Ground-Referenced Signal Sources Connections

A ground-referenced signal source is a signal source connected to the building system ground.

A ground-referenced signal source is already connected to a common ground point with respect to the USB-6453, assuming that the computer is plugged into the same power system as the source. Non-isolated outputs of instruments and devices that plug into the building power system fall into this category.

The difference in ground potential between two instruments connected to the same building power system is typically between 1 and 100 mV, but the difference can be much higher if power distribution circuits are improperly connected. If a grounded signal source is incorrectly measured, this difference can appear as measurement error. Follow the connection instructions for grounded signal sources to eliminate this ground potential difference from the measured signal.

Differential Connections for Ground-Referenced 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-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-6453 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.

The following figure shows how to connect a ground-referenced signal source to the USB-6453 configured in differential mode.

Figure 25. Differential Connections for Ground-Referenced Signal Sources


With this type of connection, the NI-PGIA rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground, shown as Vcm in the figure.

AI+ and AI- must both remain less than the maximum working range specified in the USB-6453 Specifications.

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

Only use non-referenced signal-ended 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-6453 are less than 3 m (10 ft).
  • The input signal can share a common reference point with other signals.

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 ground-reference signal sources in NRSE mode.

Figure 26. Single-Ended Connections for 
Ground-Referenced Signal Sources (NRSE Configuration)


AI<0..31> and AI SENSE must both remain less than the maximum working range specified in the USB-6453 Specifications.

In this mode, NR (AI SENSE) is internally connected to the negative input of the NI-PGIA. Therefore, the ground point of the signal connects to the negative input of the NI-PGIA.

Any potential difference between the device ground and the signal ground appears as a common-mode signal at both the positive and negative inputs of the NI-PGIA, and this difference is rejected by the amplifier. If the input circuitry of a device were referenced to ground, as it is in the RSE ground-reference setting, this difference in ground potentials would appear as an error in the measured voltage.

Using the DAQ Assistant, you can configure the channels for RSE or NRSE input modes.

Referenced Single-Ended (RSE) Connections with Ground-Referenced Signal Sources

Do not use RSE connections with ground-referenced signal sources. Use non-referenced single-ended or differential connections instead.

As illustrated in Analog Input Signal Connections, there can be a potential difference between AI GND and the ground of the sensor. In RSE mode, this ground loop causes measurement errors.