AD7709 Analog Devices, AD7709 Datasheet - Page 12

• Manufacturer Part Number: AD7709
• Description: 16-Bit Sigma Delta ADC with Current Sources, Switchable Reference Inputs and I/O Port
• Manufacturer: Analog Devices

Specifications of AD7709

• Resolution (bits): 16bit
• # Chan: 4
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

AD7709
As shown in the block diagram, the Sinc
contain +V
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.
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.
–100
–120
–140
–160
–180
–200
–100
–120
–140
–160
–20
–40
–60
–80
–20
–40
–60
–80
0
0
OS
0
0
SF = 13
OUTPUT DATA RATE = 105Hz
INPUT BANDWIDTH = 25.2Hz
FIRST NOTCH = 52.5Hz
50Hz REJECTION = –23.6dB, 50Hz
60Hz REJECTION = –14.6dB, 60Hz
SF = 82
OUTPUT DATA RATE = 16.65Hz
INPUT BANDWIDTH = 4Hz
50Hz REJECTION = –171dB, 50Hz
60Hz REJECTION = –58dB, 60Hz
Figure 5. Filter Profile with SF = 13
Figure 6. Filter Profile with SF = 82
and –V
50 100 150 200 250 300 350 400 450 500 550 600
10
t
SETTLE
20
OS
, where V
30
=
Ê
Á
Ë
FREQUENCY – Hz
FREQUENCY – Hz
40
f
ADC
2
OS
50
is the respective channel offset.
ˆ
˜ = ¥
¯
1Hz REJECTION = –53dB
2
1Hz REJECTION = –20.5dB
1Hz REJECTION = –13.6dB
1Hz REJECTION = –100dB
60
3
filter outputs alternately
t
ADC
70
80
90
650
100
700
–12–
SF Word
13
69 (Default)
255
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.
Figure 7. Filter Profile with Default SF = 69 Giving Filter
Notches at Both 50 Hz and 60 Hz
Table I. ADC Conversion and Settling Times for Various
SF Words
–100
–120
–140
–160
–100
–120
–140
–160
–20
–40
–60
–80
–20
–40
–60
–80
0
0
0
0
SF = 69
OUTPUT DATA RATE = 19.8Hz
INPUT BANDWIDTH = 4.74Hz
FIRST NOTCH = 9.9Hz
50Hz REJECTION = –66dB, 50Hz
60Hz REJECTION = –117dB, 60Hz
SF = 255
OUTPUT DATA RATE = 5.35Hz
INPUT BANDWIDTH = 1.28Hz
50Hz REJECTION = –93dB, 50Hz
60Hz REJECTION = –74dB, 60Hz
Figure 8. Filter Profile with SF = 255
10
10
20
20
Data Update Rate
f
105.3
19.79
5.35
ADC
30
30
(Hz)
FREQUENCY – Hz
40
FREQUENCY – Hz
40
50
50
1Hz REJECTION = –60dB
1Hz REJECTION = –93dB
1Hz REJECTION = –68dB
1Hz REJECTION = –94dB
60
60
70
70
80
80
Settling Time
t
19.04
101.07
373.54
SETTLE
90
90
100
REV. A
100
(ms)