Understanding converter specifications: bandwidth first

22nd October 2014

Analog Devices

Nat Bowers

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When starting a new design, the first parameter that must be chosen is bandwidth. Three types of front ends can be used, depending on the application: baseband, bandpass (or super-Nyquist or narrowband) and wideband, as shown in the figure (left).

By Rob Reeder, Senior System Application Engineer, Analog Devices.

Baseband designs require a bandwidth from DC (or low MHz) to the Nyquist frequency, typically about 100MHz or less. Designs like these can employ an amplifier or transformer (balun).

A bandpass design means a small piece of the converter’s bandwidth is going to be used at a high IF, about 20-60MHz. The centre frequency can be as low as a 100MHz, but most cases is at 140, 170, or 190MHz. Even higher IFs can be used with newer GSPS converter offerings. Typically a transformer or balun is used here. However an amplifier can be used if the dynamic performance is still adequate at the higher frequencies.

A wideband design usually refers to those designs that need as much performance as the converter can provide. These are the most challenging front-end designs since they have the widest bandwidth of the three. These applications require DC (or low MHz regions) to the GHz region, and often employ a wideband balun.

Keep in mind that a converter’s full-power bandwidth is different than its useable or sample bandwidth. The full-power bandwidth is needed to acquire signals accurately and to properly settle the internal front end. Selecting an IF in this region is not a good idea since performance results will vary widely in the system.

Based on the rated resolution and performance stated in the converter’s data sheet, the full-power bandwidth is much bigger than the sample bandwidth, and could possibly be twice as big. The design should use the sample bandwidth, not the highest frequency portions of the rated full-power bandwidth to avoid derating the dynamic performance (SNR/SFDR). The data sheet typically specifies production-tested frequencies that guarantee performance within the converter’s sample bandwidth.

Author Profile: Rob Reeder is a senior system application engineer with Analog Devices in Greensboro, N.C. He has published numerous papers on converter interfaces, converter testing, and analog signal chain design for a variety of applications. Rob received his MSEE and BSEE from Northern Illinois University in DeKalb, Illinois.