Datasheet

APPLICATION INFORMATION
TRANSITION NOISE
29
484
4099
7595
3975
201
1
7FFC 7FFD 7FFE 7FFF 8000 8001 8002
AVERAGING
QSPI™ INTERFACE
ADS8513
www.ti.com ...................................................................................................................................................... SLAS486C – JUNE 2007 – REVISED JANUARY 2009
Apply a dc input to the ADS8513 and initiate 1000 conversions. The digital output of the converter varies in
output codes because the internal noise of the ADS8513. This condition is true for all 16-bit SAR converters. The
transition noise specification found in the Electrical Characteristics table is a statistical figure that represents the
1 σ 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% of all codes. Multiplying transition noise 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 ADS8513 has a transition noise of 0.8 LSBs which yields five output codes for a ± 3 σ
distribution. Figure 49 shows 16,384 conversion histogram results.
Figure 49. Histogram of 16384 Conversions With V
IN
= 0V in ± 10V Bipolar Range
Converter noise 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 transition noise by half to 0.4 LSBs. Note that 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 3dB.
Figure 50 shows a simple interface between the ADS8513 and any QSPI-equipped microcontroller. This interface
assumes that the convert pulse does not originate from the microcontroller and that the ADS8513 is the only
serial peripheral.
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Product Folder Link(s): ADS8513