National Instruments SCXI -1125 用户手册

You can have an application where you wish to measure only AC voltages 
and remove the DC component of a signal before amplification and 
sampling. In such cases, you can connect a capacitor in series with one or 
both input terminals of the SCXI-1125, as shown in Figures 2-3 and 2-4. 
A resistor is connected across the input terminals of the channel to DC 
reference the input stage of the SCXI-1125. You do not need to use a bias 
resistor with any high-voltage terminal blocks, since the terminal blocks 
already have a resistor between the input terminals, or with the SCXI-1305 
BNC connectivity terminal block, since this terminal block already has an 
AC-coupling option built in.

Figure 2-3.  Connecting a Floating AC-Coupled Signal

Connecting a signal source to chassis ground in Figures 2-2 and 2-3, breaks the 

isolation barrier.

Figure 2-4.  Connecting a Ground-Referenced AC-Coupled Signal

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