REV. 0

AD7738

–9–

OUTPUT NOISE AND RESOLUTION SPECIFICATION

The AD7738 can be operated with chopping enabled or disabled, allowing the ADC to be programmed either to optimize the

throughput rate and channel switching time or to optimize offset drift performance. Noise tables for these two primary modes of

operation are outlined below for a selection of output rates and settling times.

CHOPPING ENABLED

The first mode, in which the AD7738 is configured with chopping enabled (CHOP = 1), provides very low noise numbers with lower

output rates. Tables I to III show the –3 dB frequencies and typical performance versus channel conversion time or equivalent output

data rate, respectively. Table I shows the typical output rms noise. Table II shows the typical effective resolution based on the rms

noise. Table III shows the typical output peak-to-peak resolution, representing values for which there will be no code flicker within a

six-sigma limit. The peak-to-peak resolutions are not calculated based on rms noise, but on peak-to-peak noise.

These typical numbers are generated from 4096 data samples acquired in Continuous Conversion mode with an analog input voltage

set to 0 V and MCLK = 6.144 MHz. The Conversion Time is selected via the Channel Conversion Time register.

Table I. Typical Output RMS Noise in

V vs. Conversion Time and Input Range with

Chopping Enabled

Conversion

Conversion

Output

–3 dB

Input Range

Time

Time

Data Rate

Frequency

FW

Register

( s)

(Hz)

(Hz)

2.5 V, +2.5 V

1.25 V, +1.25 V,

625 mV, +625 mV

127

FFh

2686

372

194

1.8

1.1

46

AEh

999

1001

521

3.0

1.8

17

91h

395

2534

1317

5.1

3.0

8

88h

207

4826

2510

8.1

4.5

4

84h

124

8074

4198

9.3

5.3

2

82h

82

12166

6326

17.0

10.6

Table II. Typical Effective Resolution in Bits vs. Conversion Time and Input Range with

Chopping Enabled

Conversion

Conversion

Output

–3 dB

Input Range

Time

Time

Data Rate

Frequency

FW

Register

( s)

(Hz)

(Hz)

2.5 V

+2.5 V

1.25 V

+1.25 V

625 mV

+625 mV

127

FFh

2686

372

194

21.4

20.4

21.1

20.1

20.1

19.1

46

AEh

999

1001

521

20.6

19.6

20.4

19.4

19.4

18.4

17

91h

395

2534

1317

19.9

18.9

19.6

18.6

18.6

17.6

8

88h

207

4826

2510

19.2

18.2

19.0

18.0

18.0

17.0

4

84h

124

8074

4198

19.0

18.0

18.8

17.8

17.8

16.8

2

82h

82

12166

6326

18.1

17.1

17.8

16.8

16.8

15.8

Table III. Typical Peak-to-Peak Resolution in Bits vs. Conversion Time and Input Range with

Chopping Enabled

Conversion

Conversion

Output

–3 dB

Input Range

Time

Time

Data Rate

Frequency

FW

Register

( s)

(Hz)

(Hz)

2.5 V

+2.5 V

1.25 V

+1.25 V

625 mV

+625 mV

127

FFh

2686

372

194

18.4

17.4

18.2

17.2

17.2

16.2

46

AEh

999

1001

521

17.8

16.8

17.5

16.5

16.5

15.5

17

91h

395

2534

1317

16.8

15.8

16.7

15.7

15.7

14.7

8

88h

207

4826

2510

16.5

15.5

16.2

15.2

15.2

14.2

4

84h

124

8074

4198

16.0

15.0

16.0

15.0

15.0

14.0

2

82h

82

12166

6326

15.0

14.0

15.0

14.0

14.0

13.0