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Fully Differential Feedback Amplifiers (FDA) such as National Semiconductor‘s LMH6550, LMH6551, and LMH6552 are used to provide balanced low distortion amplification and level shifting to wide bandwidth differential signals. A simplified conceptual diagram of an FDA is shown in the following schematics :

Two forward paths amplify the two complementary halves of the differential signal. A separate common mode feedback circuit controlled by the Vcm control input sets the output common mode voltage independent of the input common mode, as well as forcing the On and Op outputs to be equal in magnitude and opposite in phase.

The LMH6552 is a 1.5 GHz device using National Semiconductor’s proprietary differential Current Feedback (CFB) architecture to allow operation at gains greater than unity with exceptional gain flatness, without sacrificing the bandwidth. With 450 MHz of 0.1 dB unity gain flatness the device is ideally suited to driving a range of 8 to 14 bit high-speed ADCs, including National Semiconductor’s Giga Sample 8 bit family and the ADC14DS105, in a variety of wideband Nyquist applications. In designing an FDA to drive an ADC it is required to ensure that the FDA does not degrade the ADC’s signal to noise and distortion (SINAD) performance. A key element of this analysis is determining and optimizing the noise performance of the FDA. The remainder of this article will show how the FDA output noise spectral density and noise figure can be calculated for factoring into the overall system noise analysis. Voltage Feedback (VFB) FDAs have historically been constrained to operating at low gain due to their poor noise performance at higher gains. This article will show that the LMH6552 CFB architectures overcomes this constraint, delivering a noise advantage as well as a gain bandwidth advantage over alternative VFB devices. [Source : National Semiconductor Application Notes]

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