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Dear all, A very basic question actually. Just found an issue with IIR low pass filter in decimation low pass chain after a 3rd order SINC filter in a Sigma Delta A/D system. The one bit input code from Sigma Delta modulator is 1010101010... forever which is supposed to produce all 0 PCM code. However, SINC decimation after the modulator filter went through a period of transient stage (with a few non-zero output) then its output settles to be all zeros. The IIR low pass filter following the SINC filter takes a hit from the very short period of non-zero input from SINC then always spelling out non-zero values even when SINC filters settles to zero output. This is very anonying since IIR low pass filter is supposed to output all ZERO PCM code. But it doesn't coz it is IIR and it can't recover from any unwanted input. Can any of you shed some lights on how to make this SINC+IIR decimation filters work? Or is IIR so frigile that it can't overcome any errant input happened centuary ago without a reset? Thank a bunch, Gordon |