Project: 10 MHz, Lumped Element, Anti-Aliasing Filter Design




Design Services Performed:


This classical, 5 stage, 50 ohm, Anti-Aliasing filter was designed for a high performance 40 MHz, 12 bit sampling A/D converter. During the design phase, careful models were made of the inductor and capacitor Q's so that the design could be optimized without having to resort to many trial and error breadboards. One breadboard was made to characterize the components and a second breadboard was done to finalize the design. Monte Carlo analysis was preformed to verify the design over temperature and component variations. Similar filters can be built with discrete, lumped components well into the GHz range. Careful component selection and characterization is the key to passive, lumped element filter design. Lumped element filters while old technology still offer the ultimate in low noise and ultimate rejection. OPAMP's will never have as low a noise floor as basic resistors and capacitors. In fact the lowly inductors used here have a dynamic range approaching 180 dB - from the resistor noise of the inductor to the saturation of the core. These designs are going to be hard to match with any active filter topology.

  • Anti-Alias filter for high performance 12 bit, 40 Megasample A/D converter system.
  • DC-10 MHz passband with less than 0.5 dB ripple with clean rolloff to 3 dB bandedge.
  • Low group delay variation for minimum ringing.
  • Stopband rejection > 72 dB from 14 MHz to 50 MHz.
  • 50 Ohm I/O impedance (other impedances possible) with > 15 dB return loss in passband.
  •  Transmission simulation and Monte Carlo variation analysis to ensure a repeatable design.
  • Environmentally rugged design, 0 to 70 Deg C operating range with minor performance degradation.
  • Low cost, easy to manufacture, minimum parts count, small board space when integrated into a system layout .
  • Fast cycle time.
  • Also available - Elliptic Lowpass Filters, Bandpass filters and Highpass filter designs.


A transmission (S21) plot of the filter as designed and tested. Passband extends from DC-10 MHz with the stopband extending from 14 to 50 MHz. Fast design time was achieved because of the accurate models of the inductors and capacitors that were developed by careful measurement of actual parts. A little time up front always saves many hours later!

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