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# ADC12J1600: FIR filter details

Hi

In the data sheet 7.3.6.3 it is stated that the output of the FIR filters are 15-bit complex. Is that the output of the final FIR filter or is that the output of each individual filter? How many bits are used between the filters and how are bits removed, dropped, rounded, truncated or saturated?

• Oddbjorn,

The 15-bit complex data is the final output of the final FIR filter. I am looking into your other questions.

Regards,

Jim

• Oddbjorn,

The resolution varies depending on the filter and decimation mode. Each decimation mode uses a cascade of 2 or more decimation filters.  The table below shows the resolution of each filter in the chain, and its decimation (M).

 Total Decimation 1st filter 2nd Filter 3rd Filter 4th Filter 5th Filter 4 M=2, 16-bit output M=2, 15-bit output n/a n/a n/a 8 M=2, 16-bit output M=2, 16-bit output M=2, 15-bit output n/a n/a 10 M=2, 16-bit output M=5, 15-bit output n/a n/a n/a 16 M=2, 16-bit output M=2, 16-bit output M=2, 18-bit output M=2, 15-bit output n/a 20 M=2, 16-bit output M=2, 16-bit output M=5, 15-bit output n/a n/a 32 M=2, 16-bit output M=2, 16-bit output M=2, 17-bit output M=2, 15-bit output n/a

At the output of each filter, saturation is performed.  Rounding also occurs, and the rounding is convergent rounding with ties rounded toward the nearest even output value (to avoid introducing a DC bias).

Regards,

Jim

• Please use this table instead. The other had an error.

 Total Decimation 1st filter 2nd Filter 3rd Filter 4th Filter 5th Filter 4 M=2, 16-bit output M=2, 15-bit output n/a n/a n/a 8 M=2, 16-bit output M=2, 16-bit output M=2, 15-bit output n/a n/a 10 M=2, 16-bit output M=5, 15-bit output n/a n/a n/a 16 M=2, 16-bit output M=2, 16-bit output M=2, 18-bit output M=2, 15-bit output n/a 20 M=2, 16-bit output M=2, 16-bit output M=5, 15-bit output n/a n/a 32 M=2, 16-bit output M=2, 16-bit output M=2, 17-bit output M=2, 18-bit output M=2, 15-bit output