
Digital filter for sigma delta modulators (DFSDM)
RM0351
718/1830
DocID024597 Rev 5
Injected conversions can be launched by software or by a trigger. They are never
interrupted by regular conversions.
The
regular channel
is a selection of just one of the 8 channels. RCH[2:0] in the
DFSDM_FLTxCR1 register indicates the selected channel.
Regular conversions can be launched only by software (not by a trigger). A sequence of
continuous regular conversions is temporarily interrupted when an injected conversion is
requested.
Performing a conversion on a disabled channel (CHEN=0 in DFSDM_CHyCFGR1 register)
causes that the conversion will never end - because no input data is provided (with no clock
signal). In this case, it is necessary to enable a given channel (CHEN=1 in
DFSDM_CHyCFGR1 register) or to stop the conversion by DFEN=0 in DFSDM_FLTxCR1
register.
24.4.8 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 DFSDM_FLTxFCR register):
–
FastSinc
–
Sinc
1
–
Sinc
2
–
Sinc
3
–
Sinc
4
–
Sinc
5
•
Filter oversampling/decimation ratio (see FOSR[9:0] bits in DFSDM_FLTxFCR
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:
H z
( )
1 z
FOSR
–
–
1 z
1
–
–
-----------------------------
⎝
⎠
⎜
⎟
⎛
⎞
x
=
H z
( )
1 z
FOSR
–
–
1 z
1
–
–
-----------------------------
⎝
⎠
⎜
⎟
⎛
⎞
2
1 z
2
FOSR
⋅
(
)
–
+
(
)
⋅
=