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VCA2616 Datasheet(PDF) 10 Page - TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS |
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VCA2616 Datasheet(HTML) 10 Page - TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS |
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10 / 23 page  VCA2616, VCA2611 10 SBOS234E www.ti.com the circuit. This reduces the susceptibility to power-supply variation, ripple, and noise. In addition, separate power supply and ground connections are provided for each chan- nel and for the reference circuitry, further reducing interchannel crosstalk. Further details regarding the design, operation, and use of each circuit block are provided in the following sections. LOW-NOISE PREAMPLIFIER (LNP)—DETAIL The LNP is designed to achieve a low-noise figure, espe- cially when employing active termination. Figure 4 is a simplified schematic of the LNP, illustrating the differential input and output capability. The input stage employs low resistance local feedback to achieve stable low-noise, low- distortion performance with very high input impedance. Nor- mally, low noise circuits exhibit high power consumption as a result of the large bias currents required in both input and output stages. The LNP uses a patented technique that combines the input and output stages such that they share the same bias current. Transistors Q4 and Q5 amplify the signal at the gate-source input of Q4, the +IN side of the LNP. The signal is further amplified by the Q1 and Q2 stage, and then by the final Q3 and RL gain stage, which uses the same bias current as the input devices Q4 and Q5. Devices Q6 through Q10 play the same role for signals on the –IN side. The differential gain of the LNP is given in Equation 1: Gain R R L S =× 2 FIGURE 4. Schematic of the Low-Noise Preamplifier (LNP). (1) PGA OVERVIEW AND OVERALL DEVICE CHARACTERISTICS The differential output of the VCA attenuator is then amplified by the PGA circuit block. This post-amplifier is programmed by the same MGS bits that control the VCA attenuator, yielding an overall swept-gain amplifier characteristic in which the VCA × PGA gain varies from 0dB (unity) to a program- mable peak gain of 24-, 27-, 30-, 33-, 36-, 39-, 42-, or 45dB. The Gain vs VCACNTL curve in the Typical Characteristics shows the composite gain control characteristic of the entire VCA2616. Setting VCACNTL to 3.0V causes the digital MGS gain control to step in 3dB increments. Setting VCACNTL to 0V causes all the MGS-controlled gain curves to converge at one point. The gain at the convergence point is the LNP gain less 6dB, because the measurement setup looks at only one side of the differential PGA output, resulting in 6dB lower signal amplitude. ADDITIONAL FEATURES—OVERVIEW Overload protection stages are placed between the attenua- tor and the PGA, providing a symmetrically clipped output whenever the input becomes large enough to overload the PGA. A comparator senses the overload signal amplitude and substitutes a fixed DC level to prevent undesirable overload recovery effects. As with the previous stages, the VCA is AC-coupled into the PGA. In this case, the coupling time constant varies from 5 µs at the highest gain (45dB) to 59 µs at the lowest gain (25dB). The VCA2616 includes a built-in reference, common to both channels, to supply a regulated voltage for critical areas of R L 93 Ω R S1 105 Ω Q 3 Q 4 Q 5 Q 2 Q 1 R S2 34 Ω R S3 17 Ω LNP GS2 LNP INP LNP INN To Bias Circuitry LNP GS1 LNP GS3 R L 93 Ω COMP2A V DD COMP1A LNP OUTNLNPOUTP Buffer Buffer Q 8 Q 7 To Bias Circuitry Q 6 Q 9 Q 10 R W R W C COMP 4.7pF (External Capacitor) |
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Similar Description - VCA2616 |
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