Hello all, 

A simple question actually. Just found an issue with IIR low pass 
filter in decimation low pass after the 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