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3.1
Analog Input
3.1.1
Analog Input Resolution
Analog input voltage or current is measured by an ADC, which has a finite resolution.
For example, a 16-bit ADC divides the input range into 65,536, which is 216, codes.
These codes represent input voltage or current which are equally distributed across
the input range. If the input range is ±10 V, the resolution is
Because resolution depends on input range, as stated in the previous equation, a
smaller input range has higher (finer) resolution, and vice versa. Choose an input
range that matches the maximum possible range of the input signal. If a larger-than-
required range is used, the resolution is reduced; if a smaller-than-required range is
used, on the other hand, the required input range cannot be covered.
3.1.2
Analog Input Acquisition Methods
The device supports both instant (software-timed) and buffered (hardware-timed)
analog input acquisitions.
3.1.2.1
Instant (Software-Timed) Analog Input Acquisition
With instant acquisition, the software controls the rate and time of acquisition, which
is thus also called software-timed acquisition. Whenever the software sends a read
command, the current value of analog input channel is returned as shown in Figure
3.1.
Figure 3.1 Instant (software-timed) analog input acquisition.
The advantage of instant acquisition is low latency. It is typically used for reading a
single sample of analog input.
3.1.2.2
Buffered (Hardware-Timed) Analog Input Acquisition
With buffered acquisition, a hardware signal called conversion clock controls the rate
and time of acquisition. The ADC begins to convert the analog input voltage at each
rising edge of the conversion clock. After the ADC begins conversion, the analog
multiplexer routes the next buffered acquisition enabled analog input channel to the
ADC, which will be converted at the next rising edge of the conversion clock. Figure
3.2 shows an example of analog input buffered acquisition which AI0, AI1, and AI2
are enabled.
PCI-1716_Series User Manual
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