Philips Semiconductors

Product specification

SA575

Low voltage compandor

1997 Nov 07

4

LIMITS

SYMBOL

PARAMETER

TEST CONDITIONS

SA575

UNITS

MIN

TYP

MAX

Crosstalk

-80

-65

dB

For operational amplifier

Output swing

V

Output load

1kHz

600

Ω

CMR

Input common-mode range

0

V

CMRR

Common-mode rejection ratio

60

80

dB

Input bias current

-1

1

µA

Input offset voltage

3

mV

Open-loop gain

80

dB

SR

Slew rate

Unity gain

1

V/

µs

GBW

Bandwidth

Unity gain

3

MHz

Input voltage noise

BW = 20kHz

2.5

µV

PSRR

Power supply rejection ratio

1kHz, 250mV

60

dB

NOTES:

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,

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

dynamic range was from -34dB to +12dB where 0dB is equal to

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 20k

Ω 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.

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