ML33111
LANSDALE Semiconductor, Inc.
FUNCTIONAL DESCRIPTION
Introduction
The ML33111 compander (COMpressor and exPANDER)
is composed of two variable gain circuits which provide
compression and expansion of a signal’s dynamic range. The
compressor will take a signal with a 60 dB dynamic range
(1.0 mV to 1.0 Vrms), and reduce that to a 30 dB dynamic
range (10 mV to 316 mV) by attenuating strong signals,
while amplifying low level signals. The expander does the
opposite in that the 30 dB signal range is increased to a
dynamic range of 60 dB by amplifying strong signals and
attenuating low level signals. The 0 dB level is internally set
at 100 mVrms — that is the signal level which is neither
amplified nor attenuated. Both circuits contain the necessary
precision full wave rectifier, variable gain cell, and tempera-
ture compensated references required for accurate and stable
performance.
Both the compressor and expander can be muted independ-
ently by the use of Pins 4 and 12, respectively. A minimum
of 55 dB of muting is guaranteed for the compressor, and 60
dB for the expander. A passthrough function (Pin 8) is pro-
vided which sets both sections to unity gain, regardless of
input level.
Two uncommitted op amps are provided which can be used
for perpherial functions. Each is internally biased at Vb
(≈ +1.5 V), and has a bandwidth of
≈300
kHz.
NOTE: All dB values mentioned in this data sheet, unless
otherwise noted, are referenced to 100 mVrms.
Figure 15. Compressor
5
1.0
µF
Rectifier
Iref
VCC
7.5 k
Input
3
10 k
Vb
2
Output
ICONTROL
∆
Gain
40 k
Compressor
The compressor is a noninverting amplifier with a fixed
input resistor and a variable gain cell in its feedback path as
shown in Figure 15.
The amplifier output is sampled by the precision rectifier
which, in turn, supplies a DC signal (ICONTROL), repre-
sentative of the rectifier’s AC signal, to the variable gain
cell. The reference current (IREF) is an internally generated
precision current. The effective impedance of the variable
gain cell varies with the ratio of the two currents, and
decreases as ICONTROL increases, thereby providing com-
pression. The output is related to the input by the following
equation (Vin and Vout are rms volts):
V out
0.3162 x V
in
In terms of dB levels, the relationship is:
Vo(dB) = 0.5 x Vi(dB)
where 0 dB = 100 mVrms (See Figures 3 and 4).
(1)
(2)
The input and output are internally biased at Vb (≈ +1.5 V),
and must therefore be capacitor coupled to external circuitry.
Pin 3 input impedance is nominally 10 kΩ (±20%), and the
maximum functional input signal is listed in the
Recommended Operating Conditions table. Bias currents
required by the op amp and the variable gain cell are inter-
nally supplied. Due to clamp diodes at the input (to VCC and
ground), the input signal must be maintained between the
supply rails. If the input signal goes more than 0.5 V above
VCC or below ground, excessive currents will flow, and dis-
tortion will show up at the output and possibly in other parts
of the circuit.
When AC signals are not present at the input, the variable
gain cell will attempt to set a very high gain to comply with
Equation 2. An internal clamp limits the maximum gain to
≈26
dB to prevent instabilities.
The output of the rectifier is filtered by the capacitor at Pin
5, which, in conjunction with an internal 20 k resistor, pro-
vides the time constant for the attack and decay times.The
attack and decay times listed in the Electrical Characteristics
were determined using the test procedure defined in EIA-553.
Figure 9 indicates how the times vary with the capacitor value.
If the attack and decay times are decreased using a smaller
capacitor, performance at low frequencies will degrade.
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