SSM2018TS AD [Analog Devices], SSM2018TS Datasheet - Page 15

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

REV. A
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 46.
The voltage output labeled V
put just as for an op amp’s feedback. The V
fied or attenuated depending upon the control voltage. Because
the OVCE works just like an op amp, the feedback could just 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 47. Notice that the amplifier whose output (Pin 16)
was originally connected to V
back. 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.
If a symmetry trim is to be performed, it should precede the
control feedthrough trim and be done as follows:
1. Apply a 1 kHz sine wave of +10 dBu to the input, with the
2. Adjust the symmetry trim potentiometer to minimize distor-
V
V
IN+
IN–
control voltage set for unity gain.
tion of the output signal.
Figure 45. Single Supply Operation of SSM2018T
1 F 18k
1 F
18k
V+
100k
100k
47pF
V+
1
2
3
4
5
6
7
8
10 F
SSM2018T
50pF
18k
OP176
V+
1–G
MINUS
16
15
14
13
12
11
10
is fed back to the inverting in-
9
is now the output for feed-
1 F
R
B
V
G
OUT
output is ampli-
V+
1k
3k
V
CONTROL
–15–
Next the control feedthrough trim is done as follows:
1. Ground the input signal port and apply a 60 Hz sine wave
2. Adjust the control feedthrough potentiometer to null the sig-
100k
to the control port. The sine wave should have its high and
low peaks correspond to the highest gain to be used in the
application and 30 dB of attenuation, respectively. For ex-
ample, a range of +20 dB gain to 30 dB attenuation requires
that the sine wave amplitude ranges between –560 mV and
+840 mV on Pin 11.
nal seen at the output.
V+
V–
INPUTS
CONTROL
FEEDTHROUGH
TRIM
Figure 46. OVCE Follower/VCA Connection
10M
V
IN
Figure 47. OVCE Application Circuit
500k
220pF
V+
SYMMETRY
TRIM
R
NC = NO CONNECT
1
3
5
2
4
6
7
8
B
: 30k FOR CLASS A
150k FOR CLASS AB
SSM2018T
SSM2018T/SSM2118T
50pF
18k
18k
50pF
V
C
470k
16
15
14
13
12
11
10
9
NC
V–
R
B
1µF
V
V
V+
1–G
G
V
V
G
1k
1– G
3k
V
CONTROL