Datasheet

REV. 0
–16–
AD7708/AD7718
Table I. ADC Conversion and Settling Times for Various
SF Words with CHOP = 0
SF Data Update Rate Settling Time
Word f
ADC
(Hz) t
SETTLE
(ms)
13 105.3 19.04
23 59.36 33.69
27 50.56 39.55
45 30.3 65.9
69 (Default) 19.79 101.07
91 15 133.1
182 7.5 266.6
255 5.35 373.54
The overall frequency response is the product of a sinc
3
and a
sinc response. There are sinc
3
notches at integer multiples of
3 × f
ADC
and there are sinc notches at odd integer multiples
of f
ADC
/2. The 3 dB frequency for all values of SF obeys the
following equation:
f (3 dB) = 0.24 × f
ADC
Normal-mode rejection is the major function of the digital filter
on the AD7708/AD7718. The normal mode 50 ±1 Hz rejection
with an SF word of 82 is typically –100 dB. The 60 ± 1 Hz
rejection with SF = 68 is typically –100 dB. Simultaneous 50 Hz
and 60 Hz rejection of better than 60 dB is achieved with an SF
of 69. Choosing an SF word of 69 places notches at both 50 Hz
and 60 Hz. Figures 5 to 9 show the filter rejection for a selection
of SF words.
The frequency response of the filter H (f) is as follows:
1
8
81
2
2
3
SF
SF f f
ff
ff
ff
MOD
MOD
OUT
OUT
×
×
×××
×






××
××
×






sin ( / )
sin ( / )
sin ( / )
sin ( / )
π
π
π
π
where
f
MOD
= 32,768 Hz,
SF = value programmed into SF Register,
f
OUT
= f
MOD
/(SF × 8 × 3).
The following plots show the filter frequency response for a
variety of update rates from 5 Hz to 105 Hz.
FREQUENCY – Hz
0
–140
–200
0
650
50
100
150 200 250 300 350 400
450 500 550 600
700
ATTENUATION – dB
–20
–120
–160
–180
–60
–100
–40
–80
SF = 13
OUTPUT DATA RATE = 105Hz
INPUT BANDWIDTH = 25.2Hz
FIRST NOTCH = 52.5Hz
50Hz REJECTION = –23.6dB, 50Hzⴞ1Hz REJECTION = –20.5dB
60Hz REJECTION = –14.6dB, 60Hzⴞ1Hz REJECTION = –13.6dB
Figure 5. Filter Profile with SF = 13
FREQUENCY – Hz
0
–80
–160
0 10010
ATTENUATION – dB
20 30 40 50 60 70 80 90
–20
–40
–120
–140
–60
–100
SF = 82
OUTPUT DATA RATE = 16.65Hz
INPUT BANDWIDTH = 4Hz
50Hz REJECTION = –171dB, 50Hzⴞ1Hz REJECTION = –100dB
60Hz REJECTION = –58dB, 60Hzⴞ1Hz REJECTION = –53dB
Figure 6. Filter Profile with SF = 82