Chapter3 HardwareOverview
6023E/6024E/6025EUser Manual 3-4 ni.com
Dithering
When you enable dithering, 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
yourdevice, as in calibration or spectral analysis. In such applications,
noisemodulation is decreased and differential linearity is improved by the
addition of dithering. When taking DC measurements, such as when
checking the device calibration, enable dithering 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 involvingaveraging or spectral
analysis,you may want to disable dithering to reduce noise. Your software
enables and disables the dithering circuitry.
Figure 3-3 illustrates the effectof ditherin g on signal acquisition.
Figure 3-3a showsa smal l (±4 LSB) sine waveacquired with dithering off.
TheADC quantization is clearly visible. Figure 3-3b shows what happens
when 50 such acquisitions are averagedt ogether; quantization is still
plainlyvisible. In Figu re 3-3c, the sine wave is acquired with dithering on.
There is a considerable amount of visible noise, but averagingabout 50
suchacquisitions, as shown in Figure 3-3 d,el iminates both the added noise
and theeffects of quantization. Dithering has the effect of forcing
quantization noiseto become a zero-mean random variable rather than a
deterministic function of the input signal.