National Instruments PCI-6023E - Manual (Page 23)

Chapter 3 Hardware Overview
© National Instruments Corporation 3-3 PCI-6023E/6024E/6025E User Manual
12-bit analog-to-digital converter (ADC) resolution. With the proper gain
setting, you can use the full resolution of the ADC to measure the input
signal. Table 3-2 shows the input range and precision according to the gain
used.
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 applications involving averaging to increase the resolution of
your 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 3-2 illustrates the effect of dither on signal acquisition. Figure 3-2a
shows a small (±4 LSB) sine wave acquired with dither off. The ADC
quantization 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 visible noise, 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.
Table 3-2. Measurement Precision
Gain Input Range Precision *
0.5
1.0
10.0
100.0
-10 to +10V
-5 to +5V
-500 to +500 mV
-50 to +50 mV
4.88 mV
2.44 mV
244.14
µ
V
24.41
µ
V
*The value of 1 LSB of the 12-bit ADC; that is, the voltage increment corresponding to a
change of one count in the ADC 12-bit count.
Note: See Appendix A, Specifications , for absolute maximum ratings.
PCI.book Page 3 Wednesday, September 16, 1998 9:09 AM