©
National Instruments Corporation
5
SCXI-1305 Installation Guide
Configuring the Channels for AC or DC Coupling
You can configure each channel for either AC or DC coupling using the
switch settings shown in Table 2. Change any switch settings before you
connect signals to the terminal block. Each channel requires setting two
switches in the same position for the coupling circuitry to operate properly.
Configure the channels as follows:
•
For AC coupling, set both switches to the AC position. The AC
position places single-pole highpass filters in the paths of both the
positive and negative inputs for the channel. Each filter consists of a
1 µF DC blocking capacitor and a 1 M
Ω
resistor to analog ground
1
,
giving a –3 dB cutoff frequency of 0.16 Hz. The capacitors can block
up to 50 VDC; however, voltages applied to the terminal block must
not
exceed ±42 VDC.
•
For DC coupling, set both switches to the DC position. DC coupling
removes the filters from the signal paths and connects the BNCs
directly to the module inputs. Always use this position on the
SCXI-1121 excitation channels.
Refer to Figure 2 for the AC/DC coupling configuration switch locations.
1
For the SCXI-1120/D, SCXI-1121, SCXI-1125, and SCXI-1126, there is an effective 2 M
Ω
resistor instead of two 1 M
Ω
resistors. This 2 M
Ω
resistor is referenced to the negative input of the SCXI-1120/D, SCXI-1121, SCXI-1125, and SCXI-1126
instead of ground to maintain the SCXI-1120/D, SCXI-1121, SCXI-1125, and SCXI-1126 isolation.
Table 2.
AC/DC Coupling Configuration
Switch Position
Description
Use for AC coupling of the input
signal.
Use for DC coupling of the input
signal (factory setting). Use this
position for the SCXI-1121
excitation channels.
AC
DC
AC
DC
