Chapter 2
Theory of Operation
©
National Instruments Corporation
2-9
hardware, a sampling ADC, a 16-bit wide data FIFO, and a configuration
memory.
The configuration memory defines the parameters to use for each
conversion. Each entry in the configuration memory includes channel type,
channel number, bank, gain, polarity, dither, general trigger, and last
channel. The configuration memory is a 512-entry deep FIFO that is
initialized prior to the start of the acquisition sequence. It can be
incremented after every conversion, allowing the analog input
configuration to vary on a per conversion basis. Once the FIFO is empty,
the DAQ-STC asserts the FIFO retransmit signal, which restores the FIFO
data to its original state.
The channel type field indicates the resource type to be used during the
conversion and controls the multiplexer mode selection switches.
These resources include calibration channels, analog input channels in
differential, referenced single-ended, or nonreferenced single-ended mode,
or a ghost channel. The ghost type indicates that a conversion should occur
but that the data should not be stored in the data FIFO. This type is useful
for multirate scanning, which is described later in this chapter.
The channel number indicates which channel of the specified type will be
used during the conversion, while the bank field indicates which bank of
16 channels is active. This bank field is used on boards that have more
than 16 channels. These bits control the input multiplexers.
The programmable gain instrumentation amplifier (PGIA) serves two
purposes on the AT E Series boards. The PGIA applies gain to the input
signal, amplifying an analog input signal before sampling and conversion
to increase measurement resolution and accuracy. This gain is determined
by the gain field in the configuration memory. It also provides polarity
selection for the input signal, which is also controlled by the configuration
memory. In unipolar mode, the input range includes only positive voltages.
In bipolar mode, the input signal may also be a negative voltage.
The PGIA provides gains of 0.5, 1, 2, 5, 10, 20, 50, and 100, except on
the AT-MIO-16XE-50, AT-MIO-16XE-10, and AT-AI-16XE-10. On the
AT-MIO-16XE-50, the PGIA supports gains of 1, 2, 10, and 100; on
the AT-MIO-16XE-10 and AT-AI-16XE-10, the PGIA supports gains
of 1, 2, 5, 10, 20, 50, and 100.
The dither circuitry adds approximately 0.5 LSB rms of white Gaussian
noise to the signal to be converted by the ADC. This addition is useful for
applications, such as calibration, involving averaging to increase the
resolution of the board to more than the resolution of the ADC. In such
applications, which are often lower frequency in nature, adding the dither
Содержание AT-AI-16XE-10
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