3

5/90, Rev. B1

SSM-2120/SSM-2122 DYNAMIC RANGE PROCESSOR/DUAL VCA

VOLTAGE-CONTROLLED AMPLIFIERS

The two voltage-controlled amplifiers are full Class A current in/

current out devices with complementary dB/V gain control ports.

The control sensitivities are +6mV/dB and –6mV/dB. A resistor

divider (attenuator) is used to adapt the sensitivity of an external

control voltage to the range of the control port. It is best to use

200

Ω or less for the attenuator resistor to ground.

VCA INPUTS

The signal inputs behave as virtual grounds. The input current

compliance range is determined by the current into the reference

current pin.

REFERENCE PIN

The reference current determines the input and output current

compliance range of the VCAs. The current into the reference

pin is set by connecting a resistor to V+. The voltage at the refer-

ence pin is about two volts above V– and the current will be

The current consumption of the VCAs will be directly proportional

to I

REF

which is nominally 200

µA. The device will operate at lower

current levels which will reduce the effective dynamic range of

the VCAs. With a 200

µA reference current, the input and output

clip points will be

±400µA. In general:

VCA OUTPUTS

The VCA outputs are designed to interface directly with the vir-

tual ground inputs of external operational amplifiers configured

as current-to-voltage converters. The outputs must operate at

virtual ground because of the output stage’s finite output imped-

ance. The power supplies and selected compliance range de-

termines the values of input and output resistors needed. As an

example, with

±15V supplies and ±400µA maximum input and

output current, choose R

IN

= R

OUT

= 36k

Ω for an output compli-

ance range of

±14.4 V. Note that the signal path through the

VCA including the output current-to-voltage converter is nonin-

verting.

I

CLIP

=

±2I

REF

BLOCK DIAGRAM (SSM-2120)

+

OUTPUT 1

INPUT 1

CFT 1

–

2V

+

FULL

WAVE

RECTIFIER

V+

LOG AV 1

+

–

THRESH 1

V–

+

OUTPUT 2

INPUT 2

CFT 2

–

2V

+

FULL

WAVE

RECTIFIER

V+

LOG AV 2

+

–

THRESH 2

V–

SSM-2122

I

REF

=

V+ –

V– +2V

OBSOLETE