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Chapter 2 Hardware Overv iew of the NI 78x x Multifunction Reconfigurable I/O U ser Manual 2-14 ni.com Y ou can configure the NI 783 x /784 x /785 x device channels in softw are for RSE or NRSE input modes. Use the RSE i…

Chapter 2 Hardware Overview of the NI 78xx

line to AIGND, connect it to AIGND through a resistor that is about

100 times the equivalent source impedance. The resistor puts the signal

path nearly in balance. About the same amount of noise couples onto both

connections, which yields better rejection of electrostatically coupled

noise. Also, this input mode does not load down 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 2-7.

This fully balanced input mode offers slightly better noise rejection but has

the disadvantage of loading down the source 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

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, connect the

negative input directly to AIGND. If the source has high output impedance,

balance the signal path as previously described using the same value

resistor on both the positive and negative inputs. Loading down the source

causes some gain error.

Single-Ended Connection Considerations

When an NI 783x/784x/785x AI signal is referenced to a ground that can be

shared with other input signals, it forms a single-ended connection. The

input signal connects to the positive input of the instrumentation amplifier

and the ground connects 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 device are less than 3 m (10 ft).

• The input signal can share a common reference point with other

signals.

Use DIFF input connections for greater signal integrity for any input signal

that does not meet the preceding conditions.

Chapter 2 Hardware Overview of the NI 78xx

Multifunction Reconfigurable I/O User Manual 2-14 ni.com

You can configure the NI 783x/784x/785x device channels in software for

RSE or NRSE input modes. Use the RSE input mode for floating signal

sources. In this case, the device provides the reference ground point for the

external signal. Use the NRSE input mode for ground-referenced signal

sources. In this case, the external signal supplies its own reference ground

point and the device should not supply one.

In single-ended input modes, electrostatic and magnetic noise couples into

the signal connections more than in differential input modes. The coupling

is the result of differences in the signal path. Magnetic coupling

is proportional to the area between the two signal conductors. Electrical

coupling is a function of how much the electric field differs between the

two conductors.

Single-Ended Connections for Floating Signal

Sources (RSE Input Mode)

Figure 2-8 shows how to connect a floating signal source to a channel on

the NI 783x/784x/785x configured for RSE input mode.

Figure 2-8. Single-Ended Input Connections for Nonreferenced or Floating Signals

–

I/O Connector

AISENSE

AIGND

AI+

AI–

Floating

Signal

Source

Instrumentation

Amplifier

Measured

Voltage

RSE Input Mode Selected

Chapter 2 Hardware Overview of the NI 78xx

Single-Ended Connections for Grounded Signal

Sources (NRSE Input Mode)

To measure a grounded signal source with a single-ended input mode, you

must configure the NI 783x/784x/785x in the NRSE input mode. Then

connect the signal to the positive input of the NI 783x/784x/785x

instrumentation amplifier and connect the signal local ground reference to

the negative input of the instrumentation amplifier. The ground point of the

signal should be connected to AISENSE. Any potential difference between

the NI 783x/784x/785x ground and the signal ground appears as a

common-mode signal at both the positive and negative inputs of the

instrumentation amplifier. The instrumentation amplifier rejects this

difference. If the input circuitry of a NI 783x/784x/785x is referenced to

ground in RSE input mode, this difference in ground potentials appears as

an error in the measured voltage.

Figure 2-9 shows how to connect a grounded signal source to a channel on

the NI 783x/784x/785x configured for NRSE input mode.

Figure 2-9. Single-Ended Input Connections for Ground-Referenced Signals

–

I/O Connector

AISENSE

AIGND

AI+

AI–

Instrumentation

Amplifier

Measured

Voltage

Ground-

Referenced

Signal

Source

Common-

Mode

Noise and

Ground

Potential

NRSE Input Mode Selected