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SSM2018T Datasheet(PDF) 8 Page - Analog Devices |
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SSM2018T Datasheet(HTML) 8 Page - Analog Devices |
8 / 16 page REV. B –8– SSM2018T normal and actually disables that output amplifier ensuring that it will not oscillate and cause interference problems. Shorting the output to the negative supply does not cause the supply current to increase. This amplifier is only used in the “OVCE” application explained later. The control port follows a –30 mV/dB control law. The applica- tion circuit shows a 3 k W and 1 kW resistor divider from a control voltage. The choice of these resistors is arbitrary and could be any values to properly scale the control voltage. In fact, these resistors can be omitted if the control voltage has been properly scaled. The 1 mF capacitor is in place to provide some filtering of the control signal. Although the control feedthrough is trimmed at the factory, the feedthrough increases with frequency (TPC 16). Thus, high frequency noise can feed through and add to the noise of the VCA. Filtering the control signal helps minimize this noise source. Theory of Operation of the SSM2018T The SSM2018T has the same internal circuitry as the original SSM2018. The detailed diagram in Figure 2 shows the main components of the VCA. The essence of the SSM2018T 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. Notice that the bases of Q2 and Q3 are connected to ground via a 200 W 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 k W 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. 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 SSM2018T The SSM108 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 SSM2018T has three compensation pins: COMP1, COMP2, and COMP3. COMP3 is normally left open. Grounding this pin actually defeats the adap- tive 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 APPLICATIONS The SSM2018T is a trimless Voltage Controlled Amplifier (VCA) for volume control in audio systems. The SSM2018T is identi- cal to the original SSM2018 in functionality and pinout; how- ever, it is the first professional quality audio VCA in the marketplace that does not require an external trimming potenti- ometer to minimize distortion. Instead, the SSM2018T is laser trimmed before it is packaged to ensure the specified THD and control feedthrough performance. This has a significant savings in not only the cost of external trimming potentiometers, but also the manufacturing cost of performing the trimming during production. Basic VCA Configuration The primary application circuit for the SSM2018T is the basic VCA configuration, which is shown in Figure 1. This configura- tion uses differential current feedback to realize the VCA. A complete description of the internal circuitry of the VCA, and this configuration, is given in the Theory of Operation section below. The SSM2018T is trimmed at the factory for operation in the basic VCA configuration with class AB biasing. Thus, for optimal distortion and control feedthrough performance, the same con- figuration and biasing should be used. All of the graphs for the SSM2018T in the data sheet have been measured using the circuit of Figure 1. V+ 1 F RB 150k 18k V+ 18k VIN+ 1 F 18k VIN– 47pF 1 F 50pF 1k VCONTROL 3k VOUT V– SSM2018T Figure 1. Basic VCA Application Circuit In the simple VCA configuration, the SSM2018T inputs are at a virtual ground. Thus, 18 k W resistors are required to convert the input voltages to input currents. The schematic also shows ac coupling capacitors. These are inserted to minimize dc offsets generated by bias current through the resistors. Without the capacitors, the dc offset due to the input bias current is typically 5 mV. The input stage has the flexibility to run either inverting, noninverting, or balanced. The most common configuration is to run it in the noninverting single-ended mode. If either input is unused, the associated 18 k W resistor and coupling capacitor should be removed to prevent any additional noise. The common-mode rejection in balanced mode is typically 55 dB up to 1 kHz, decreasing at higher frequencies as shown in TPC 21. To ensure good CMRR in the balanced configuration, the input resistors must be balanced. For example, a 1% mis- match results in a CMRR of 40 dB. To achieve 55 dB, these resistors should have an absolute tolerance match of 0.1%. The output of the basic VCA is taken from Pin 14, which is the output of an internal amplifier. Notice that the second voltage output (Pin 16) is connected to the negative supply. This is |
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