FIR Filter II, not able to get unity gain

Hi Chee,

thanks for your reply.

My filter is working fine in Matlab. What I am trying to figure out is how to use the FIR II IP correctly: I.e. how to select the 24bit output range from the filter which gives me maximum dynamic without overflow.

The DC (zero Hz) gain of the filter equals the sum of the filter taps (SUM(h(n)). If SUM(h(n)) = 1 then filter has gain=1 at DC. This is the case for my filter.

In FIR II I have this setup:

24bit input

24bit coefficients

Number of coefficients: 26

With these numbers in FIR II the full output range is 53.

I assume this is based on: 24 + 24 + ceil(log2(26)) = 24 + 24 + 5 = 53. Is this correct?

I notice in FIR II that if one enables the auto-scale the center tap is scaled to 2^23 when using 24bit filter taps. SUM(h(n)) then typical gets larger than 2^24.

In my case the sum of the filter coefficients after the FIR II auto scaling is approximately: +29E6

Assuming a range for both + and - values this adds 2 bits. (26bit : +/- 33E6)

Based on these numbers the 24bit output range with max resolution without overflow is:

MSB to remove: 3 (5-2)

LSB to remove: 26 (53-3-24)

Is this the correct understanding of the FIR II IP filter?

BR

AD