SA575
Expandor
The typical expandor configuration is shown in Figure 2.
The variable gain cell and the rectifier cell are in the signal
input path. The V
REF
is always 1/2 V
CC
to provide the
maximum headroom without clipping. The 0 dB ref is
100 mV
RMS
. The input is AC coupled through C
5
, and the
output is AC coupled through C
3
. If in a system the inputs
and outputs are AC coupled, then C
3
and C
5
can be
eliminated, thus requiring only one external component,
C
4
. The variable gain cell and rectifier cell are DC coupled
so any offset voltage between Pins 4 and 9 will cause small
offset error current in the rectifier cell. This will affect the
accuracy of the gain cell. This can be improved by using an
extra capacitor from the input to Pin 4 and eliminating the
DC connection between Pins 4 and 9.
The expandor gain expression and the attack and release
time constant is given by Equation 1 and Equation 2,
respectively.
Expandor gain =
4V
IN
(avg)
3.8 kW x 100
mA
2
(eq. 1)
where V
IN
(avg) = 0.95V
IN(RMS)
t
R
=
t
A
= 10 kW x C
RECT
= 10 kW x C
4
(eq. 2)
7, 13
C
5
EXP IN
10mF
10kW
9, 11
D
G
S
10kW
6, 14
C
3
EXP OUT
10mF
4, 16
3.8kW
5, 15
2.2mF
V
REF
8
C
4
Figure 2. Typical Expandor Configuration
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