Chapter 4
Analog Input
©
National Instruments Corporation
4-3
Caution
Exceeding the differential and common-mode input ranges distorts the input
signals. Exceeding the maximum input voltage rating can damage the device and the
computer. NI is
not
liable for any damage resulting from such signal connections. The
maximum input voltage ratings can be found in the specifications document for each
S Series device.
Input Polarity and Range
Input range refers to the set of input voltages that an analog input channel
can digitize with the specified accuracy. On some S Series devices, you can
individually program the input range of each AI channel.
The input range affects the resolution of the S Series device for an
AI channel. Resolution refers to the magnitude of one ADC code. For
example, a 16-bit ADC converts analog inputs into one of 65,536 (=2
16
)
codes, meaning one of 65,536 possible digital values. These values are
spread fairly evenly across the input range. So, for an input range of –5 V
to 5 V, the code width of a 16-bit ADC is:
S Series devices support bipolar input ranges. A bipolar input range means
that the input voltage range is between –V
ref
and V
ref
.
The instrumentation amplifier applies a different gain setting to the
AI signal depending on the input range. Gain refers to the factor by which
the instrumentation amplifier multiplies (amplifies) the input signal before
sending it to the ADC.
On S Series devices with programmable input ranges, choose an input
range that matches the expected input range of your signal. A large input
range can accommodate a large signal variation, but reduces the voltage
resolution. Choosing a smaller input range improves the voltage resolution,
but may result in the input signal going out of range. For more information
about programming these settings, refer to the
NI-DAQmx Help
or the
LabVIEW Help
in version 8.0 or later.
5 V
5 V
–
(
)
–
2
16
--------------------------------
153
μ
V
=
