National Instruments NI USB-621x - Referenced Single-Ended (RSE) Measurements

Chapter 6 Connecting AI Signals on the USB-6215/6218 Devices
© National Instruments Corporation 6-3 NI USB-621x User Manual
The differential connection configuration allows the common-mode noise
voltage, V
cm
, to be rejected during the measurement of V
1
.
You must connect the negative lead of your sensors and AI GND to a local
ground signal on your system.
Referenced Single-Ended (RSE) Measurements
Using the RSE measurement configuration allows the USB-6215/6218 to
take measurements on all AI channels when all channels share a common
ground. Refer to Figure 6-2 for an illustration of an RSE connection
configuration.
Note If you leave the AI GND pin unconnected, the signals will float outside the working
input range of the USB-6215/6218, which may result in unreliable measurements because
there is no way to ensure that the input signal is within 10 V of AI GND.
Figure 6-2. Connecting a Device to a USB-6215/6218 Using RSE Connections
In an RSE connection configuration, each input channel is measured with
respect to AI GND.
AI1
AI2
AI GND
USB-6215/6218
MUX
ADC
PGIA