Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

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.

When to Use Differential Connections with Ground-Referenced Signal Sources

Use DIFF 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.

DIFF signal connections reduce noise pickup and increase common-mode noise rejection. DIFF signal connections also allow input signals to float within the common-mode limits of the NI-PGIA.

Refer to the Using Differential Connections for Ground-Referenced Signal

Sources section for more information about differential connections.

When to Use Non-Referenced Single-Ended (NRSE) Connections with Ground-Referenced Signal Sources

Only use non-referenced single-ended 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.

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