AD7708 datasheet(16/44 Pages) AD

16 / 44 page

REV. 0

–16–

AD7708/AD7718

Table I. ADC Conversion and Settling Times for Various

SF Words with

CHOP = 0

Data Update Rate

Settling Time

Word

fADC (Hz)

tSETTLE (ms)

105.3

19.04

59.36

33.69

50.56

39.55

30.3

65.9

69 (Default)

19.79

101.07

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

× fADC and there are sinc notches at odd integer multiples

of fADC/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:

f f

MOD

OUT

×× ×









××









sin (

)

sin (

)

sin (

)

sin (

)

where

fMOD = 32,768 Hz,

SF = value programmed into SF Register,

fOUT = fMOD/(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

–140

–200

650

50 100 150 200 250 300 350 400 450 500 550 600

700

–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, 50Hz 1Hz REJECTION = –20.5dB

60Hz REJECTION = –14.6dB, 60Hz 1Hz REJECTION = –13.6dB

Figure 5. Filter Profile with SF = 13

FREQUENCY – Hz

–80

–160

100

–20

–40

–120

–140

–60

–100

SF = 82

OUTPUT DATA RATE = 16.65Hz

INPUT BANDWIDTH = 4Hz

50Hz REJECTION = –171dB, 50Hz 1Hz REJECTION = –100dB

60Hz REJECTION = –58dB, 60Hz 1Hz REJECTION = –53dB

Figure 6. Filter Profile with SF = 82

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