF ceramic capacitor be placed directly

at the HUSH chip. Both power rails should be decoupled this

manner.