AD7794/AD7795

Rev. D | Page 13 of 36

RMS NOISE AND RESOLUTION SPECIFICATIONS

The AD7794/AD7795 can be operated with chop enabled or

chop disabled, allowing the ADC to be optimized for switching

time or drift performance. With chop enabled, the settling time

is two times the conversion time. However, the offset is

continuously removed by the ADC leading to low offset and low

offset drift. With chop disabled, the allowable update rates are

the same as in chop enable mode. However, the settling time

now equals the conversion time. With chop disabled, the offset

is not removed by the ADC, so periodic offset calibrations can

be required to remove offset due to drift.

CHOP ENABLED

External Reference

Table 5 shows the AD7794/AD7795 rms noise for some update

rates and gain settings. The numbers given are for the bipolar

input range with an external 2.5 V reference. These numbers are

typical and are generated with a differential input voltage of 0 V.

Table 6 and Table 7 show the effective resolution, while the

output peak-to-peak (p-p) resolution is listed in brackets. It is

important to note that the effective resolution is calculated

using the rms noise, while the p-p resolution is calculated based

on peak-to-peak noise. The p-p resolution represents the

resolution for which there is no code flicker. These numbers are

typical and are rounded to the nearest LSB.

Table 5. RMS Noise (μV) vs. Gain and Output Update Rate Using an External 2.5 V Reference with Chop Enabled

Update Rate (Hz)

Gain of 1

Gain of 2

Gain of 4

Gain of 8

Gain of 16

Gain of 32

Gain of 64

Gain of 128

4.17

0.64

0.6

0.29

0.22

0.1

0.065

0.039

0.041

8.33

1.04

0.96

0.38

0.26

0.13

0.078

0.057

0.055

16.7

1.55

1.45

0.54

0.36

0.18

0.11

0.087

0.086

33.2

2.3

2.13

0.74

0.5

0.23

0.17

0.124

0.118

62

2.95

2.85

0.92

0.58

0.29

0.2

0.153

0.144

123

4.89

4.74

1.49

1

0.48

0.32

0.265

0.283

242

11.76

9.5

4.02

1.96

0.88

0.45

0.379

0.397

470

11.33

9.44

3.07

1.79

0.99

0.63

0.568

0.593

Table 6.

Effective Resolution (Bits) vs. Gain and Output Update Rate for the AD7794 Using an External 2.5 V Reference with Chop Enabled

Update Rate (Hz)

Gain of 1

Gain of 2

Gain of 4

Gain of 8

Gain of 16

Gain of 32

Gain of 64

Gain of 128

4.17

23 (20.5)

22 (19.5)

22 (19.5)

21.5 (19)

21.5 (19)

21 (18.5)

21 (18.5)

20 (17.5)

8.33

22 (19.5)

21.5 (19)

21.5 (19)

21 (18.5)

21 (18.5)

21 (18.5)

20.5 (18)

19.5 (17)

16.7

21.5 (19)

20.5 (18)

21 (18.5)

20.5 (18)

20.5 (18)

20.5 (18)

20 (17.5)

19 (16.5)

33.2

21 (18.5)

20 (17.5)

20.5 (18)

20 (17.5)

20.5 (18)

20 (17.5)

19 (16.5)

18.5 (16)

62

20.5 (18)

19.5 (17)

20.5 (18)

20 (17.5)

20 (17.5)

19.5 (17)

19 (16.5)

18 (15.5)

123

20 (17.5)

19 (16.5)

19.5 (17)

19 (16.5)

19.5 (17)

19 (16.5)

18 (15.5)

17 (14.5)

242

18.5 (16)

18 (15.5)

18 (15.5)

18 (15.5)

18.5 (16)

18.5 (16)

17.5 (15)

16.5 (14)

470

18.5 (16)

18 (15.5)

18.5 (16)

18.5 (16)

18 (15.5)

18 (15.5)

17 (14.5)

16 (13.5)

Table 7.

Effective Resolution (Bits) vs. Gain and Output Update Rate for the AD7795 Using an External 2.5 V Reference with Chop Enabled

Update Rate (Hz)

Gain of 1

Gain of 2

Gain of 4

Gain of 8

Gain of 16

Gain of 32

Gain of 64

Gain of 128

4.17

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

8.33

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16.7

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

33.2

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

62

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (15.5)

123

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (16)

16 (15.5)

16 (14.5)

242

16 (16)

16 (15.5)

16 (15.5)

16 (15.5)

16 (16)

16 (16)

16 (15)

16 (14)

470

16 (16)

16 (15.5)

16 (16)

16 (16)

16 (15.5)

16 (15.5)

16 (14.5)

16 (13.5)