Hello all,
I recently learned/heard that when implementing a IIR digital filter of direct form I, the gain has to be finely adjusted with an additionnal module on the output. Otherwise, the 0dB level will not be perfectly reached in your passband (filter is low pass) and e.g. consequently a digital value that is suppose to be 40000.0000 will be
4000.0001. Essentially, I imagine this output module to be a multiplier of 1.0000000XXXX or 0.99999XXXI was led to believe this problem does not arise with digital filter implementations of direct form II.
Is this true ? Looking over the flow diagrams of the 2 type of filters, I do not understand why one form would require a gain adjustement and the other would not.
P.S The filter is being implemented in an FPGA, not in a DSP. P.S The passband ripple is tiny, below 1e-4dB
All help will be appreciated. Regards
-Roger