SSM2018 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
SSM2018 Datasheet PDF : 16 Pages
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SSM2018
Data Sheet
THEORY OF OPERATION
The SSM2018 has the same internal circuitry as the original
SSM2018. The detailed diagram in Figure 28 shows the main
components of the VCA. The essence of the SSM2018 is the
gain core, which comprises two differential pairs (Q1–Q4).
When the control voltage, VC, is adjusted, current through the
gain core is steered to one side or the other of the two differential
pairs. The tail current for these differential pairs is set by the
mode bias of the VCA (Class A or AB), which is labeled as IM in
the diagram. IM is then modulated by a current proportional to
the input voltage, labeled IS. For a positive input voltage, more
current is steered (by the splitter) to the left differential pair; the
opposite is true for a negative input.
To understand how the gain control works, a simple example is
best. Take the case of a positive control voltage on Pin 11. Note
that the bases of Q2 and Q3 are connected to ground via a 200
Ω resistor. A positive control voltage produces a positive voltage
on the bases of Q1 and Q4. Concentrating on the left-most
differential pair, this raises the base voltage of Q1 above that of
Q2. Thus, more of the tail current is steered through Q1 than
through Q2. The current from the collector of Q2 flows through
the external 18 kΩ feedback resistor around amplifier A3.
When this current is reduced, the output voltage is also reduced.
Thus, a positive control voltage results in an attenuation of the
input signal, which explains why the gain constant is negative.
V+
COMP 2 COMP 1 VG
–IG
2
8
5
14
3
COMPENSATION
A3
NETWORK
The collector currents of Q2 and Q3 produce the output voltage.
The output of Q3 is mirrored by amplifier A1 to add to the
overall output voltage. On the other hand, the collector currents
of Q1 and Q4 are used for feedback to the differential inputs.
Because Pins 6 and 4 are shorted together, any input voltage
produces an input current which flows into Pin 4. The same is
true for the inverting input, which is connected to Pin 1. The
overall feedback ensures that the current flowing through the
input resistors is balanced by the collector currents in Q1 and Q4.
COMPENSATING THE SSM2018
The SSM2018 has a network that uses an adaptive compensation
scheme that adjusts the optimum compensation level for a given
gain. The control voltage not only adjusts the gain core steering,
it also adjusts the compensation. The SSM2018 has three
compensation pins: COMP1, COMP2, and COMP3. COMP3 is
normally left open. Grounding this pin actually defeats the adaptive
compensation circuitry, giving the VCA a fixed compensation
point. The only time 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.
+I1–G
1
A1
A2
15 BAL
4 –I1–G
A4
16 V1–G
–IN 7
+IN 6
V– 10
1–G
G
Q1 Q2
Im
+
(
Is
2
)
200Ω
G
1–G
GAIN CORE
Q3 Q4
1.8kΩ
200Ω
Im
–
(
Is
2
)
SSM2018
SPLITTER
9
COMP 3
Figure 28. Detailed Functional Diagram
VREF
Im
11 VC
13 GND
12 MODE
Rev. C | Page 10 of 16
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