REV. A

–12–

SSM2018T/SSM2118T

A2

A4

Q3

Q4

Q1

Q2

200

1–G

G

G

1–G

200

1.8k

GAIN

CORE

14

8

5

16

COMP 2

COMP 1

9

Im

SPLITTER

3

1

15

4

2

11

13

12

BAL

GND

MODE

COMP 3

7

6

10

V–

+IN

–IN

V+

COMPENSATION

NETWORK

Im–(Is)

2

Im+(Is)

2

A4

–0.1 V to +6.0 V. The negative compliance is much smaller be-

cause the gain core transistors (Q1 and Q3) begin to saturate

when the collector potential is brought below their base poten-

tial. These outputs have high immunity to capacitive loads. In

fact, the load on either or both outputs can be as large as 10 nF

with no change in the distortion performance. For values above

10 nF, the distortion does start to increase. For example, a

100 nF load causes the distortion to increase from 0.006% to

0.02% at 1 kHz.

The noise performance of a single SSM2118T with an OP275

output amplifier is shown in Figure 20. When multiple

SSM2118T parts are operated in parallel, the noise does in-

crease by a factor equal to the square root of the number of

parts paralleled. For example, if five parts are in parallel, the

total output noise is 100 nV

√(Hz) × √5 = 220 nV/√Hz.

Compensating the SSM2018T and SSM2118T

Both parts employ the same compensation network. This net-

work uses an adaptive compensation scheme that adjusts the op-

timum compensation level for a given gain. The control voltage

not only adjusts the gain core steering, it also adjusts the com-

pensation. The SSM2018T and SSM2118T have three com-

pensation pins: COMP1, COMP2, and COMP3. COMP3 is

normally left open. Grounding this pin actually defeats the

adaptive compensation circuitry, giving the VCA a fixed com-

pensation point. The only time that this is desirable is when the

VCA has fixed feedback, such as the Voltage Controlled Panner

(VCP) circuit shown later in the data sheet. Thus, for the Basic

VCA circuit or the OVCE circuit, COMP3 should be left open.

A compensation capacitor does need to be added between

COMP1 and COMP2. Because the VCA operates over such a

wide gain range, ideally the compensation should be optimized

for each gain. When the VCA is in high attenuation, there is

very little “loop gain,” and the part needs to have high compen-

sation. On the other hand, at high gain, the same compensation

capacitor would overcompensate the part and roll off the high

frequency performance. Thus, the SSM2018T and SSM2118T

employ a patented adaptive compensation circuit. The compen-

sation capacitor is “Miller” connected between the base and col-

lector of an internal transistor. By changing the gain of this

transistor via the control voltage, the compensation is changed.

Increasing the compensation capacitor causes the frequency re-

sponse and slew rate to decrease, which will tend to cause high

frequency distortion to increase. For the basic VCA circuit,

47 pF was chosen as the optimal value. The OVCE circuit de-

scribed later uses a 220 pF capacitor. The reason for the in-

crease is to compensate for the extra phase shift from the

additional output amplifier used in the OVCE configuration.

The compensation capacitor can be adjusted over a practical

range from 47 pF to 220 pF, if desired. Below 47 pF, the parts

may oscillate, and above 220 pF the frequency response is sig-

nificantly degraded.

Control Section

As mentioned before, the control voltage on Pin 11 steers the

current through the gain core transistors to set the gain. The

output gain formula is as follows:

V

(–aV

The exponential term arises from the standard Ebers-Moll

equation describing the relationship of a transistor’s collector

current as a function of the base-emitter voltage:

I

(V

.

due to the resistor divider of the 200

Ω and 1.8 kΩ resistors

shown in Figures 38 and 40. The resulting expression for “a” is

as follows: a = 1/(10

× V

at room temperature. Substituting a = 4 in the above equation

results in a –28.8 mV/dB control law at room temperature.

The –28.8 mV/dB number is slightly different from the data

sheet specification of –30 mV/dB. The difference arises from

is known as the thermal voltage, and it has a direct dependency

Figure 40. SSM2118T Detailed Functional Diagram