SA572 Datasheet, PDF(7/12 Page) NXP Semiconductors

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SA572 Datasheet, PDF (7/12 Pages) NXP Semiconductors – Programmable analog compandor

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SA572

Basic Expandor

Figure 6 shows an application of the circuit as a simple

expandor. The gain expression of the system is given by:

Ç Ç VOUT

VIN

R3 @ VIN(AVG)

R2 @ R1

(eq. 4)

(I1 = 140 mA)

Both the resistors R1 and R2 are tied to internal summing

nodes. R1 is a 6.8 kW internal resistor. The maximum input

current into the gain cell can be as large as 140 mA. This

corresponds to a voltage level of 140 mAâ¢6.8 kW = 952 mV

peak. The input peak current into the rectifier is limited to

300 mA by the internal bias system. Note that the value of

R1 can be increased to accommodate higher input level. R2

and R3 are external resistors. It is easy to adjust the ratio of

R3/R2 for desirable system voltage and current levels. A

small R2 results in higher gain control current and smaller

static and dynamic tracking error. However, an impedance

buffer A1 may be necessary if the input is voltage driven

with large source impedance.

The gain cell output current feeds the summing node of

the external OPA A2. R3 and A2 convert the gain cell output

current to the output voltage. In high-performance

applications, A2 has to be low-noise, high-speed and wide

band so that the high-performance output of the gain cell

will not be degraded. The non-inverting input of A2 can be

biased at the low noise internal reference Pin 6 or 10.

Resistor R4 is used to bias up the output DC level of A2 for

maximum swing. The output DC level of A2 is given by:

Ç Ç VOUT DC

V REF

)

(eq. 5)

VB can be tied to a regulated power supply for a dual

supply system and be grounded for a single supply system.

CA sets the attack time constant and CR sets the recovery

time constant.

+VB

17.3kW

CIN1

VIN

2.2mF

100kW

CIN2

(7,9)

2.2mF

6.8kW

CIN3

2.2mF

(5,11)

VREF

BUFFER

(6,10) R6

1kW

(2,14)

(4,12)

3.3kW

(3,13)

CA CR

1mF 10mF

2.2mF

(8)

(16)

+VCC

Figure 6. Basic Expandor Schematic

VOUT

Basic Compressor

Figure 7 shows the hook-up of the circuit as a

compressor. The IC is put in the feedback loop of the OPA

A1. The system gain expression is as follows:

Ç Ç VOUT

VIN

R2 @ R1

R3 @ VIN(AVG)

(eq. 6)

(I1 = 140 mA)

RDC1, RDC2, and CDC form a DC feedback for A1. The

output DC level of A1 is given by:

Ç Ç VOUT DC

V REF

)

RDC1 ) RDC2

Ç Ç * VB @

RDC1 ) RDC2

(eq. 7)

The zener diodes D1 and D2 are used for channel

overload protection.

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