Connect Analog Voltage Signals to a Data Acquisition (DAQ) Device

To measure analog signals, you need to know the signal source—grounded or floating. You also must consider the measurement system—differential, referenced single-ended, or nonreferenced single-ended.

• Floating Signal Sources – In a floating source, the voltage signal is not connected to any absolute reference or any common ground, such as earth or building ground. Floating signal sources are also called nonreferenced signal sources. Some common examples of floating signal sources are batteries, thermocouples, transformers, and isolation amplifiers.
• Grounded Signal Sources – A grounded source is one in which the voltage signals are referenced to a system ground, such as earth or building ground. Because such sources use the system ground, they share a common ground with the measurement device. The most common examples of grounded sources are devices that plug into the building ground through wall outlets, such as signal generators and power supplies.
• Differential Mode – A differential measurement system has neither of its inputs tied to a fixed reference, such as earth or building ground. A differential measurement system is similar to a floating signal source in that the measurement is made with respect to a floating ground that is different from the measurement system ground. Differential connections may require two analog input terminals to make a signal connection, reducing the overall number of signals you can attach to your DAQ device.
• Referenced Single-Ended Mode – A referenced single-ended (RSE) measurement system measures voltage with respect to the ground, AIGND in the figure, which is directly connected to the measurement system ground. This option typically allows for more signal connections per device (up to 2x) than differential mode.
• Non-Referenced Single-Ended Mode – In an NRSE measurement system, all measurements are still made with respect to a single-node analog input sense (AISENSE), but the potential at this node can vary with respect to the measurement system ground. This option typically allows for more signal connections per device (up to 2x) than differential mode.
• Pseudodifferential Mode –Like a differential input channel, a pseudodifferential measurement system exposes both the positive and negative sides of the channel. You connect the positive and negative inputs to the respective outputs of the unit under test. The negative input is tied to system ground through a relatively small impedance The positive and negative sides of the input channel are separated by a larger impedance.

The following table provides an application-independent summary of analog input connections.

Table 1. Analog Input Configurations

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

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 device 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.

Figure 1. Differential Connections for Floating 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 instrumentation amplifier on the DAQ device.

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 device are less than 3 m (10 ft).

Figure 2. NRSE Connections for Floating Signal Sources

Differential input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions. However, differential input connections may reduce the overall number of signals you can attach to your DAQ device.

With this type of single-ended connection, the instrumentation amplifier rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

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 device are less than 3 m (10 ft).

Figure 3. RSE Connections for Floating Signal Sources

Differential input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions. However, differential input connections may reduce the overall number of signals you can attach to your DAQ device.

With this type of single-ended connection, the instrumentation amplifier rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

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

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 device 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.

Figure 4. 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 instrumentation amplifier on the DAQ device.

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 device are less than 3 m (10 ft).
• The input signal can share a common reference point with other signals.

Figure 5. Single-Ended Connections for Ground-Referenced Signal Sources (NRSE Configuration)

Differential input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions. However, differential input connections may reduce the overall number of signals you can attach to your DAQ device.

With this type of single-ended connection, the instrumentation amplifier rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

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

As shown in the bottom-rightmost cell of Table 1, there can be a potential difference between AI GND and the ground of the sensor. In RSE mode, this ground loop causes measurement errors.

Environmental noise can seriously affect the measurement accuracy of the device if you do not take proper care when running signal wires between signal sources and the device.

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 instrumentation amplifier rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

The following recommendations apply mainly to analog input signal routing to the device, although they also apply to signal routing in general. Minimize noise pickup and maximize measurement accuracy by taking the following precautions:

• Use differential analog input connections to reject common-mode noise.
• Use individually shielded, twisted-pair wires to connect AI signals to the device. With this type of wire, the signals attached to the positive and negative input channels are twisted together and then covered with a shield. You then connect this shield only at one point to the signal source ground. This kind of connection is required for signals traveling through areas with large magnetic fields or high electromagnetic interference.

Refer to the NI Developer Zone document, Field Wiring and Noise Considerations for Analog Signals, for more information.