National Instruments SCXI-1121 Manuel D’Utilisation

© National Instruments Corporation

Figure 2-6.  Floating AC-Coupled Signal Connection

Figure 2-7.  AC-Coupled Signal Connection with High Common-Mode Voltage

For AC-coupled signals, you should connect an external resistor from the 
positive input channel to the signal reference. This is needed to provide the 
DC path for the positive input bias current. Typical resistor values range 
from 100 k

Ω

 to 1 M

Ω

. This solution, although necessary in this case, lowers 

the input impedance of the input channel amplifier and introduces an 
additional offset voltage proportional to the input bias current and to the 
resistor value used. The typical input bias current of the amplifier consists 
of ±80 pA and a negligible offset drift current. When a 100 k

Ω

 resistor is 

used, this will result into ±8 µV of offset, which is insignificant in most 
applications. However, if larger resistors are used, significant input offset 
may result. To determine the maximum offset introduced by the biasing 
resistor, use the following equation:

The input signal range of an SCXI-1121 input channel is ±5 V/ G

total 

referenced to its negative input, where G

total 

is equal to the product of the 

first-stage and second-stage gains. In addition, the input channels are 
overvoltage protected to 250 V

rms

 with power on or off at a maximum of 

4.5 m

Arms

 sink or source.

V

s

V

out

+

–

+

–

R

bias

+

Module

V

s

V

out

+

–

+

–

R

bias

V

cm

High 
CMV

+

–

+

Module

ofsbias

bias

bias

×

=