Filter output value is lower than expected

[Altera_Forum]

Hi, 

 I have a fir compiler with the following settings:  Input Bit Width: 16 Coefficient Scaling: Auto with 16 Bits Filter Type: LP with 37 Coefficients Sample Rate: 50 MHz Cut Off Frequency: 10 MHz Passband Ripple: near 0 dB  A NCO core with a 16 Bit output supplies the filter with a 1 MHz sinus signal. I analyzed the full resolution output (38 Bits) of the filter in Signal Tap II. 6 MSB Bits were identical so I truncated 5 MSB Bits. I also truncated 17 LSB Bits to get 16 Bits output.  But the filter outputs a sinus signal that is attenuated by about -4 dB. In the Frequency Response Tab in fir compiler parameterize GUI it should be near 0 dB. Does anyone have an Idea what the problem could be?

[Altera_Forum]

since you are truncating 17 bits,in effect you are dividing the sum of internal products by 2^17. Thus if you scale your coeffs as: 

 coeff = coeff/sum(coeff) *2^17 then you should get unity dc gain. but what actually happens at 1MHz may be due to your response

[Altera_Forum]

thanks kaz! I scaled the coefficients as you said and now I get the right output signal.