SA575
Functional Description
This section describes the basic subsystems and
applications of the SA575 Compandor. More theory of
operation on compandors can be found in AND8159 and
AND8160. The typical applications of the SA575 low
voltage compandor in an Expandor (1:2), Compressor (2:1)
and Automatic Level Control (ALC) function are
explained. These three circuit configurations are shown in
Figures 2, 3, and 4 respectively.
The SA575 has two channels for a complete companding
system. The left channel, A, can be configured as a 1:2
Expandor while the right channel, B, can be configured as
either a 2:1 Compressor, a 1:2 Expandor or an ALC. Each
channel consists of the basic companding building blocks
of rectifier cell, variable gain cell, summing amplifier
and V
REF
cell. In addition, the SA575 has two additional
high performance uncommitted op amps which can be
utilized for application such as filtering, pre-emphasis/
de-emphasis or buffering.
Figure 5 shows the complete schematic for the
applications demo board. Channel A is configured as an
expandor while channel B is configured so that it can be
used either as a compressor or as an ALC circuit. The
switch, S
1
, toggles the circuit between compressor and
ALC mode. Jumpers J
1
and J
2
can be used to either include
the additional op amps for signal conditioning or exclude
them from the signal path. Bread boarding space is
provided for R
1
, R
2
, C
1
, C
2
, R
10
, R
11
, C
10
and C
11
so that
the response can be tailored for each individual need. The
components as specified are suitable for the complete
audio spectrum from 20 Hz to 20 kHz.
The most common configuration is as a unity gain
non-inverting buffer where R
1
, C
1
, C
2
, R
10
, C
10
and C
11
are
eliminated and R
2
and R
11
are shorted. Capacitors C
3
, C
5
,
C
8
, and C
12
are for DC blocking. In systems where the
inputs and outputs are AC coupled, these capacitors and
resistors can be eliminated. Capacitors C
4
and C
9
are for
setting the attack and release time constant.
C
6
is for decoupling and stabilizing the voltage reference
circuit. The value of C
6
should be such that it will offer a
very low impedance to the lowest frequencies of interest.
Too small a capacitor will allow supply ripple to modulate
the audio path. The better filtered the power supply, the
smaller this capacitor can be. R
12
provides DC reference
voltage to the amplifier of channel B. R
6
and R
7
provide a
DC feedback path for the summing amp of channel B,
while C
7
is a short-circuit to ground for signals. C
14
and C
15
are for power supply decoupling. C
14
can also be
eliminated if the power supply is well regulated with very
low noise and ripple.
Demonstrated Performance
The applications demo board was built and tested for a
frequency range of 20 Hz to 20 kHz with the component
values as shown in Figure 5 and V
CC
= 5.0 V. In the
expandor mode, the typical input dynamic range was from
-34 dB to +12 dB where 0 dB is equal to 100 mV
RMS
. The
typical unity gain level measured at 0 dB @ 1.0 kHz input
was
"0.5
dB and the typical tracking error was
"0.1
dB
for input range of -30 to +10 dB.
In the compressor mode, the typical input dynamic range
was from -42 dB to
"18
dB with a tracking error +0.1 dB
and the typical unity gain level was
"0.5
dB.
In the ALC mode, the typical input dynamic range was
from -42 dB to +8.0 dB with typical output deviation of
"0.2
dB about the nominal output of 0 dB. For input
greater than +9.0 dB in ALC configuration, the summing
amplifier sometimes exhibits high frequency oscillations.
There are several solutions to this problem. The first is to
lower the values of R
6
and R
7
to 20 kW each. The second
is to add a current limiting resistor in series with C12 at
Pin 13. The third is to add a compensating capacitor of
about 22 to 30 pF between the input and output of summing
amplifier (Pins 12 and 14). With any one of the above
recommendations, the typical ALC mode input range
increased to +18 dB yielding a dynamic range of over
60 dB.
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