National Instruments NI USB-621x - Manual (Page 64)

© National Instruments Corporation 6-1 NI USB-621x User Manual
6
Connecting AI Signals on the
USB-6215/6218 Devices
You can connect the USB-6215/6218 directly to a variety of devices and
other signal sources. Make sure the devices you connect to the
USB-6215/6218 are compatible with the input specifications of the
module.
When connecting various sources to the USB-6215/6218, you can use
differential, single-ended, or a combination of single-ended and differential
connections. Refer to Figures 6-1, 6-2, and 6-3 for diagrams of each
connection type.
Note You must always connect AI GND to a local ground signal in your system using a
low impedance connection. If you leave AI GND unconnected, you cannot ensure that
AI <0..31> are within 10 V of AI GND, and your measurement may be unreliable.
Differential Measurements
To attain more accurate measurements and less noise, use a differential
measurement configuration. A differential measurement configuration
requires two inputs for each measurement.
Differential Pairs
Table 6-1 the signal pairs that are valid for differential connection
configurations with the USB-6215/6218.
Table 6-1. I/O Connector Signals
Channel Signal + Signal –
0 AI 0 AI 8
1 AI 1 AI 9
2 AI 2 AI 10