New 4-Pole Filter from 0.02 Hz to 1.00 Hz in 32-Pin DIP Package Click Here for more information on this product D61 Series 4-pole low-pass and high-pass fixed frequency filters provide low distortion and wide signal-to-noise ratio performance in a small 32-pin DIP footprint. These plug-in ready to use Butterworth and Bessel filter modules are avaialable with corner frequencies (fc) from 0.02 Hz to 1.00 Hz.