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

1 Hz

60 Hz

1 Hz

50 Hz

1 Hz

60 Hz

1 Hz

SF

Rejection

Rejection

Rejection

Rejection

Word

(dB)

(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.