SPRUHM8K December 2013 – May 2024 TMS320F28374D , TMS320F28375D , TMS320F28376D , TMS320F28377D , TMS320F28377D-EP , TMS320F28377D-Q1 , TMS320F28378D , TMS320F28379D , TMS320F28379D-Q1
Both the comparator filter and data filter available in SDFM have the SincN filter as the core. The SincN filter is essentially a low-pass filter that converts the input bit stream into digital data by digital filtering and decimation. This filtered digital data represents analog input given to the sigma delta modulator. Simplified SincN architecture consists of cascaded integrators and differentiators separated by a down-sampler as shown in Figure 14-5. The Z-transfer function of the Sinc filter of order N is shown in Figure 14-6.
Effective resolution of the Sinc filter (ENOB) depends upon filter type, OSR and sigma-delta modulator frequency. Typically, higher resolution or ENOB can be achieved by higher OSR for a given filter type; however, the tradeoff is increased filter delay. It is important to choose the right sigma delta modulator by studying the optimal speed versus resolution tradeoff. Refer to the corresponding sigma delta modulator data sheet to determine the effective resolution for a given Sinc filter configuration. Figure 14-7 shows the frequency response of different filter structures when OSR = 32 and when the sigma delta modulator frequency is 10MHz.
The order of different sinc filter is shown in Table 14-2.
Filter Type | Order of Sinc Filter |
---|---|
Sinc1 | 1 |
Sinc2 | 2 |
Sinc3 | 3 |
SincFast | 3 |