REV. A

–14–

SSM2018T/SSM2118T

such 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.

0.1

µF

+15V

18k

Ω

6

DAC8512

8

7

CS

CLR

2

1

0.1

µF

18k

Ω

50pF

47pF

150k

Ω

+15V

–15V

0.1

µF

+5V

1

µF

R6

825

Ω

R7

1k

Ω

0V

C

5

LD

3

SCLK

4

SDI

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

SSM2018T

NC

NC

NC

NC

NC = NO CONNECT

Figure 44. 12-Bit DAC Controls the VCA Gain

Supply Considerations and Single Supply Operation

The SSM2018T and SSM2118T have a wide operating supply

range. Many of the graphs in this data sheet show the perfor-

mance of the part from

±5 V to ±18 V. These graphs offer typi-

cal 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 sup-

ply is

±5 V.

The circuits in the data sheet do not show supply decoupling for

simplicity; however, to ensure best performance, each supply

pin should be decoupled with a 0.1

µF ceramic (or other low re-

sistance and inductance type) capacitor as close to the package

as possible. This minimizes the chance of supply noise feeding

through the part and causing excessive noise in the audio fre-

quency range.

The SSM2018T and SSM2118T can be operated in single sup-

ply mode as long as the circuit is properly biased. Figure 45

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 OP176

or AD797 should be used to ensure a very low impedance

ground over the full audio frequency range. The minimum op-

erating supply for the SSM2018 is

±5 V, which gives a mini-

mum 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.

1µF

2k

Ω

SSM2018T OR SSM2118T

1k

Ω*

3500ppm/

°C

1k

Ω*

3500ppm/

°C

CONTROL

VOLTAGE

*PRECISION RESISTOR CO.

LARGO, FL 34647

(813) 541-5771

Figure 42. Two TCRs Compensate for Temperature Drift

of Gain Constant

+15V

–15V

R3

10k

Ω

50pF

R4

1k

Ω

R5

9k

Ω

R1

10k

Ω

OP176

R2

10k

Ω

1k

Ω*

3500ppm/

°C

SSM2018T OR SSM2118T

CONTROL

VOLTAGE

Figure 43. Current Source Allows Temperature Compen-

sation with One TCR

One of the resistors in the divider to the control port can be sub-

stituted with an appropriately chosen TCR to compensate the

SSM2018T or the SSM2118T as shown in Figure 42. Because

the resistor divider effectively cuts the temperature coefficient in

half, two TCRs must be used. The combined drift of the two is

7000 ppm/

°C, given an effective drift for to the control voltage

of –3500 ppm/

°C. Of course, a single TCR with the appropriate

coefficient can be used. The 3500 ppm parts were chosen be-

cause they are a standard item and do not need to be special

ordered.

In many applications, an op amp is used to drive the control

voltage. If this is the case, it may be more economical to use the

op amp and a single TCR for temperature compensation. The

op amp is configured as a Howland current source as shown in

Figure 43. The current then flows through a single TCR to

create the control voltage. Because the resistor divider is not

present, the temp coefficient is equivalent to the TCR’s coef-

ficient. Using this technique, the drift was reduced from

–3500 ppm/

°C to –150 ppm/°C, which results in a total com-

pensated gain shift of 0.4 dB over the full temperature range at a

gain of

±20 dB.

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 44

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

(or SSM2118T). The DAC8512 is a complete 12-bit converter

in an 8-pin package. It includes an on board reference and a

output amplifier to produce an output voltage from 0 V to

+4.095 V, which is 1 mV/bit. Since the voltage is always posi-

tive, this circuit only provides attenuation. The resistor divider

on the output of the DAC8512 is set to scale the output voltage