SSM2018P Datasheet, PDF (15/16 Pages) Analog Devices – Trimless Voltage Controlled Amplifiers

SSM2018T/SSM2118T
50pF
Next the control feedthrough trim is done as follows:
18k
V_OUT
1. Ground the input signal port and apply a 60 Hz sine wave
to the control port. The sine wave should have its high and
1
16
V+
2
15
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
3
14
that the sine wave amplitude ranges between –560 mV and
4 ^SSM2018T 13
R_B
+840 mV on Pin 11.
1µF _18k
5
12
V+
2. Adjust the control feedthrough potentiometer to null the sig-
V_IN+
V_IN–
6
11
V_CONTROL
3k
nal seen at the output.
7
10
1µF
1k
1µF 18k
8
9
47pF
V_C
V_G
OBSOLETE V+
100k
V+
OP176
100k
10µF
Figure 45. Single Supply Operation of SSM2018T
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_1–G is fed back to the inverting in-
V_IN
V_1–G
Figure 46. OVCE Follower/VCA Connection
V+
100kΩ
CONTROL
FEEDTHROUGH
TRIM
10MΩ
500kΩ
V–
50pF
18kΩ
SYMMETRY
TRIM
50pF
470kΩ
18kΩ
V_1–G
VG
put just as for an op amp’s feedback. The V_G output is ampli-
1
16
fied or attenuated depending upon the control voltage. Because
V+ 2
15
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
3
14
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_MINUS is now the output for feed-
back. As mentioned before, because the SSM2018T is trimmed
for the basic VCA configuration, potentiometers are needed for
INPUTS
4 _SSM2018T 13
5
12
6
11
7
10
RB
V+
3kΩ
1µF
V–
1kΩ
VCONTROL
the OVCE configuration to ensure the best THD and control
8
9 NC
feedthrough performance.
If a symmetry trim is to be performed, it should precede the
control feedthrough trim and be done as follows:
220pF
R_B: 30kΩ FOR CLASS A
150kΩ FOR CLASS AB
NC = NO CONNECT
1. Apply a 1 kHz sine wave of +10 dBu to the input, with the
control voltage set for unity gain.
Figure 47. OVCE Application Circuit
2. Adjust the symmetry trim potentiometer to minimize distor-
tion of the output signal.
REV. A
–15–