AD7708 Analog Devices, AD7708 Datasheet - Page 43

• Manufacturer Part Number: AD7708
• Description: 16-Bit 8/10-Channel, Low Voltage, Low Power, Sigma Delta ADC
• Manufacturer: Analog Devices

Specifications of AD7708

• Resolution (bits): 16bit
• # Chan: 10
• 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: SOIC,SOP

Optimizing Throughput while Maximizing 50 Hz and 60 Hz
Rejection in a Multiplexed Data Acquisition System
The AD7708/AD7718 can be optimized for one of two modes
of operation. Operating the AD7708/AD7718 with chopping
enabled (CHOP = 0) optimizes the AD7708/AD7718 for analog
performance over channel throughput. Output data rates vary
from 5.35 Hz (186.77 ms) to 105.03 Hz (9.52 ms). Optimum
performance, in terms of minimizing offset error and offset and
gain drift performance, is achieved as a result of chopping the
signal chain.
With chopping disabled the available output rates vary from
16.06 Hz (62.26 ms) to 1365.33 Hz (0.73 ms). The range of
applicable SF words is from 3 to 255. When the chopping is
disabled the channel output data rate is increased by a factor of
3 compared to the situation when chopping is enabled is
disabled. When used in multiplexed applications, operation
with chop disabled will offer the best throughput time when
SF
Word
03
13
69
75
82
151
255
Table XXI. Normal Mode 50 Hz and 60 Hz Rejection vs. Settling Time and Update Rate for a Selection of SF Words
f
1365.33
315.08
59.36
54.6
49.95
27.13
16.06
ADC
(Hz)
CHOP Disabled
t
2.20
9.52
50.54
54.93
60
110.6
186.76
SETTLE
(ms)
50 Hz
Rejection
(dB)
0.05
1
42
57
100
60
77
1 Hz
60 Hz
Rejection
(dB)
0.08
1.5
94
60
60
68
1 Hz
cycling through all channels. The drawback with chopping
disabled is that the drift performance is degraded and calibra-
tion is required following gain and temperature changes. One
of the key requirements in these applications is to optimize the SF
word to obtain the maximum filter rejection at 50 Hz and 60 Hz
while minimizing the channel throughput rate. This is achieved
with an SF word of 75 giving 57 dB rejection at 50 Hz and
60 dB rejection at 60 Hz while offering a channel throughput
time of 55 ms.
Using a higher SF word of 151, 50 Hz and 60 Hz rejection can
be maximized at 60 dB with a channel throughput rate of 110 ms.
An SF word of 255 gives maximum rejection at both 50 Hz
and 60 Hz, but the channel throughput rate is restricted to
186 ms. Table XXI gives a quick comparison of normal mode 50 Hz
and 60 Hz rejection, settling time, and update rate for a selec-
tion of SF words with both chopping enabled and disabled.
f
N/A
105.03
19.79
18.2
16.65
9.04
5.35
ADC
(Hz)
t
N/A
19.04
101
110
180
221
373.5
SETTLE
CHOP Enabled
(ms) (dB)
50 Hz
Rejection
N/A
21
60
62
100
72
93
AD7708/AD7718
1 Hz
Rejection
(dB)
N/A
13.6
94
66
53
63
68
60 Hz
1 Hz