Connecting Analog Input Signals On Usb-6215/6216/6218 Devices; Taking Differential Measurements - National Instruments Data Acquisition Device NI USB-621x User Manual

Chapter 4 Analog Input
Connecting Analog Input Signals on
USB-6215/6216/6218 Devices
You must always connect AI GND to a local ground signal in your system using
Note
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

NI USB-621x User Manual
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.
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
differential connection configurations with USB-621x devices.
Figure 4-24 shows a differential connection configuration.
V
1
V V
cm 2
* This signal name indicates the differential pair. Refer to Table 3-1,
Signals, for a list of differential signal pairs.
Figure 4-24. Connecting to the USB-6215/6216/6218 in Differential Mode
Signals, lists the signal pairs that are valid for
AI 0+
AI 0– (AI 8)*
Mux
AI 1+
AI 1– (AI 9)*
AI GND
4-36
ADC
PGIA
USB-6215/6216/6218
I/O Connector
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