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VCA2612 Datasheet(PDF) 11 Page - Texas Instruments |
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VCA2612 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 25 page  VCA2612 11 SBOS117C www.ti.com VCA—OVERVIEW The magnitude of the differential VCA input signal (from the LNP or an external source) is reduced by a programmable attenuation factor, set by the analog VCA Control Voltage (VCACNTL) at pin 43. The maximum attenuation factor is further programmable by using the three MGS bits (pins 40- 42). Figure 3 illustrates this dual-adjustable characteristic. Internally, the signal is attenuated by having the analog VCACNTL vary the channel resistance of a set of shunt- connected FET transistors. The MGS bits effectively adjust the overall size of the shunt FET by switching parallel components in or out under logic control. At any given maximum gain setting, the analog variable gain characteris- tic is linear in dB as a function of the control voltage, and is created as a piecewise approximation of an ideal dB-linear transfer function. The VCA gain control circuitry is common to both channels of the VCA2612. FIGURE 1. Simplified Block Diagram of the VCA2612. FIGURE 2. Recommended Circuit for Coupling an External Signal into the VCA Inputs. 0 –24 –45 Control Voltage 0 Maximum Attenuation Minimum Attenuation 3.0V FIGURE 3. Swept Attenuator Characteristic. THEORY OF OPERATION The VCA2612 is a dual-channel system consisting of three primary blocks: a Low Noise Preamplifier (LNP), a Voltage Controlled Attenuator (VCA), and a Programmable Gain Amplifier (PGA). For greater system flexibility, an onboard multiplexer is provided for the VCA inputs, selecting either the LNP outputs or external signal inputs. Figure 1 shows a simplified block diagram of the dual-channel system. LNP—OVERVIEW The LNP input may be connected to provide active-feedback signal termination, achieving lower system noise perfor- mance than conventional passive shunt termination. Even lower noise performance is obtained if signal termination is not required. The unterminated LNP input impedance is 600k Ω. The LNP can process fully differential or single- ended signals in each channel. Differential signal processing results in significantly reduced 2nd-harmonic distortion and improved rejection of common-mode and power supply noise. The first gain stage of the LNP is AC-coupled into its output buffer with a 44 µs time constant (3.6kHz high-pass charac- teristic). The buffered LNP outputs are designed to drive the succeeding VCA directly or, if desired, external loads as low as 135 Ω with minimal impact on signal distortion. The LNP employs very low impedance local feedback to achieve stable gain with the lowest possible noise and distortion. Four pin-programmable gain settings are available: 5dB, 17dB, 22dB, and 25dB. Additional intermediate gains can be programmed by adding trim resistors between the Gain Strap programming pins. The common-mode DC level at the LNP output is nominally 2.5V, matching the input common-mode requirement of the VCA for simple direct coupling. When external signals are fed to the VCA, they should also be set up with a 2.5VDC common-mode level. Figure 2 shows a circuit that demon- strates the recommended coupling method using an external op amp. The VCM node shown in the drawing is the VCM output (pin 19). Typical R and C values are shown, yielding a high-pass time constant similar to that of the LNP. If a different common-mode referencing method is used, it is important that the common-mode level be within 10mV of the VCM output for proper operation. V CM (+2.5V) 1k Ω 1k Ω 47nF To VCA IN Input Signal VCA LNP Channel A Input VCA Control PGA Channel A Output External In A Maximum Gain Select MGS Analog Control VCA LNP Channel B Input PGA Channel B Output External In B |
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Similar Description - VCA2612 |
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