ADS8341_14 datasheet(16/25 Pages) TI1

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ADS8341

SBAS136D

NOISE

The noise floor of the ADS8341 itself is extremely low, as

can be seen from Figures 10 thru 13, and is much lower than

competing A/D converters. The ADS8341 was tested at both

5V and 2.7V and in both the internal and external clock

modes. A low-level DC input was applied to the analog

input pins and the converter was put through 5,000 conver-

sions. The digital output of the A/D converter will vary in

output code due to the internal noise of the ADS8341. This

is true for all 16-bit SAR-type A/D converters. Using a

histogram to plot the output codes, the distribution should

appear bell-shaped with the peak of the bell curve represent-

ing the nominal code for the input value. The

±1σ, ±2σ, and

±3σ distributions will represent the 68.3%, 95.5%, and

99.7%, respectively, of all codes. The transition noise can be

calculated by dividing the number of codes measured by 6

and this will yield the

±3σ distribution or 99.7% of all codes.

Statistically, up to 3 codes could fall outside the distribution

when executing 1000 conversions. The ADS8341, with < 3

output codes for the

±3σ distribution, will yield a < ±0.5

LSB transition noise at 5V operation. Remember, to achieve

this low noise performance, the peak-to-peak noise of the

input signal and reference must be < 50

µV.

FIGURE 10. Histogram of 5,000 Conversions of a DC Input at the

Code Transition, 5V operation external clock mode.

FIGURE 11. Histogram of 5,000 Conversions of a DC Input at the

Code Center, 5V operation internal clock mode.

FIGURE 12. Histogram of 5,000 Conversions of a DC Input at the

Code Transition, 2.7V operation external clock mode.

FIGURE 13. Histogram of 5,000 Conversions of a DC Input at the

Code Center, 2.7V operation internal clock mode.

AVERAGING

The noise of the A/D converter can be compensated by

averaging the digital codes. By averaging conversion results,

transition noise will be reduced by a factor of 1/

√n, where n

is the number of averages. For example, averaging 4 conver-

sion results will reduce the transition noise by 1/2 to

±0.25

LSBs. Averaging should only be used for input signals with

frequencies near DC.

For AC signals, a digital filter can be used to low-pass filter

and decimate the output codes. This works in a similar

manner to averaging; for every decimation by 2, the signal-

to-noise ratio will improve 3dB.

4606

194

200

7FFC

7FFE

7FFF

8000

8001

Code

7FFC

7FFE

7FFF

4614

8000

8001

Code

203

183

683

3619

638

7FFD

7FFE

7FFF

8000

8001

Code

3572

586

790

7FFD

7FFE

7FFF

8000

8001

Code

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