AD7709 datasheet(13/33 Pages) AD

13 / 33 page

REV. A

AD7709

–12–

As shown in the block diagram, the Sinc

3 filter outputs alternately

contain +VOS and –VOS, where VOS is the respective channel offset.

This offset is removed by performing a running average of 2, which

means that the settling time to any change in programming of

the ADC will be twice the normal conversion time, while an

asynchronous step change on the analog input will not be fully

reflected until the third subsequent output.

SETTLE

ADC

= Ê

ËÁ

¯˜

=¥

The allowable range for SF is 13 to 255, with a default of 69

(45H). The corresponding conversion rates, conversion times,

and settling times are shown in Table I. Note that the conver-

sion time increases by 0.732 ms for each increment in SF.

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

Table I. ADC Conversion and Settling Times for Various

SF Words

Data Update Rate

Settling Time

SF Word

fADC (Hz)

tSETTLE (ms)

105.3

19.04

69 (Default)

19.79

101.07

255

5.35

373.54

Normal mode rejection is the major function of the digital filter

on the AD7709. 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 8 show the filter rejection for a selection

of SF words.

FREQUENCY – Hz

–80

–160

100

–20

–40

–120

–140

–60

–100

SF = 69

OUTPUT DATA RATE = 19.8Hz

INPUT BANDWIDTH = 4.74Hz

FIRST NOTCH = 9.9Hz

50Hz REJECTION = –66dB, 50Hz

1Hz REJECTION = –60dB

60Hz REJECTION = –117dB, 60Hz

1Hz REJECTION = –94dB

Figure 7. Filter Profile with Default SF = 69 Giving Filter

Notches at Both 50 Hz and 60 Hz

FREQUENCY – Hz

–80

–160

100

–20

–40

–120

–140

–60

–100

SF = 255

OUTPUT DATA RATE = 5.35Hz

INPUT BANDWIDTH = 1.28Hz

50Hz REJECTION = –93dB, 50Hz

1Hz REJECTION = –93dB

60Hz REJECTION = –74dB, 60Hz

1Hz REJECTION = –68dB

Figure 8. Filter Profile with SF = 255

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