SSM2160_03 Datasheet, PDF (11/16 Pages) Analog Devices – 6-Channel, Serial Input Master/Balance Volume Controls

SSM2160
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 a smaller 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 9, 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
To reduce the master step size, place resistor R_M between
MSTR SET and MSTR OUT. The master step size of the
master volume control will then become
R_M
= ^{1700 X M}
1– X_M
where X_M is the desired master control step size in decibels. See
Figure 10 for practical values of R_M. Note that 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
could be some variation from lot to lot, so applications requiring
precise step size should include a fixed resistor plus a trimmer
potentiometer to span the calculated value ± 25%. In this example,
R_C is not needed since 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 resis-
tor to 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 channel step
size, place a resistor, R_C, from CH SET to V+. 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
1.3
1.2
1.1
0.8
0.6
0.4
0.2
0
10²
10³
10⁴
10⁵
RM
Figure 10. Master Step Size vs. R_M
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
the preceding equation or Figure 10, R_M is found to be 1700 Ω
and is connected between MSTR SET and MSTR OUT. There
1.0
10¹
10²
10³
RC
Figure 11. Channel Step Size vs. R_C
Example: Modifying Channel Step Size
A channel step size of 1.3 dB is desired. From Figure 11 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 indi-
vidual channel muting results in increased signal distortion in
the unmuted channels. Users should determine if this condition
is acceptable in the particular application.
REV. A
–11–