National Instruments NI PXI-7831R - Manual (Page 32)

Chapter 2 Hardware Overview of the NI PXI-7831R
© National Instruments Corporation 2-11 NI PXI-7831R User Manual
the source, other than the very high-input impedance of the instrumentation
amplifier.
You can fully balance the signal path by connecting another resistor of the
same value between the positive input and AIGND, as shown in Figure 2-6.
This fully balanced input mode offers slightly better noise rejection but has
the disadvantage of loading the source down with the series combination
(sum) of the two resistors. If, for example, the source impedance is 2 k
Ω
and each of the two resistors is 100 k
Ω, the resistors load down the source
with 200 k
Ω and produce a –1% gain error.
Both inputs of the instrumentation amplifier require a DC path to ground in
order for the instrumentation amplifier to work. If the source is AC coupled
(capacitively coupled), the instrumentation amplifier needs a resistor
between the positive input and AIGND. If the source has low-impedance,
choose a resistor that is large enough not to significantly load the source but
small enough not to produce significant input offset voltage as a result of
input bias current (typically 100 k
Ω to 1 MΩ). In this case, you can tie the
negative input directly to AIGND. If the source has high output impedance,
you should balance the signal path as previously described using the same
value resistor on both the positive and negative inputs; you should be aware
that there is some gain error from loading down the source.
Single-Ended Connection Considerations
A single-ended connection is one in which the NI PXI-7831R AI signal is
referenced to a ground that can be shared with other input signals. The input
signal is tied to the positive input of the instrumentation amplifier, and the
ground is tied to the negative input of the instrumentation amplifier.
You can use single-ended input connections for any input signal that meets
the following conditions:
• The input signal is high-level (>1 V).
• The leads connecting the signal to the NI PXI-7831R are less than
3 m (10 ft).
• The input signal can share a common reference point with other
signals.
DIFF input connections are recommended for greater signal integrity for
any input signal that does not meet the preceding conditions.
You can configure in software the NI PXI-7831R channels for two different
types of single-ended connections—RSE input mode and NRSE input
mode. The RSE input mode is used for floating signal sources; in this case,