NE571 Philipss, NE571 Datasheet - Page 5

• Manufacturer Part Number: NE571
• Description: Compandor
• Manufacturer: Philipss

signal-to-noise ratio is poor and companding is used to allow a wider
phone system specifications here are very loose. These specs could
Philips Semiconductors
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
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
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, I
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.
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 V
input of the op amp is tied to V
1997 Aug 14
Compandor
Figure 5. Restricted Dynamic Range Channel
LEVEL
0dB
INPUT
+20
–40
–80
NOISE
REF
G
, for the variable gain ( G) cell.
, and the summing nodes of the
REF
. The non-inverting
OUTPUT
LEVEL
0dB
–20
–40
–80
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5
output level at V
rectifier and G cell (located at the right of R
same potential. The THD trim pin is also at the V
Figure 7 shows how the circuit is hooked up to realize an expandor.
The input signal, V
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
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 R
determine the DC bias at the output of the op amp. The output will
bias to:
V
V
The output of the expander will bias up to:
V
V
The output will bias to 3.0V when the internal resistors are used.
External resistors may be placed in series with R
the gain), or in parallel with R
value.
V
OUT
REF
OUT
REF
IN
NOTE:
*EXTERNAL COMPONENTS
GAIN
I
B
DC
DC
= 140 A
Figure 6. Chip Block Diagram (1 of 2 Channels)
*C
1
*C
1
IN1
2 R 3 V IN (avg)
IN2
RECT
1
1
2,15
R 1 R 2 I B
R
G
3,14
20k
30k
OUT
IN
DCTOT
30k
IN
IN
10k
R
R
R
R
R
20k
, is applied to the inputs of both the rectifier and
Figure 7. Basic Expander
will thus drop 12dB, giving us the desired 2-to-1
1
2
R
DC1
R
3
4
1.8V
THD TRIM
1
2
DC
V
C
1.8V
RECT
REF
R
G
and C
4
8,9
1,16
IG
4
R
R
20k
to raise the DC bias to any desired
3.0V
DC2
3
G
30k
R
R
DC
6,11
4
*C
3
. The values of R
RECT
INV
R
V
1.8V
REF
4
IN
5,12
1
V
GND PIN 4
CC
and R
Product specification
OUTPUT
–
+
REF
PIN 13
3
R
, (which will affect
7,10
3
2
V
potential.
) have the
DC
REF
SA571
will
SR00680
SR00681
V
OUT