REV. A

–15–

SSM2018T/SSM2118T

V+

1

µF

R

B

18k

V+

18k

V

IN+

1

µF 18k

V

IN–

47pF

1

µF

50pF

1k

V

CONTROL

3k

V

OUT

1

2

16

15

5

6

7

12

11

10

3

4

14

13

89

SSM2018T

V+

OP176

100k

100k

V+

10

µF

Figure 45. Single Supply Operation of SSM2018T

Operational Voltage Controlled Element

The SSM2018T has considerable flexibility beyond the basic

VCA circuit utilized throughout this data sheet. The name

“Operational Voltage Controlled Element” comes from the fact

that the part behaves much like an operational amplifier with a

second voltage controlled output. The symbol for the OVCE

connected as a unity gain follower/VCA is shown in Figure 46.

fied or attenuated depending upon the control voltage. Because

the OVCE works just like an op amp, the feedback could just as

easily have included resistors to add gain, or a filter network to

add frequency shaping. The full circuit for the OVCE is shown

in Figure 47. Notice that the amplifier whose output (Pin 16)

back. As mentioned before, because the SSM2018T is trimmed

for the basic VCA configuration, potentiometers are needed for

the OVCE configuration to ensure the best THD and control

feedthrough performance.

If a symmetry trim is to be performed, it should precede the

control feedthrough trim and be done as follows:

1. Apply a 1 kHz sine wave of +10 dBu to the input, with the

control voltage set for unity gain.

2. Adjust the symmetry trim potentiometer to minimize distor-

tion of the output signal.

Next the control feedthrough trim is done as follows:

1. Ground the input signal port and apply a 60 Hz sine wave

to the control port. The sine wave should have its high and

low peaks correspond to the highest gain to be used in the

application and 30 dB of attenuation, respectively. For ex-

ample, a range of +20 dB gain to 30 dB attenuation requires

that the sine wave amplitude ranges between –560 mV and

+840 mV on Pin 11.

2. Adjust the control feedthrough potentiometer to null the sig-

nal seen at the output.

Figure 46. OVCE Follower/VCA Connection

18k

Ω

50pF

470k

Ω

500k

Ω

V+

100k

Ω

10M

Ω

CONTROL

FEEDTHROUGH

TRIM

V+

V–

INPUTS

220pF

NC

V–

1µF

1k

Ω

3k

Ω

150k

Ω FOR CLASS AB

NC = NO CONNECT

SYMMETRY

TRIM

V+

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

SSM2018T

18k

Ω

50pF

Figure 47. OVCE Application Circuit