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Chapter 2 Hardware Overv iew of the NI 78x x Multifunction Reconfigurable I/O U ser Manual 2-12 ni.com Differential Connections for Nonreferenced or Floating Signal Sources Figure 2-7 shows how to connect a floa ting sig…

Chapter 2 Hardware Overview of the NI 78xx

Differential Connections for Ground-Referenced

Signal Sources

Figure 2-6 shows how to connect a ground-referenced signal source to a

channel on the NI 783x/784x/785x configured in DIFF input mode.

Figure 2-6. Differential Input Connections for Ground-Referenced Signals

With this connection type, the instrumentation amplifier rejects both the

common-mode noise in the signal and the ground potential difference

between the signal source and the NI 783x/784x/785x ground, shown as

in Figure 2-6. In addition, the instrumentation amplifier can reject

common-mode noise pickup in the leads connecting the signal sources to

the device. The instrumentation amplifier can reject common-mode signals

when V+

and V–

(input signals) are both within their specified input

ranges. Refer to the Multifunction Reconfigurable I/O Specifications,

available at

ni.com/docs, for more information about input ranges.

DIFF Input Mode Selected

–

I/O Connector

AISENSE

AIGND

AI+

AI–

Ground-

Referenced

Signal

Source

Common-

Mode

Noise and

Ground

Potential

Measured

Voltage

Instrumentation

Amplifier

Chapter 2 Hardware Overview of the NI 78xx

Multifunction Reconfigurable I/O User Manual 2-12 ni.com

Differential Connections for Nonreferenced or

Floating Signal Sources

Figure 2-7 shows how to connect a floating signal source to a channel on

the NI 783x/784x/785x configured in DIFF input mode.

Figure 2-7. Differential Input Connections for Nonreferenced Signals

Figure 2-7 shows two bias resistors connected in parallel with the signal

leads of a floating signal source. If you do not use the resistors and the

source is truly floating, the source might not remain within the

common-mode signal range of the instrumentation amplifier, causing

erroneous readings. You must reference the source to AIGND by

connecting the positive side of the signal to the positive input of the

instrumentation amplifier and connecting the negative side of the signal to

AIGND and to the negative input of the instrumentation amplifier without

resistors. This connection works well for DC-coupled sources with low

source impedance, less than 100 Ω.

For larger source impedances, this connection leaves the differential signal

path significantly out of balance. Noise that couples electrostatically onto

the positive line does not couple onto the negative line because it is

connected to ground. Hence, this noise appears as a differential-mode

signal instead of a common-mode signal, and the instrumentation amplifier

does not reject it. In this case, instead of directly connecting the negative

DIFF Input Mode Selected

–

AISENSE

AIGND

AI+

AI–

–

I/O Connector

Floating

Signal

Source

Bias

Current

Return

Paths

Measured

Voltage

Instrumentation

Amplifier

Bias

Resistors

(see text)

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.