Data Sheet

SSM2018

Rev. C | Page 13 of 16

OUTPUT DRIVE

The SSM2018 is buffered by an internal op amp to provide a

low impedance output. This output is capable of driving to

within 1.2 V of either rail at 1% distortion for a 100 kΩ load.

Note that this 100 kΩ load is in parallel with the feedback

resistor of 18 kΩ, so the effective load is 15.3 kΩ. For better

than 0.01% distortion, the output should remain about 3.5 V

away from either rail as shown in Figure 6. As the graph of

output swing versus load resistance shows (Figure 13), to

maintain less than 1% distortion the output current should be

limited to approximately ±1.3 mA. If higher current drive is

required, the output should be buffered with a high quality op

amp such as the ADA4897-1 or the AD797.

The internal amplifiers are compensated for unity gain stability

and are capable of driving a capacitive load up to 4700 pF.

Larger capacitive loads should be isolated from the output of

the SSM2018 by the use of a 50 Ω series resistor.

Figure 30. Upgrading SSM2018 Sockets

UPGRADING SSM2018 SOCKETS

The SSM2018 easily replaces the SSM2018 in the basic VCA

configuration. The parts are pin for pin compatible allowing

direct replacement. At the same time, the trimming potentiometers

for symmetry and offset should be removed, as shown in Figure 30.

Upgrading immediately to the SSM2018 saves the expense of

the potentiometers and the time in production of trimming for

minimum distortion and control feedthrough.

If the SSM2018 is used in the OVCE or VCP configuration, the

SSM2018 can still directly replace it; however, the potentiometers

cannot necessarily be removed, as explained in the Operational

Voltage Controlled Element and Voltage Controlled Panner

sections.

TEMPERATURE COMPENSATION OF THE GAIN

CONSTANT

The gain constant has a −3500 ppm/°C temperature drift due to

the inherent nature of the control port. Over the full temperature

range of −40°C to +85°C, the drift causes the gain to change by

7 dB if the part is in a gain of ±20 dB. If the application requires

the gain constant to be the same over a wide temperature range,

external temperature compensation should be employed. The

simplest form of compensation is a temperature compensating

resistor (TCR) such as the PT146 from Precision Resistor Co.

These elements are different than a standard thermistor in that

they are linear over temperature to better match the linear drift

of the gain constant.

Figure 31. Two TCRs Compensate for Temperature Drift of Gain Constant

The gain constant has a −3500 ppm/°C temperature drift that is

due to the reciprocal dependence of the design on absolute

temperature. This causes the gain to vary by 7 dB over the

temperature range from −40°C to +85°C when the nominal gain

at room temperature is set to 20 dB. The gain change is quite

small if the temperature range of operation is restricted.

Nevertheless, the TC of the gain constant is easily compensated

by buffering the control voltage to the VCA with a circuit having a

3500 ppm/°C temperature coefficient. Figure 31 shows a simple

solution to the problem using an op amp with a PT146 temperature

compensating resistor from the Precision Resistor Company.

Note that this circuit is inverting, which changes the gain

constant to a positive quantity. Any other circuit that provides

the necessary positive TC works.

V+

REMOVE FOR SSM2018T

10MΩ

OFFSET

TRIM

1

V+

2

3

4

1µF

1µF

18kΩ

1kΩ

NC

18kΩ

50pF

47pF

18kΩ

3kΩ

1µF

NOTES

2. NC = NO CONNECT.

16

15

14

13

5

6

7

12

11

V–

V+

10

8

9

SSM2018

V–

500kΩ

100kΩ

470kΩ

SYMMETRY

TRIM

CONTROL

VOLTAGE

1kΩ*

2kΩ

+15V

–15V

*PT146 AVAILABLE FROM

PRECISION RESISTOR CO.

10601 75TH STREET NORTH

LARGO, FL. 34647 (727) 541-5771

OP27

PIN 11

SSM2108/

SSM2108T