Table of Contents
Advertisement
Chapter 3
Signal Connections
NI PCI-1200 User Manual
The 100 kΩ resistors shown in Figure 3-4 create a return path to ground for
the bias currents of the instrumentation amplifier. If there is no return path,
the instrumentation amplifier bias currents charge stray capacitances,
resulting in uncontrollable drift and possible saturation in the amplifier.
Typically, values from 10 to 100 kΩ are used.
A resistor from each input to ground, as shown in Figure 3-4, provides bias
current return paths for an AC-coupled input signal.
If the input signal is DC-coupled, you need only the resistor that connects
the negative signal input to ground. This connection does not lower the
input impedance of the AI channel.
Single-Ended Connection Considerations
Single-ended connections are those in which all NI PCI-1200 AI signals are
referenced to one common ground. The input signals are tied to the positive
input of the instrumentation amplifier, and the common ground point is tied
to the negative input of the instrumentation amplifier.
When the NI PCI-1200 is configured for a single-ended input mode
(NRSE or RSE), eight AI channels are available. Use single-ended input
connections when the following conditions are met by all input signals:
•
Input signals are high level (greater than 1 V).
•
Leads connecting the signals to the NI PCI-1200 are less than 10 ft.
•
All input signals share a common reference signal (at the source).
If any of the preceding criteria is not met, use the DIFF input configuration.
You can software configure the NI PCI-1200 for two types of single-ended
connections, RSE configuration and NRSE configuration. Use the RSE
configuration for floating signal sources; in this case, the NI PCI-1200
provides the reference ground point for the external signal. Use the NRSE
configuration for ground-referenced signal sources; in this case, the
external signal supplies its own reference ground point and the
NI PCI-1200 should not supply one.
3-12
ni.com
Advertisement
Table of Contents
