Designers are finding new techniques to improve system performance with Mini-Circuits’ patented reflectionless filters all the time. Among these, supplementing anti-alias filters in wideband ADCs to minimize the effect of switching transients has recently attracted interest.

During the sample-and-hold or track-and-hold operations of an ADC, the switching action of the input stage creates a rapid change in the load conditions for the driving circuit. These switching transients may also generate pulses that back-propagate through the system. In wideband ADCs, these switching transients and other non-linear products may occur at frequencies well beyond a traditional anti-aliasing filter’s operating frequency range. As these filters aren’t intended to be true broadband devices, the impedance matching at these higher frequencies may be poor enough to yield reflections of the switching transients and non-linear products back to the ADC input, create a standing wave and otherwise impact the ADC’s output performance (spur-free dynamic range and noise figure).

In this case, reflectionless filters can be employed at the input of the ADC to absorb the out-of-band signals and mitigate the effect of switching transients. Reflectionless filters can be added to both differential inputs of the ADC, eliminating differential and common-mode switching transients (see figure below). If greater selectivity is required than what the typical MMIC reflectionless filters can offer, a traditional anti-aliasing filter with higher Q elements or cascaded reflectionless filters could be added in series.

Simple schematic depicting how dual-matched reflectionless filters can be used to improve ADC anti-alias filtering.

This use case is presented in concept only and implementation may vary depending on the user’s specific requirements. If you have questions about this or other applications for reflectionless filters, contact apps@minicircuits.com to talk with one of our engineers.

Check out Mini-Circuits’ full portfolio of reflectionless high pass, low pass and band pass filters >