National Instruments E Series - Manual (Page 28)

Chapter 3 Hardware Overview
DAQCard E Series User Manual 3-6  National Instruments Corporation
Considerations for Selecting Input Ranges
Which input polarity and range you select depends on the expected
range of the incoming signal. A large input range can accommodate a
large signal variation but reduces the voltage resolution. Choosing a
smaller input range improves the voltage resolution but may result in
the input signal going out of range. For best results, you should match
the input range as closely as possible to the expected range of the input
signal. For example, if you are certain the input signal will not be
negative (below 0 V), unipolar input polarity is best. However, if the
signal is negative or equal to zero, inaccurate readings will occur if you
use unipolar input polarity.
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 DAQCard, as in calibration or spectral analysis. In such
applications, noise modulation is decreased and differential linearity is
improved by the addition of dither. When taking DC measurements,
such as when checking the DAQCard 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 3-3 illustrates the effect of dither on signal acquisition.
Figure 3-3a shows a small (
±
4 LSB) sine wave acquired with dither off.
The quantization of the ADC is clearly visible. Figure 3-3b shows what
happens when 50 such acquisitions are averaged together; quantization
is still plainly visible. In Figure 3-3c, 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 3-3d,
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.