Developing an Ultra High Intercept Last Gain Stage to a 14-Bit High SFDR ADC (Part 1 of 2)

One common RF receiver chain mixes down to a 70MHz center frequency using low cost SAW filters to define the bandwidth out of the last mixer. This relatively low signal level SAW filter output must then be raised up to be a large part of an ADC input range for digitization. It would be desirable to provide this last gain stage with very low 3rd order intermodulation (IM3) spurious to keep closely spaced carriers from creating false signals on top of desired signals in the final FFT.

While ADCs have continued to improve in their IM3 characteristics, systems of this type have often depended on relatively high power RF amps (>500mW) to provide adequate IM3 in this last stage. Recent amplifier advances have opened a path to very low power solutions where the amplifier IM3 is well below the mid -80dBc to -90dBc performance found in emerging low power ADCs.

If the IM3 is acceptable, to complete the system design, it would be desirable to include a low order bandpass filter to limit the wideband noise and other distortion terms that might be added by this last stage. While the IM3 tones are often very close to the desired signals and will get no filtering, the IM2 and simple harmonic terms can be far away from the desired bandpass edges and perhaps only modest filtering is required.

Part 1 will describe the individual ADC and amplifier characteristics, show a proposed 19dB gain interface circuit including a 2nd order RLC filter between the amplifier and ADC. Part 2 will describe some simple circuit changes to improve the single tone results and then show tested 2-tone Intermodulation (IM3) performance. Making these super low IM3 measurements (-110dBc for the amplifier itself) can benefit from some extra effort on the input signal and those approaches will be described in detail. Read part 1 here.

Michael Steffes is Sr. Applications Manager and Jian Wang is Lead Applications Engineer in the High Speed Signal Path Products at Intersil Corp.

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