National Instruments NI PXI-7831R Single-Ended Connection Considerations

Chapter 2 Hardware Overview ofthe NI PXI-7831R

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 Figure2-6.

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 kΩ

and each of the two resistors is 100 kΩ, the resistors load down the source

with 200 kΩ 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 kΩ 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,