Programming instructions

Chapter 5 Things You Should Know about Analog Input
© National Instruments Corporation 5-7 LabVIEW Data Acquisition Basics Manual
Considerations for Selecting Analog Input Settings
The resolution and device range of a DAQ device determine the smallest
detectable change in the input signal. You can calculate the smallest
detectable change, called the
code width
, using the following formula.
For example, a 12-bit DAQ device with a 0 to 10 V input range detects a
2.4 mV change, while the same device with a –10 to 10 V input range
detects only a change of 4.8 mV.
A high resolution A/D converter provides a smaller code width given a
device voltage ranges shown above.
The smaller your code width, the more accurate your measurements
will be.
There are times you must know whether your signals are unipolar or
bipolar. Unipolar signals are signals that range from 0 value to a positive
value (i.e., 0 to 5 V). Bipolar signals are signals that range from a negative
to a positive value (i.e., –5 to 5 V). To achieve a smaller code width if
your signal is unipolar, specify that the device range is unipolar, as shown
previously. If your signal range is smaller than the device range, you should
set your limit settings to values that more accurately reflect your signal
range. Table 5-1 shows how the code width of the 12-bit DAQ devices vary
with device ranges and limit settings, because your limit settings
automatically adjust the gain on your device.
codewidth
device range
2
resolution
----------------------------------=
device range
2
resolution
----------------------------------
10
2
12
-------
2.4
mV==
device range
2
resolution
----------------------------------
20
2
12
-------
4.8
mV==
device range
2
resolution
----------------------------------
10
2
16
-------
.
15
mV==
device range
2
resolution
----------------------------------
20
2
16
-------
.
3
mV==