Chapter 3 Hardware Overview
National Instruments Corporation
3-7
VXI-MIO Series User Manual
Dither
When you enable dither, you add approximately 0.5 LSB rms 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 VXI-MIO Series module, 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 module 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. You enable and disable the dither
circuitry through software.
Figure 3-2 illustrates the effect of dither on signal acquisition.
Figure 3-2a shows a small (
±
4 LSB) sine wave acquired with dither off.
The quantization of the ADC is clearly visible. Figure 3-2b shows what
happens when 50 such acquisitions are averaged together; quantization
is still plainly visible. In Figure 3-2c, the sine wave is acquired with
dither on. There is a considerable amount of noise visible. But
averaging about 50 such acquisitions, as shown in Figure 3-2d,
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.
