REV. 0
AD7708/AD7718
–43–
Table XXI. Normal Mode 50 Hz and 60 Hz Rejection vs. Settling Time and Update Rate for a Selection of SF Words
CHOP Disabled CHOP Enabled
50 Hz ⴞ 1 Hz 60 Hz ⴞ 1 Hz 50 Hz ⴞ 1 Hz 60 Hz ⴞ 1 Hz
SF Rejection Rejection Rejection Rejection
Word f
ADC
(Hz) t
SETTLE
(ms) (dB) (dB) f
ADC
(Hz) t
SETTLE
(ms) (dB) (dB)
03 1365.33 2.20 0.05 0.08 N/A N/A N/A N/A
13 315.08 9.52 1 1.5 105.03 19.04 21 13.6
69 59.36 50.54 42 94 19.79 101 60 94
75 54.6 54.93 57 60 18.2 110 62 66
82 49.95 60 100 16.65 180 100 53
151 27.13 110.6 60 60 9.04 221 72 63
255 16.06 186.76 77 68 5.35 373.5 93 68
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
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.
