SSM2120/SSM2122

REV. C

–3–

2V

FULL

WAVE

RECTIFIER

V+

THRESH 2

LOG AV 2

V–

CFT 1

INPUT 1

OUTPUT 1

CFT 2

INPUT 2

OUTPUT 2

SSM2122

2V

FULL

WAVE

RECTIFIER

V+

THRESH 1

LOG AV 1

V–

Figure 1. SSM2120 Block Diagram

VCA PERFORMANCE

Figures 2a and 2b show the typical THD and noise performance

of the VCAs over

±20 dB gain/attenuation. Full Class A operation

provides very low THD.

GAIN – dB

0.03

0.01

–20

20

–10

0

10

0.003

a. VCA THD Performance vs. Gain

(+10 dBV In/Out @ 1 kHz)

GAIN – dB

–70

–80

–20

20

–10

0

10

–90

b. VCA Noise vs. Gain (20 kHz Bandwidth)

Figure 2. Typical THD and Noise Performance

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 +6 mV/dB and –6 mV/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

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

I

[(V

+)–((V –)+ 2V )]

R

REF

The current consumption of the VCAs will be directly pro-

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

± 400 µA. In general:

VCA OUTPUTS

The VCA outputs are designed to interface directly with the virtual

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 impedance.

The power supplies and selected compliance range determines

the values of input and output resistors needed. As an example,

with

±15 V supplies and ±400 µA maximum input and output

Ω for an output compliance

range of

±14.4 V. Note that the signal path through the VCA

including the output current-to-voltage converter is noninverting.