What's the (Converter) Frequency Kenneth?

Q. How Do I Design a Converter Front-end without Compromising the Performance?

What's the (Converter) Frequency Kenneth?

A.  Designers that employ a converter for high-frequency sampling have to face many challenges. Designing a front-end isn't simple, but the following comments can guide the designer to a solution.

Designers can choose amongst three types of front-ends: baseband, narrowband, or wideband; the application determines which should be applied.

Baseband applications require bandwidth from dc or the low MHz to the Nyquist frequency of the converter. In terms of relative bandwidth, this implies about 100 MHz or less. These designs can employ either an amplifier or a transformer (balun).

Narrowband applications (narrow being relative to the ADC's full Nyquist bandwidth) usually operate at high intermediate frequencies (IF). They typically use only 5 to 20 MHz of bandwidth in the 2nd or 3rd Nyquist zone, with a center frequency ≥190 MHz. The design only needs a portion of the Nyquist bandwidth, but the unused bandwidth is often needed to implement an anti aliasing filter. A transformer or balun is typically used for these applications, but an amplifier can be used if its performance is adequate at these frequencies.

Wideband designs need it all, with the user taking as much as the converter will supply. These designs have the widest bandwidth, making the frontend design the most challenging of the three types. These applications require bandwidth from dc or low MHz to several GHz. Currently, these designs typically employ a wideband balun, but amplifiers are catching up in bandwidth and performance.

After choosing the converter, choose the front-end amplifier (active) or transformer (passive). The tradeoffs between the two are many and depend on the application, but can be distilled to a few points. Amplifiers add noise, require a power supply, and burn power, but they are not gain bandwidth dependent like a transformer. Also, they have better gain flatness within the pass band region. Transformers are passive, so they don't add noise or burn power, but their asymmetrical behavior can cause spurious issues. Transformers are not ideal devices; if not used properly their parasitics can undermine any design, particularly at higher frequencies (>100 MHz).

Hopefully, this advice will keep the design on track. For additional information, please refer to the references or send me an email.

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Contributing Writer Rob Reeder is a senior converter applications engineer working in Analog Devices high-speed converter group in Greensboro, NC since 1998. Rob received his MSEE and BSEE from Northern Illinois University in DeKalb, IL in 1998 and 1996 respectively. Rob Reeder

For more information:

Design Tools:

AD9204/AD9231/AD9251 S-Parameter Data (xls)
AD9215LFCSP Analog Input S-Parameter Data (xls)
AD9212/19/22/28/52/59/87 LFCSP Analog Input S-Parameter Data (xls)
AD9226LQFP Analog Input S-Parameter Data (xls)
AD9233/46 LFCSP Analog Input S-Parameter Data (xls)
AD9235LFCSP Analog Input S-Parameter Data (xls)
AD9236LFCSP Analog Input S-Parameter Data (xls)
AD9237LFCSP Analog Input S-Parameter Data (xls)
AD9244LQFP Analog Input S-Parameter Data (xls)
AD9245LFCSP Analog Input S-Parameter Data (xls)
AD9258/AD9268 S-Parameter Data (xls)

Application Notes:

AN-827: A Resonant Approach to Interfacing Amplifiers to Switched-Capacitor ADCs (pdf)
AN-935: Designing an ADC Transformer-Coupled Front End (pdf)


Designing Transformer Coupled Front-Ends for High Performance A/D Converters

Analog Dialogue:

Transformer-Coupled Front-End for Wideband A/D Converters (April 2005)
Wideband A/D Converter Front-End Design Considerations – When to Use a Double Transformer Configuration (July 2006)
Wideband A/D Converter Front-End Design Considerations II - Amplifier- or Transformer Drive for the ADC? (February 2007)

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