REV. B

–11–

SSM2018T

As explained above, the gain constant has a –3500 ppm/

∞C that

is due to its reciprocal dependence on absolute temperature.

This will cause 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. Of course, the gain change is quite

small if the temperature range of operation is restricted. Never-

theless, 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 4 shows a simple

solution to the problem using an op amp with a PT146 tem-

perature compensating resistor from the Precision Resistor

Company. Note that this circuit is inverting, which will change

the gain constant to a positive quantity. Any other circuit that

provides the necessary positive TC will work.

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 5

shows a 12-bit DAC, the DAC8512, controlling the SSM2018T.

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

age. 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 inter-

face is needed, then the DAC8562 may be used or, for a dual

DAC, the AD8582.

0.1 F

+15V

18k

DAC8512

CS

CLR

0.1 F

18k

50pF

47pF

150k

+15V

–15V

0.1 F

+5V

1 F

R6

825

R7

1k

0V

+2.24V

LD

SCLK

SDI

SSM2018T

NC

NC

NC

NC

NC = NO CONNECT

Figure 5. 12-Bit DAC Controls the VCA Gain

Supply Considerations and Single Supply Operation

The SSM2018T 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 specifi-

cations 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

mF 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 SSM2018T can be operated in single supply mode as long

as the circuit is properly biased. Figure 6 shows the proper con-

figuration, which includes an amplifier to create a false ground

node midway between the supplies. A high quality, wide band-

width audio amplifier, such as the OP176 or AD797, should

be used to ensure a very low impedance ground over the full

audio frequency range. The minimum operating supply for the

SSM2018T 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 SSM2018T

with

±5 V supplies.

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 7. The voltage output labeled

upon the control voltage.

V+

1 F

18k

V+

18k

1 F 18k

47pF

1 F

50pF

1k

3k

SSM2018T

V+

OP176

100k

100k

V+

10 F

Figure 6. Single Supply Operation of SSM2018T

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

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 8. Notice that the amplifier whose output (Pin 16) was

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.