Philips Semiconductors

Product specification

SA572

Programmable analog compandor

1998 Nov 03

5

SA572 BASIC APPLICATIONS

Description

The SA572 consists of two linearized, temperature-compensated

gain cells (

∆G), each with a full-wave rectifier and a buffer amplifier

as shown in the block diagram. The two channels share a 2.5V

common bias reference derived from the power supply but otherwise

operate independently. Because of inherent low distortion, low noise

and the capability to linearize large signals, a wide dynamic range

can be obtained. The buffer amplifiers are provided to permit control

of attack time and recovery time independent of each other.

Partitioned as shown in the block diagram, the IC allows flexibility in

the design of system levels that optimize DC shift, ripple distortion,

tracking accuracy and noise floor for a wide range of application

requirements.

Gain Cell

Figure 4 shows the circuit configuration of the gain cell. Bases of the

be derived from the transistor model equation in the forward active

region.

DV

BE

Q3Q4

+ D

BE

Q1Q2

V

1

2

I

1

2

I

O

I

S

* V

1

2

I

1

2

I

O

I

S

where I

V

IN

R

1

V

I

I

S

* V

I

I

S

(2)

where I

V

IN

R

1

control current of the gain cell.

can be simplified to:

I

2

I

2

@ I

1

I

2

I

@ I

G

(3)

The first term of Equation 3 shows the multiplier relationship of a

linearized two quadrant transconductance amplifier. The second

term is the gain control feedthrough due to the mismatch of devices.

In the design, this has been minimized by large matched devices

and careful layout. Offset voltage is caused by the device mismatch

and it leads to even harmonic distortion. The offset voltage can be

trimmed out by feeding a current source within

±25µA into the THD

trim pin.

The residual distortion is third harmonic distortion and is caused by

gain control ripple. In a compandor system, available control of fast

attack and slow recovery improve ripple distortion significantly. At

the unity gain level of 100mV, the gain cell gives THD (total harmonic

distortion) of 0.17% typ. Output noise with no input signals is only

6

must feed the virtual ground input of an operational amplifier with a

resistor from output to inverting input. The non-inverting input of the

VREF

THD

TRIM

V+

R1

6.8k

1

2

I

G )

1

2

I

O

I1

140

µA

280

µA

I2

IG

IO

Q4

Q3

Q1

Q2

VIN

+

–

A1

SR00697

Figure 4. Basic Gain Cell Schematic