Datasheet

ADuC836
–26–
ADuC836
–27–
Digital Filter
The output of the - modulator feeds directly into the digital
lter. The digital lter then band-limits the response to a frequency
signicantly lower than one-half of the modulator frequency. In
this manner, the 1-bit output of the comparator is translated into
a band-limited, low noise output from the ADuC836 ADCs.
The ADuC836 lter is a low-pass, SIN
3
or (SINx/x)3 lter whose
primary function is to remove the quantization noise introduced
at the modulator. The cutoff frequency and decimated output
data rate of the lter are programmable via the SF (Sinc Filter)
SFR, as described in Table VIII.
Figure 11 shows the frequency response of the ADC channel at
the default SF word of 69 dec or 45H, yielding an overall output
update rate of just under 20 Hz.
It should be noted that this frequency response allows frequency
components higher than the ADC Nyquist frequency to pass
through the ADC, in some cases without signicant attenuation.
These components may, therefore, be aliased and appear in-band
after the sampling process.
It should also be noted that rejection of mains related frequency
components, i.e., 50 Hz and 60 Hz, is seen to be at a level of
>65 dB at 50 Hz and >100 dB at 60 Hz. This conrms the data
sheet specications for 50 Hz/60 Hz Normal Mode Rejection
(NMR) at a 20 Hz update rate.
0 20 30 50 70 80 90 100 110
FREQUENCY – Hz
GAIN – dB
0
–20
–40
–70
–80
–90
–100
–110
–120
10 40 60
–10
–30
–60
–50
Figure 11. Filter Response, SF = 69 dec
The response of the lter, however, will change with SF word, as
can be seen in Figure 12, which shows >90 dB NMR at 50 Hz
and >70 dB NMR at 60 Hz when SF = 255 dec.
0 20 30 50 70 80 90 110
FREQUENCY – Hz
GAIN – dB
0
–20
–40
–70
–80
–90
–100
–110
–120
10 40 60
–10
–30
–60
–50
Figure 12. Filter Response, SF = 255 dec
Figures 13 and 14 show the NMR for 50 Hz and 60 Hz across
the full range of SF word, i.e., SF = 13 dec to SF = 255 dec.
10 50 70 110 150 170 190 210
SF – Decimal
GAIN – dB
0
–20
–40
–70
–80
–90
–100
–110
–120
30 90 130
–10
–30
–60
–50
230 250
Figure 13. 50 Hz Normal Mode Rejection vs. SF
10 50 70 110 150 170 190 210
SF – Decimal
GAIN – dB
0
–20
–40
–70
–80
–90
–100
–110
–120
30 90 130
–10
–30
–60
–50
230 250
Figure 14. 60 Hz Normal Mode Rejection vs. SF
REV. A
REV. A