National Instruments NI 783xR Single-Ended Connection Considerations

Chapter 2 Hardware Overview ofthe NI 783xR

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 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 783xR 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 NI 783xR are less than

3m(10ft).

• 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.

You can configure the NI 783xR channels in software for RSE or NRSE

input modes. Use the RSE input mode for floating signal sources. In this

case, the NI 783xR 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

NI7 83xR should not supply one.