Datasheet
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APPLICATION INFORMATION
TRANSITION NOISE
2
7FFD
H
7FFE
H
7FFF
H
8000
H
8001
H
8002
H
8003
H
190
3142
9732
3075
242
1
AVERAGING
QSPI™ INTERFACE
ADS8507
SLAS381 – DECEMBER 2006
Apply a DC input to the ADS8507 and initiate 1000 conversions. The digital output of the converter varies in
output codes due to the internal noise of the ADS8507. This is true for all 16-bit SAR converters. The transition
noise specification found in the electrical characteristics section is a statistical figure which 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% of all codes. Multiplying TN by 6 yields the ±3 σ distribution or 99.7% of all
codes. Statistically, up to 3 codes could fall outside the 5 code distribution when executing 1000 conversions.
The ADS8507 has a TN of 0.8 LSBs which yields 5 output codes for a ±3 σ distribution. Figure 48 shows 16384
conversion histogram results.
Figure 48. Histogram of 16384 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/ √Hz where n is the number of averages. For example, averaging four
conversion results reduces the TN by ½ 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 works in a
similar manner to averaging: for every decimation by 2, the signal-to-noise ratio improves 3 dB.
Figure 49 shows a simple interface between the ADS8507 and any QSPI equipped microcontroller. This
interface assumes that the convert pulse does not originate from the microcontroller and that the ADS8507 is the
only serial peripheral.
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