Analog Input Circuitry - National Instruments PCI-1200 User Manual

Chapter 4 Theory of Operation

Analog Input Circuitry

NI PCI-1200 User Manual
The AI circuitry consists of two AI input multiplexers, multiplexer (mux)
counter/gain select circuitry, a software-programmable gain amplifier,
a 12-bit ADC, and a 16-bit sign-extended FIFO memory.
One of the input multiplexers has eight AI channels (channels 0 through 7).
The other multiplexer is connected to channels 1, 3, 5, and 7 for differential
mode. The input multiplexers provide input overvoltage protection of
±35 V powered on and ±25 V powered off.
The mux counters control the input multiplexers. The NI PCI-1200 can
perform either single-channel data acquisition or multichannel scanned
data acquisition. These two modes are software-selectable. For
single-channel data acquisition, select the channel and gain before
initiating data acquisition. These gain and multiplexer settings remain
constant during the entire DAQ process. For multichannel scanned data
acquisition, select the highest numbered channel and gain before initiating
data acquisition. Then the mux counter decrements from the highest
numbered channel to channel 0 and repeats the process. Thus, you can scan
from two to eight channels. Notice that you use the same gain setting for all
channels in the scan sequence.
The programmable gain amplifier applies gain to the input signal, allowing
an input analog signal to be amplified before being sampled and converted,
thus increasing measurement resolution and accuracy. The instrumentation
amplifier gain is software-selectable. The NI PCI-1200 provides gains
of 1, 2, 5, 10, 20, 50, and 100.
The dither circuitry, when enabled, adds approximately 0.5 LSBrms of
white Gaussian noise to the signal to be converted to the ADC. This
addition is useful for applications involving averaging, to increase the
resolution of the NI PCI-1200 to more than 12 bits, as in calibration.
In such applications, which are often lower frequency in nature, noise
modulation is decreased and differential linearity is improved by the
addition of the dither. For high-speed 12-bit applications not involving
averaging, you should disable dither because it only adds noise.
When taking DC measurements, such as when calibrating the device,
enable dither and average about 1,000 points to take a single reading.
This process removes the effects of 12-bit quantization and reduces
measurement noise, resulting in improved resolution. Dither, or additive
white noise, has the effect of forcing quantization noise to become a
zero-mean random variable rather than a deterministic function of input.
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