REV. A

–13–

SSM2018T/SSM2118T

1.38E-23, q = electron charge = 1.6E-19, and T = absolute

temperature in Kelvin). This temperature dependency leads to

the –3500 ppm/

°C drift of the control law. It also means that

the control law changes as the part warms up. Thus, our speci-

fication for the control law states that the part has been powered

up for 60 seconds.

When the part is initially turned on, the temperature of the die

is still at the ambient temperature (25

°C for example), but the

power dissipation causes the die to warm up. With

±15 V sup-

plies and a supply current of 11 mA, 330 mW is dissipated.

This number is multiplied by

θ

die’s temperature. In this case, the die increases from 25

°C to

approximately 50

°C. A 25°C temperature change causes a

8.25% increase in the gain constant, resulting in a gain constant

of 30 mV/dB. The graph in Figure 31 shows how the gain con-

stant varies over the full temperature range.

Proper Operating Mode for the SSM2018T and SSM2118T

Both parts have the flexibility of operating in either Class A or

Class AB. This is accomplished by adjusting the amount of cur-

trade-off between the two classes is that Class A tends to have

lower THD but higher noise than Class AB. However, by utiliz-

ing well matched gain core transistors, distortion compensation

circuitry, and laser trimming, the SSM2018T and SSM2118T

have excellent THD performance in Class AB. Thus, the parts

offer the best of both worlds in having the low noise of Class AB

with low THD.

Because the parts operate optimally in Class AB, the distortion

trim is performed for this class. To guarantee conformance to the

data sheet THD specifications, both the SSM2018T and SSM2118T

must be operated in Class AB. This does not mean that the parts

cannot be operated in Class A, but the optimal THD trim point

is different for the two classes. Using Class A operation results

in a shift of THD performance from a typical value of 0.006%

to 0.05% without trim. An external potentiometer could be

added to change the trim back to its optimal point as shown in

the OVCE application circuit, but this adds the expense and

time in adjusting a potentiometer.

MODE input (Pin 12). For class AB operation with

±15 V

µA. For other supply voltages, adjust the value of R

current remains at 95

µA. This current follows the formula:

I

(V

R

B

The factor of 0.7 V arises from the fact that the dc bias on Pin

12 is a diode drop above ground.

Output Drive

The SSM2018T 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 k

Ω load.

(Note: This 100 k

Ω load is in parallel with the feedback resistor

of 18 k

Ω, 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 3. As the graph of output

swing versus load resistance shows (Figure 10), to maintain less

than 1% distortion, the output current should be limited to

approximately

±1.3 mA. If higher current drive is required,

then the output should be buffered with a high quality op amp

such as the OP176 or 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

SSM2018T by the use of a 50

Ω series resistor.

Upgrading SSM2018 Sockets

The SSM2018T easily replaces the SSM2018 in the basic VCA

configuration. The parts are pin for pin compatible allowing di-

rect replacement. At the same time, the trimming potentiom-

eters for symmetry and offset should be removed, as shown in

Figure 41. Upgrading to the SSM2018T immediately saves the

expense of the potentiometers and the time in production of

trimming for minimum distortion and control feedthrough.

18k

Ω

50pF

V+

47pF

NC

1µF

1k

Ω

3k

Ω

V–

V+

1µF 18kΩ

1µF 18kΩ

NC = NO CONNECT

1

2

3

4

5

6

7

8

16

15

14

13

12

10

9

SSM2018T

11

470k

Ω

500k

Ω

100k

Ω

10M

Ω

OFFSET

TRIM

V+

V–

SYMMETRY

TRIM

REMOVE FOR SSM2018T

Figure 41. Upgrading SSM2018 Sockets

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

SSM2018T can still directly replace it. However, the potenti-

ometers cannot necessarily be removed, as explained in the

OVCE and VCP sections.

Temperature Compensation of the Gain Constant

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

°C tem-

perature 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 that the gain constant be the

same over a wide temperature range, then external temperature

compensation should be employed. The simplest form of com-

pensation is a temperature compensating resistor (TCR), such

as the PT146 from Precision Resistor Co. These elements are

different from a standard thermistor in that they are linear over

temperature to better match the linear drift of the gain constant.