Chapter3 HardwareOverview
ATE Series User Manual 3-10 ni.com
Considerations for Selecting Input Ranges
Which input polarity and range you select depends on the expected range
ofthe incoming signal. A large input range can accommodate a large signal
variationbut reduces the voltage resolution. Choosing a smaller input range
improvesthe voltage resolution but may result in the input signal goi ng out
ofrange. 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
certainthe input s ignal is not negative (below 0 V), unipolar input polarity
is best. However, if the signal is negative or equal to zero, inaccurate
readings occur if you use unipolar input polarity.
Dither
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 applicationsinvolving averaging to increase the resolution
ofthe A TE Seriesdevice, as in calibration or spectral analysis. In such
applications,n oise modulation is decreased and differential linearityis
improved by the addition of the dither. When taking DC measurements,
suchas when checking the device calibration, you should enable dither and
averageab out 1,000 points to take a single reading. This process removes
the effects of quantizationand reduces measurement noise, resulting in
improved resolution. For high-speed applications not involving averaging
orspectral analysis, you may want to disable the dither to reduce noise. You
enable and disable the dither circuitry through software.
Figure 3-7 illustrates the effect of dither on signal acquisition. Figure 3-7a
showsa small (±4 LSB) sine wave acquired with dither off. The
quantizationof the ADC is clearly visible. Figure 3-7bshows what happens
when 50 such acquisitions are averagedt ogether; quantization is still
plainly visible. In Figure 3-7c, the sine waveis acquired with dither on .
There is a considerable amount of noise visible. But averaging about 50
suchacquisitions, as shown in Figure 3-7 d,el iminates both the added noise
andthe 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.