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PDF SA571N Data sheet ( Hoja de datos )
| Número de pieza | SA571N | |
| Descripción | Compandor | |
| Fabricantes | Philips | |
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INTEGRATED CIRCUITS
SA571
Compandor
Product specification
IC17 Data Handbook
Philips
Semiconductors
1997 Aug 14
1 page  
Philips Semiconductors
Compandor
Product specification
SA571
This paper describes an inexpensive integrated circuit, the SA571
Compandor, which offers a pair of high performance gain control
circuits featuring low distortion (<0.1%), high signal-to-noise ratio
(90dB), and wide dynamic range (110dB).
CIRCUIT BACKGROUND
The SA571 Compandor was originally designed to satisfy the
requirements of the telephone system. When several telephone
channels are multiplexed onto a common line, the resulting
signal-to-noise ratio is poor and companding is used to allow a wider
dynamic range to be passed through the channel. Figure 5
graphically shows what a compandor can do for the signal-to-noise
ratio of a restricted dynamic range channel. The input level range of
+20 to -80dB is shown undergoing a 2-to-1 compression where a
2dB input level change is compressed into a 1dB output level
change by the compressor. The original 100dB of dynamic range is
thus compressed to a 50dB range for transmission through a
restricted dynamic range channel. A complementary expansion on
the receiving end restores the original signal levels and reduces the
channel noise by as much as 45dB.
The significant circuits in a compressor or expander are the rectifier
and the gain control element. The phone system requires a simple
full-wave averaging rectifier with good accuracy, since the rectifier
accuracy determines the (input) output level tracking accuracy. The
gain cell determines the distortion and noise characteristics, and the
phone system specifications here are very loose. These specs could
have been met with a simple operational transconductance
multiplier, or OTA, but the gain of an OTA is proportional to
temperature and this is very undesirable. Therefore, a linearized
transconductance multiplier was designed which is insensitive to
temperature and offers low noise and low distortion performance.
These features make the circuit useful in audio and data systems as
well as in telecommunications systems.
BASIC CIRCUIT HOOK-UP AND OPERATION
Figure 6 shows the block diagram of one half of the chip, (there are
two identical channels on the IC). The full-wave averaging rectifier
provides a gain control current, IG, for the variable gain (∆G) cell.
The output of the ∆G cell is a current which is fed to the summing
node of the operational amplifier. Resistors are provided to establish
circuit gain and set the output DC bias.
INPUT
LEVEL
+20
0dB
OUTPUT
LEVEL
–20
0dB
–40
NOISE
–40
–80 –80
SR00679
Figure 5. Restricted Dynamic Range Channel
The circuit is intended for use in single power supply systems, so
the internal summing nodes must be biased at some voltage above
ground. An internal band gap voltage reference provides a very
stable, low noise 1.8V reference denoted VREF. The non-inverting
input of the op amp is tied to VREF, and the summing nodes of the
rectifier and ∆G cell (located at the right of R1 and R2) have the
same potential. The THD trim pin is also at the VREF potential.
Figure 7 shows how the circuit is hooked up to realize an expandor.
The input signal, VIN, is applied to the inputs of both the rectifier and
the ∆G cell. When the input signal drops by 6dB, the gain control
current will drop by a factor of 2, and so the gain will drop 6dB. The
output level at VOUT will thus drop 12dB, giving us the desired 2-to-1
expansion.
Figure 8 shows the hook-up for a compressor. This is essentially an
expandor placed in the feedback loop of the op amp. The ∆G cell is
setup to provide AC feedback only, so a separate DC feedback loop
is provided by the two RDC and CDC. The values of RDC will
determine the DC bias at the output of the op amp. The output will
bias to:
VOUT
DC
+
1
)
RDC1 ) RDC2
R4
THD TRIM R3 INVIN
GIN
R2
20k
3,14
RECTIN
8,9 6,11 5,12
R3
20k
∆G
IG
R4
30k
VREF
1.8V
OUTPUT
7,10
2,15 R1
10k
1,16
CRECT
VCC PIN 13
GND PIN 4
SR00680
Figure 6. Chip Block Diagram (1 of 2 Channels)
R3
*CIN1
R2
VIN
*CIN2
R1
∆G
R4
–
+ VOUT
VREF
NOTE:
2 R3 VIN (avg)
GAIN + R1 R2 IB
IB = 140µA
*CRECT
*EXTERNAL COMPONENTS
Figure 7. Basic Expander
ǒ ǓVREF +
1
)
RDCTOT
30k
1.8V
SR00681
The output of the expander will bias up to:
VOUT DC
+
1
)
R3
R4
VREF
ǒ ǓVREF +
1
)
20k
30k
1.8V + 3.0V
The output will bias to 3.0V when the internal resistors are used.
External resistors may be placed in series with R3, (which will affect
the gain), or in parallel with R4 to raise the DC bias to any desired
value.
1997 Aug 14
5
5 Page 
Philips Semiconductors
Compandor
Product specification
SA571
DEFINITIONS
Data Sheet Identification
Product Status
Definition
Objective Specification
Formative or in Design
This data sheet contains the design target or goal specifications for product development. Specifications
may change in any manner without notice.
Preliminary Specification
Preproduction Product
This data sheet contains preliminary data, and supplementary data will be published at a later date. Philips
Semiconductors reserves the right to make changes at any time without notice in order to improve design
and supply the best possible product.
Product Specification
Full Production
This data sheet contains Final Specifications. Philips Semiconductors reserves the right to make changes
at any time without notice, in order to improve design and supply the best possible product.
Philips Semiconductors and Philips Electronics North America Corporation reserve the right to make changes, without notice, in the products,
including circuits, standard cells, and/or software, described or contained herein in order to improve design and/or performance. Philips
Semiconductors assumes no responsibility or liability for the use of any of these products, conveys no license or title under any patent, copyright,
or mask work right to these products, and makes no representations or warranties that these products are free from patent, copyright, or mask
work right infringement, unless otherwise specified. Applications that are described herein for any of these products are for illustrative purposes
only. Philips Semiconductors makes no representation or warranty that such applications will be suitable for the specified use without further testing
or modification.
LIFE SUPPORT APPLICATIONS
Philips Semiconductors and Philips Electronics North America Corporation Products are not designed for use in life support appliances, devices,
or systems where malfunction of a Philips Semiconductors and Philips Electronics North America Corporation Product can reasonably be expected
to result in a personal injury. Philips Semiconductors and Philips Electronics North America Corporation customers using or selling Philips
Semiconductors and Philips Electronics North America Corporation Products for use in such applications do so at their own risk and agree to fully
indemnify Philips Semiconductors and Philips Electronics North America Corporation for any damages resulting from such improper use or sale.
Philips Semiconductors
811 East Arques Avenue
P.O. Box 3409
Sunnyvale, California 94088–3409
Telephone 800-234-7381
© Copyright Philips Electronics North America Corporation 1997
All rights reserved. Printed in U.S.A.
Philips
Semiconductors
1997 Aug 14
11
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