SA572N datasheet(7/12 Pages) PHILIPS

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Philips Semiconductors

Product specification

SA572

Programmable analog compandor

1998 Nov 03

Q10

Q17

Q16

Q18

10k

D13

Q14

D15

10k

D11

D12

TRACKING

TRIM

IR1

IR2

IQ = 2IR2

V+

R +

–

SR00699

Figure 6. Buffer Amplifier Schematic

Basic Expandor

Figure 7 shows an application of the circuit as a simple expandor.

The gain expression of the system is given by

OUT

3 @ VIN(AVG)

2 @ R1

(5)

(I1=140µA)

Both the resistors R1 and R2 are tied to internal summing nodes. R1

is a 6.8k internal resistor. The maximum input current into the gain

cell can be as large as 140

µA. This corresponds to a voltage level of

140

µA • 6.8k=952mV peak. The input peak current into the rectifier

is limited to 300

µA 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 drive 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

ODC + VREF 1 )

* V

(6)

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. *5COL

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