DocID024597 Rev 3 619/1693
RM0351 Digital filter for sigma delta modulators (DFSDM)
657
21.3.8 Digital filter configuration
DFSDM contains a Sinc
x
type digital filter implementation. This Sinc
x
filter performs an input
digital data stream filtering, which results in decreasing the output data rate (decimation)
and increasing the output data resolution. The Sinc
x
digital filter is configurable in order to
reach the required output data rates and required output data resolution. The configurable
parameters are:
• Filter order/type: (see FORD[2:0] bits in DFSDMx_FCR register):
–FastSinc
–Sinc
1
–Sinc
2
–Sinc
3
–Sinc
4
–Sinc
5
• Filter oversampling/decimation ratio (see FOSR[9:0] bits in DFSDMx_FCR register):
– FOSR = 1-1024 - for FastSinc filter and Sinc
x
filter x = F
ORD
= 1..3
– FOSR = 1-215 - for Sinc
x
filter x = F
ORD
= 4
– FOSR = 1-73 - for Sinc
x
filter x = F
ORD
= 5
The filter has the following transfer function (impulse response in H domain):
• Sinc
x
filter type:
• FastSinc filter type:
Figure 152. Example: Sinc
3
filter response
Hz()
1z
FOSR–
–
1z
1–
–
-----------------------------
⎝⎠
⎜⎟
⎛⎞
x
=
Hz()
1z
FOSR–
–
1z
1–
–
-----------------------------
⎝⎠
⎜⎟
⎛⎞
2
1z
2FOSR⋅()–
+()⋅=
1RUPDOL]HGIUHTXHQF\I,1I'$7$
*DLQG%
069
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