SSM2160/SSM2161

–11–

REV. 0

present the attenuation and gain as two separate items, in fact,

the VCA can be operated smoothly from a gain condition to an

attenuation. The master and channel step sizes default to 1 dB

in the absence of external components. The step sizes can be

changed by the addition of external resistors if finer resolution is

desired.

Control Range vs. Step Size

Before adjusting step sizes from the standard 1 dB, consider the

effect on control range. The master control and the channel

control provide 1 dB step sizes, which may be modified by the

addition of external resistors. As the total number of steps is

unchanged, reduction of the step size results in less control

range. The range of the control is:

Range = Step Size (dB)

× (Number of Levels Used)

Since the master volume control operates from a 7-bit word, its

DAC has 128 levels (including 0). The channel volume control

DAC is a 5-bit input, so there are 32 levels for volume control

(including 0). As can be seen in Figure 21, the practical control

range is set by the noise floor. It can be advantageous to reduce

the master step size to give finer steps from zero attenuation

down to the noise floor.

Reducing Master Step Size

MSTR SET and MSTR OUT. The master step size of the

master volume control will then become:

the step size for the master control can only be adjusted to less

than 1 dB. No resistor is required for the default value of 1 dB

per step. For larger step sizes, use digital control. Noninteger

dB step sizes can be obtained by using digital control and a

reduced step size.

1.0

0.8

0

0.6

0.4

0.2

Example: Modifying Master Step Size to 0.5 dB

A master step size of 0.5 dB is desired for the master control,

while a 1 dB step size is adequate for the channel control. Using

Ω and

is connected between MSTR SET and MSTR OUT. There

could be some variation from lot to lot, so applications requiring

precise step size should include a fixed plus a trimmer to span

the calculated value

±25%. In this example, R

as the default channel step size is already 1 dB. CH SET is left

floating. With this step size, the dynamic range of the master

control is:

DNR = 0.5

× 127 dB = 63.5 dB

In this configuration, the maximum master volume is 0 dB,

while the minimum volume is –63.5 dB. Since the channel

volume can still provide 0 dB to 31 dB of gain, the total system

gain can vary between –63.5 dB and 32 dB. Note that a 0 dB

command setting to the master control always results in unity

gain, regardless of the step size.

Channel Step Size

The channel DACs’ full-scale current is set by an internal

resistor to the V+. By shunting this resistor, the full-scale

current, and therefore the step size, will increase. No provisions

are available for reducing the channel step size. To increase the

Note that a 0 dB setting for a channel will always give unity

gain, regardless of how large or small the step size is. This is

true for both the master and channel volume controls.

1.5

1.4

1.0

1.3

1.2

1.1

Example: Modifying Channel Step Size

A channel step size of 1.3 dB is desired. From Figure 23 we see

that a 40

Ω resistor (approximately) connected from CH SET to

V+ is required. As this varies from lot to lot, the exact value

should be determined empirically, or a fixed resistor plus

trimmer potentiometer should be used. Take care not to short

Pin 24 to Pin 1 as damage will result.

Muting

The SSM2160 offers master and channel muting. On power

up, the master mute is activated, thus preventing any transients

from entering the signal path and possibly overloading amplifi-

ers down the signal path. Mute is typically better than –95 dB

relative to a 0 dBu input. Due to design limitations, the individual

channel muting results in increased signal distortion in the

unmuted channels. Users should determine if this condition is

acceptable in the particular application.