39.6.3.2 Decimation Filter

The sigma-delta architecture of the SDADC implies a filtering and a decimation of the bitstream at the output of the SDADC. The decimation filter decimates the bitstream by 64, 128 or 256, 512, 1024. To perform the decimation operation, a 3rd order SINC filter with programmable Over Sampling Ratio is implemented with the following transfer function:

H (z) = \frac{1}{OS R^{3}} {(\sum_{i = 0}^{OSR + - 1} z^{- i})}^{3}

OSR is the Over Sampling Ratio which can be modified to change the output data rate (See CTRLC for the setting of this parameter).

The DC gain of this filter is unity and does not depend on its OSR. However, as it generates a 3rd order zero at (CLK_SDADC_FS / OSR) frequency multiples, its frequency response depends on the OSR parameter. See next section for frequency plots.

Figure 39-3. Spectral Mask of an OSR = 64, CLK_SDADC_FS = 1 MHz, 3rd Order Sinc Filter Overall Response (Continuous Line) and 0–1600Hz Bandwidth Response (Dashed Line)

The zeros of this filter are located at multiples of CLK_SDADC_FS/64.

Figure 39-4. Spectral Mask of an OSR = 128, CLK_SDADC_FS = 1 MHz, 3rd Order Sinc Filter Overall Response (Continuous Line) and 0–1600 Hz Bandwidth Response (Dashed Line)

The zeros of this filter are located at multiples of CLK_SDADC_FS/128.

Figure 39-5. Spectral Mask of an OSR = 256, CLK_SDADC_FS = 1 MHz, 3rd Order Sinc Filter Overall Response (Continuous Line) and 0–1600 Hz Bandwidth Response (Dashed Line)

The zeros of this filter are located at multiples of CLK_SDADC_FS/256.