The sinc filter is a variable decimation rate, third-order, low-pass filter. Data are supplied to this section of the filter from the modulator at the rate of fMOD. The sinc filter attenuates the high-frequency noise of the modulator, then decimates the data stream into parallel data. The decimation rate affects the overall data rate of the converter.
Equation 7 shows the scaled Z-domain transfer function of the sinc filter.
The frequency domain transfer function of the sinc filter is shown in Equation 8.
The sinc filter has notches (or zeroes) that occur at the output data rate and multiples thereof. At these frequencies, the filter has infinite attenuation. Figure 25 shows the sinc filter frequency response and Figure 26 shows the sinc filter roll-off. With a step change at input, the filter takes 3 tDR to settle. After a START signal rising edge, the filter takes tSETTLE time to give the first data output. The filter settling times at various data rates are discussed in the START subsection of the SPI Interface section. Figure 27 and Figure 28 show the filter transfer function until fMOD / 2 and fMOD / 16, respectively, at different data rates. Figure 29 shows the transfer function extended until 4 fMOD. It can be seen that the ADS1291, ADS1292, and ADS1292R passband repeats itself at every fMOD. The input R-C anti-aliasing filters in the system should be chosen such that any interference in frequencies around multiples of fMOD are attenuated sufficiently.