Datasheet
APPLICATION INFORMATION
TRANSITION NOISE
7FFD 7FFE
16
288
7FFF
3855
8000
7740
8001 8002
4230
247
8003
8
AVERAGING
ADS8517
SLAS527A – SEPTEMBER 2008 – REVISED JUNE 2009 .................................................................................................................................................
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Apply a dc input to the ADS8517 and initiate 1000 conversions. The digital output of the converter varies in
output codes because of the internal noise of the ADS8517. This variance is true for all 16-bit SAR converters.
The transition noise specification found in the Electrical Characteristics section is a statistical figure that
represents the one sigma limit or rms value of these output codes.
Using a histogram to plot the output codes, the distribution should appear bell-shaped, with the peak of the bell
curve representing the nominal output code for the input voltage value. The ± 1 σ , ± 2 σ , and ± 3 σ distributions
represent 68.3%, 95.5%, and 99.7%, respectively, of all codes. Multiplying the transition noise (TN) by 6 yields
the ± 3 σ distribution, or 99.7%, of all codes. Statistically, up to three codes could fall outside the five-code
distribution when executing 1000 conversions. The ADS8517 has a TN of 0.8 LSBs, which yields five output
codes for a ± 3 σ distribution. Figure 43 shows 16,384 conversion histogram results.
Figure 43. Histogram of 16,384 Conversions with V
IN
= 0 V in ± 10 V Bipolar Range
The noise of the converter can be compensated by averaging the digital codes. By averaging conversion results,
transition noise is reduced by a factor of 1/ √ n where n is the number of averages. For example, averaging four
conversion results reduces the TN by 1/2 to 0.4 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 action works in a
similar manner to averaging: for every decimation by 2, the signal-to-noise ratio improves by 3 dB.
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