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!
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
- DC-10 MHz passband with
less than 0.5 dB ripple with clean rolloff to 3 dB
- 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
simulation and Monte Carlo variation analysis to ensure a
- Environmentally rugged
design, 0 to 70 Deg C operating range with minor
- 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