SSM2018TS AD [Analog Devices], SSM2018TS Datasheet - Page 12

• Manufacturer Part Number: SSM2018TS
• Description: Trimless Voltage Controlled Amplifiers
• Manufacturer: AD [Analog Devices]

SSM2018T/SSM2118T
–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)
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
–IN
V+
+IN
V–
16
10
6
7
COMP 2
5 = 220 nV/ Hz.
COMPENSATION
Figure 40. SSM2118T Detailed Functional Diagram
NETWORK
8
COMP 3
9
COMP 1
5
1–G
Im+( Is )
–12–
Q1
2
–I
15
Q2
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:
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:
The factor “a” is a function of not only V
due to the resistor divider of the 200
shown in Figures 38 and 40. The resulting expression for “a” is
as follows: a = 1/(10
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
the temperature dependency of the control law. The term V
is known as the thermal voltage, and it has a direct dependency
G
G
200
SPLITTER
G
+I
14
Q3
G
+I
Im–( Is )
Q4
3
1–G
1–G
2
GAIN
CORE
A2
200
V
V
Im
V
OUT
REF
I
C
T
) which is approximately equal to four
1.8k
I
V
S
IN
e
(V
A4
A4
e
BE
(–aV
/V
and 1.8 k resistors
T
)
C
.
T
13
11
12
1
2
4
)
but also the scaling
BAL
–I
V
GND
MODE
V
C
1–G
1–G
REV. A
T