A floating signal source is a signal source that is not connected to the building ground system, but has an isolated ground-reference point.

Some examples of floating signal sources are outputs of transformers, thermocouples, battery-powered devices, optical isolators, and isolation amplifiers. An instrument or device that has an isolated output is a floating signal source.

Differential Connections for Floating Signal Sources

Differential signal connections reduce noise pickup and increase common-mode noise rejection. Differential signal connections also allow input signals to float within the common-mode limits of the NI programmable gain instrumentation amplifier (NI-PGIA).

Use differential input connections for any channel that meets any of the following conditions:

  • The input signal is low level (less than 1 V).
  • The leads connecting the signal to the USB-6451 are greater than 3 m (10 ft).
  • The input signal requires a separate ground-reference point or return signal.
  • The signal leads travel through noisy environments.
  • Two analog input channels, AI+ and AI-, are available for the signal.

It is important to connect the negative lead of a floating source to AI GND (either directly or through a bias resistor). Otherwise, the source may float out of the maximum working voltage range of the NI-PGIA and the USB-6451 returns erroneous data.

The easiest way to reference the source to AI GND is to connect the positive side of the signal to AI+ and connect the negative side of the signal to AI GND as well as to AI- without using resistors. This connection works well for DC-coupled sources with low source impedance (less than 100 Ω).

Figure 19. Differential Connections for Floating Signal Sources 
without Bias Resistors


However, for larger source impedances, this connection leaves the differential 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. This noise appears as a differential mode signal instead of a common-mode signal, and thus appears in your data. In this case, instead of directly connecting the negative line to AI GND, 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 NI-PGIA).

Figure 20. Differential Connections for Floating Signal Sources 
with Single Bias Resistor


You can fully balance the signal path by connecting another resistor of the same value between the positive input and AI GND, as shown in the following figure. 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 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.

Figure 21. Differential Connections for Floating Signal Sources 
with Balanced Bias Resistors


Both inputs of the NI-PGIA require a DC path to ground in order for the NI-PGIA to work. If the source is AC coupled (capacitively coupled), the NI-PGIA 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 100 kΩ 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, as shown in the following figure.

Figure 22. Differential Connections for AC Coupled Floating Sources 
with Balanced Bias Resistors


Non-Referenced Single-Ended (NRSE) Connections for Floating Signal Sources

Only use NRSE input connections if the input signal meets the following conditions:

  • The input signal is high-level (greater than 1 V).
  • The leads connecting the signal to the USB-6451 are less than 3 m (10 ft).

Differential input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions.

In the single-ended modes, more electrostatic and magnetic noise couples into the signal connections than in differential configurations. 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.

With this type of connection, the NI programmable gain instrumentation amplifier (NI-PGIA) rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

It is important to connect the negative lead of a floating signals source to AI GND (either directly or through a resistor). Otherwise the source may float out of the valid input range of the NI-PGIA and the USB-6451 returns erroneous data.

The following figure shows a floating source connected to the USB-6451 in NRSE mode.

Figure 23. NRSE Connections for Floating Signal Sources


All of the bias resistor configurations discussed in the Differential Connections for Floating Signal Sources section apply to the NRSE bias resistors as well. Replace AI- with AI SENSE in the figures included in that section for configurations with zero to two bias resistors. The noise rejection of NRSE mode is better than RSE mode because the AI SENSE connection is made remotely near the source. However, the noise rejection of NRSE mode is worse than differential mode because the AI SENSE connection is shared with all channels rather than being cabled in a twisted pair with the AI+ signal.

You can use the DAQ Assistant to configure the channels for RSE or NRSE input modes.

Referenced Single-Ended (RSE) Connections for Floating Signal Sources

Only use RSE input connections if the input signal meets the following conditions:

  • The input signal can share a common reference point, AI GND, with other signals that 
use RSE.
  • The input signal is high-level (greater than 1 V).
  • The leads connecting the signal to the USB-6451 are less than 3 m (10 ft).

Differential input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions.

In the single-ended modes, more electrostatic and magnetic noise couples into the signal connections than in differential configurations. 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.

With this type of connection, the NI programmable gain instrumentation amplifier (NI-PGIA) rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

The following figure shows how to connect a floating signal source to the USB-6451 configured for RSE mode.

Figure 24. RSE Connections for Floating Signal Sources


You can use the DAQ Assistant to configure the channels for RSE or non-referenced single-ended (NRSE) input modes.