SA575 Datasheet, PDF (4/14 Pages) NXP Semiconductors – Low voltage compandor

Philips Semiconductors
Low voltage compandor
Product specification
SA575
DC ELECTRICAL CHARACTERISTICS (cont.)
SYMBOL
PARAMETER
TEST CONDITIONS
Crosstalk
For operational amplifier
V_O
R_L
CMR
Output swing
Output load
Input common-mode range
CMRR Common-mode rejection ratio
I_B
V_OS
A_VOL
SR
Input bias current
Input offset voltage
Open-loop gain
Slew rate
GBW
Bandwidth
E_NI
PSRR
Input voltage noise
Power supply rejection ratio
1kHz, 0dB, C_REF = 220µF
R_L = 10kΩ
1kHz
V_IN = 0.5V to 4.5V
R_L = 10kΩ
Unity gain
Unity gain
BW = 20kHz
1kHz, 250mV
LIMITS
SA575
MIN
TYP
-80
V_CC-0.4
V_CC
600
0
60
80
-1
3
80
1
3
2.5
60
MAX
-65
V_CC
1
UNITS
dB
V
Ω
V
dB
µA
mV
dB
V/µs
MHz
µV
dB
NOTES:
1. Operation down to V_CC = 2V is possible, but performance is reduced. See curves in Figure 7a and 7b.
2. Reference voltage, V_REF, is typically at 1/2V_CC.
FUNCTIONAL DESCRIPTION
This section describes the basic subsystems and applications of the
SA575 Compandor. More theory of operation on compandors can
be found in AN174 and AN176. 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 3, 4, 5
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 6 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, S1, toggles the circuit between compressor
and ALC mode. Jumpers J1 and J2 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 R1, R2, C1,
C2, R10, R11, C10 and C11 so that the response can be tailored for
each individual need. The components as specified are suitable for
the complete audio spectrum from 20Hz to 20kHz.
The most common configuration is as a unity gain non-inverting
buffer where R1, C1, C2, R10, C10 and C11 are eliminated and R2
and R11 are shorted. Capacitors C3, C5, C8, and C12 are for DC
blocking. In systems where the inputs and outputs are AC coupled,
these capacitors and resistors can be eliminated. Capacitors C4
and C9 are for setting the attack and release time constant.
C6 is for decoupling and stabilizing the voltage reference circuit.
The value of C6 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.
R12 provides DC reference voltage to the amplifier of channel B.
R6 and R7 provide a DC feedback path for the summing amp of
channel B, while C7 is a short-circuit to ground for signals. C14 and
C15 are for power supply decoupling. C14 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 20Hz to 20kHz with the component values as shown in
Figure 6 and V_CC = 5V. In the expandor mode, the typical input
dynamic range was from -34dB to +12dB where 0dB is equal to
100mV_RMS. The typical unity gain level measured at 0dB @ 1kHz
input was +0.5dB and the typical tracking error was +0.1dB for input
range of -30 to +10dB.
In the compressor mode, the typical input dynamic range was from
-42dB to +18dB with a tracking error +0.1dB and the typical unity
gain level was +0.5dB.
In the ALC mode, the typical input dynamic range was from -42dB to
+8dB with typical output deviation of +0.2dB about the nominal
output of 0dB. For input greater than +9dB 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 R6 and R7 to 20kΩ 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 30pF 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 +18dB yielding a dynamic range of over 60dB.
EXPANDOR
The typical expandor configuration is shown in Figure 3. 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 0dB ref is 100mV_RMS. The input is AC
coupled through C5, and the output is AC coupled through C3. If in
a system the inputs and outputs are AC coupled, then C3 and C5
can be eliminated, thus requiring only one external component, C4.
The variable gain cell and rectifier cell are DC coupled so any offset
1997 Nov 07
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