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APPLICATION INFORMATION

TRANSITION NOISE

2

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, ±2σ, and ±3σ 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.

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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