National Instruments Data Acquisition Device NI USB-621x - Manual (Page 69)

Chapter 4 Analog Input
NI USB-621x User Manual 4-36 ni.com
Connecting Analog Input Signals on
USB-6215/6216/6218 Devices
You can connect the USB-6215/6216/6218 directly to a variety of devices
and other signal sources. Make sure the devices you connect to the
USB-6215/6216/6218 are compatible with the input specifications of the
module.
When connecting various sources to the USB-6215/6216/6218, you can use
differential, single-ended, or a combination of single-ended and differential
connections.
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.
Taking 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. The AI <0..31> description in
Table 3-1,
I/O Connector Signals , lists the signal pairs that are valid for
differential connection configurations with USB-621x devices.
Figure 4-24 shows a differential connection configuration.
Figure 4-24. Connecting to the USB-6215/6216/6218 in Differential Mode
* This signal name indicates the differential pair. Refer to Table 3-1, I/O Connector
Signals , for a list of differential signal pairs.
AI 0+
AI 0– (AI 8)*
Mux
AI 1+
AI 1– (AI 9)*
AI GND
V
1
V
2
V
cm
ADC PGIA
USB-6215/6216/6218