SSM2018

Data Sheet

Rev. C | Page 14 of 16

DIGITAL CONTROL OF THE GAIN

A common method of controlling the gain of a VCA is to use a

digital-to-analog converter to set the control voltage. Figure 32

shows a 12-bit DAC, the DAC8512, controlling the SSM2018.

The DAC8512 is a complete 12-bit converter in an 8-pin package.

It includes an on-board reference and an output amplifier to

produce an output voltage from 0 V to 4.095 V, which is 1 mV/bit.

Since the voltage is always positive, this circuit only provides

attenuation. The resistor divider on the output of the DAC8512

is set to scale the output voltage so that full scale produces 80 dB

of attenuation. The resistor divider can be adjusted to provide

other attenuation ranges. If a parallel interface is needed, then

the DAC8562 may be used or, for a dual DAC, the AD8582.

Figure 32. 12-Bit DAC Controls the VCA Gain

SUPPLY CONSIDERATIONS AND SINGLE-SUPPLY

OPERATION

The SSM2018 has a wide operating supply range. Many of

the graphs in this data sheet show the performance of the part

from ±5 V to ±18 V. These graphs offer typical performance

specifications and are a good indication of the parts’ capabilities.

The minimum operating supply voltage is ±4.5 V. Below this

voltage, the parts are inoperable. Thus, to account for supply

variations, the recommended minimum supply is ±5 V.

For simplicity, the circuits in the data sheet do not show supply

decoupling; however, to ensure best performance, each supply pin

should be decoupled with a 0.1 µF ceramic (or other low resistance

and inductance type) capacitor as close to the package as possible.

This minimizes the chance of supply noise feeding through the

part causing excessive noise in the audio frequency range.

The SSM2018 can be operated in single-supply mode as long as

the circuit is properly biased. Figure 33 shows the proper

configuration, which includes an amplifier to create a false

ground node midway between the supplies. A high quality, wide

bandwidth audio amplifier, such as the ADA4897-1 or the

AD797, should be used to ensure a very low impedance ground

over the full audio frequency range. The minimum operating

supply for the SSM2018 is ±5 V, which gives a minimum single-

supply of +10 V and ground. The performance of the circuit

with +10 V is identical to graphs that show operation of the

SSM2018 with ±5 V supplies.

Figure 33. Single-Supply Operation of SSM2018

OPERATIONAL VOLTAGE CONTROLLED ELEMENT

The SSM2018 has considerable flexibility beyond the basic VCA

circuit utilized throughout this data sheet. The name

operational voltage controlled element (OVCE) 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

is amplified or attenuated depending upon the control voltage.

Because the OVCE works similarly to an op amp, the feedback

could as easily have included resistors to add gain, or a filter

network to add frequency shaping. The full application circuit

for the OVCE is shown in Figure 35. Note that the amplifier

whose output (Pin 16) was originally connected to VMINUS is

now the output for feedback. As mentioned before, because the

SSM2018 is trimmed for the basic VCA configuration,

potentiometers are needed for the OVCE configuration to

ensure the best THD and control feedthrough performance.

1

2

3

4

0.1µF

18kΩ

150kΩ

NC

NC = NO CONNECT

18kΩ

50pF

47pF

NC

NC

NC

16

15

14

13

5

6

7

12

11

+15V

10

8

9

–15V

+15V

SSM2018T

0.1µF

0.1µF

R7

1kΩ

1µF

+5V

R6

825Ω

DAC8512

2

1

8

7

6

5

3

4

CS

CLR

LD

SCLK

SDI

1

V+

2

3

4

1µF

1µF

18kΩ

1kΩ

18kΩ

50pF

47pF

10µF

18kΩ

100kΩ

100kΩ

3kΩ

1µF

16

15

14

13

5

6

7

12

11

V+

10

8

9

SSM2018T

V+

V+

OP176