Dither - National Instruments DAQ PCI E Series User Manual

Dither

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When you enable dither, you add approximately 0.5 LSBrms of white
Gaussian noise to the signal to be converted by the ADC. This addition
is useful for applications involving averaging to increase the resolution
of your PCI E Series board, as in calibration or spectral analysis. In such
applications, noise modulation is decreased and differential linearity is
improved by the addition of the dither. When taking DC measurements,
such as when checking the board calibration, you should enable dither
and average about 1,000 points to take a single reading. This process
removes the effects of quantization and reduces measurement noise,
resulting in improved resolution. For high-speed applications not
involving averaging or spectral analysis, you may want to disable the
dither to reduce noise. Your software enables and disables the dither
circuitry.
Figure 3-5 illustrates the effect of dither on signal acquisition.
Figure 3-5a shows a small (±4 LSB) sine wave acquired with dither off.
The ADC quantization is clearly visible. Figure 3-5b shows what
happens when 50 such acquisitions are averaged together; quantization
is still plainly visible. In Figure 3-5c, the sine wave is acquired with
dither on. There is a considerable amount of visible noise. But
averaging about 50 such acquisitions, as shown in Figure 3-5d,
eliminates both the added noise and the effects of quantization. Dither
has the effect of forcing quantization noise to become a zero-mean
random variable rather than a deterministic function of the input signal.
3-9
Chapter 3 Hardware Overview
PCI E Series User Manual