National Instruments DAQ S AC-Coupled, Field Wiring Considerations

Chapter 4 Analog Input

•High Source Impedance—For larger source impedances, this

connection leaves the DIFF signal path significantly off 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 DIFF-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 line to AIGND, connect the

negative line to AI GND through a resistor that is about 100 times the

equivalent source impedance. The resistor puts the signal path nearly

in balance, so that about the same amount of noise couples onto both

connections, yielding better rejection of electrostatically coupled

noise. This configuration 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. This fully

balanced configuration 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

2kΩ and each of the two resistors is 100kΩ, the resistors load down

the source with 200kΩ and produ ce a –1% gain error.

AC-Coupled

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 AI GND. 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 100kΩ to 1 MΩ). In this case, connect the

negative input directly to AI GND. 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; be aware that there

is some gain error from loading down the source.

Field Wiring Considerations

Environmental noise can seriously affect the measurement accuracy of the

S Series device if you do not take proper care when running signal wires

between signal sources and the device. The following recommendations

apply mainly to AI signal routing, although they also apply to signal

routing in general.