Filter
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FX2N-5A Special function block Buffer Memory (BFM) 7
7-26
Formula for filter cut-off frequency f
L
: f
L
= F
1
/(sampling time
×
no. of active channels) [Hz] with
cut-off frequency factor F
1
=0.1, 0.05, 0.025 or 0.01.
There are two filter parameters that can be changed, the filter-level and the cut-off frequency.
This selection is done in bit0 to bit7. The selected filter values apply to all channels 1 through 4
in the same way. Selecting a non existing filter mode or selecting a filter mode without
specifying a cut-off frequency will lead to an error in BFM 29. In this case the previously valid
settings will be kept.
The filter settings for the module also have a direct influence on the sampling rate and
conversion speed of the module. The relation between selected filter level and sampling rate is
as follows:
* In these cases b4 to b7 must be set to a value between K1 to K4.
The value of BFM 25 is stored non-volatile in the internal EEPROM. There is a safety function
to protect the internal EEPROM from being destroyed by accidentally writing the same value
continuously to BFM 25.
Table 7.3: Setting range
Filter Level
(value in BFM25, b0 to b3)
sampling time per input
channel
refresh rate of analog output
channel
filter off (K0) 1ms / ch update every 2 ms
2nd level digital filter (K1)* 3ms / ch update every 6 ms
5th level digital filter (K2)* 4ms / ch update every 8 ms
7th level digital filter (K3)* 4.5ms / ch update every 9 ms