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FREQUENTLY ASKED QUESTIONS
What kind of high-speed analog-to-digital converters (ADCs) is this analysis appropriate for?
Some newer 12- and 14-bit ADCs have conversion rates through 500 Msamples/s and first Nyquist zone applications exceeding 150 MHz. The challenge is driving them and achieving maximum dynamic range.
In assessing circuit dynamic range, it isn't realistic simply to convert those ADCs' specs for signal-to-noise ratio (SNR) to equivalent number of bits (ENOB). That's because they're based on characterization tests performed using narrow-bandwidth lab filters, while typical applications involve broadband signals of low amplitude that must be amplified to match the input range of the ADC.
This article is about optimizing the analog front end (AFE) of the signal chain to minimize SNR. It's a heuristic analysis based on a rigorous derivation that is available at www.intersil.com/data/rd/isl55210/isl55210_high_dynamic_range_adc_interface_issues.pdf. One outcome of the analysis is the advantage of using a fully differential amplifier (FDA) with certain minimum performance characteristics for the gain stage.
What kind of FDA performance?
Recently, FDAs have become available with spurious-free dynamic range (SFDR) greater than 100 dBc to greater than 100 MHz with less than 1-nV input-referred noise. Such an FDA can provide a very clean differential I/O gain stage if it's carefully interfaced to the ADC in such a way as to limit its output broadband noise spectrum and higher-frequency distortion terms.
What's the basis of the analysis?
The analysis is focused on reducing second-harmonic distortion (HD2). The circuit being analyzed uses a transformer and ac coupling (see the figure).
What are the advantages to using an input transformer?
When the amplifier is the common voltage feedback type (VFA), the signal gain can be higher than the noise gain for the amplifier itself:
Here, the source impedance (matched by Z^sub i^above) is reflected through the transformer as another R^sub g^term on the secondary for the noise gain calculation. Assuming that, the differential voltage noise gain for this configuration is: